Full interview-prep notes, ##Episode 1 through 29.
Episode 1 : Execution Context
==============================
Everything in JS happens inside the execution context. Imagine a sealed-off container inside which JS runs. It is an abstract concept that hold info about the env. within the current code is being executed.
In the container the first component is memory component and the 2nd one is code component
Memory component has all the variables and functions in key value pairs. It is also called Variable environment.
Code component is the place where code is executed one line at a time. It is also called the Thread of Execution.
JS is a synchronous, single-threaded language
Synchronous:- In a specific synchronous order.
Single-threaded:- One command at a time.
Episode 2 : How JS is executed & Call Stack
=============================================
When a JS program is ran, a global execution context is created.
The execution context is created in two phases.
- Memory creation phase - JS will allocate memory to variables and functions.
- Code execution phase
Let's consider the below example and its code execution steps:
var n = 2;
function square(num) {
var ans = num * num;
return ans;
}
var square2 = square(n);
var square4 = square(4);
The very first thing which JS does is memory creation phase, so it goes to line one of above code snippet, and allocates a memory space for variable 'n' and then goes to line two, and allocates a memory space for function 'square'. When allocating memory for n it stores 'undefined', a special value for 'n'. For 'square', it stores the whole code of the function inside its memory space. Then, as square2 and square4 are variables as well, it allocates memory and stores 'undefined' for them, and this is the end of first phase i.e. memory creation phase.
Now, in 2nd phase i.e. code execution phase, it starts going through the whole code line by line. As it encounters var n = 2, it assigns 2 to 'n'. Until now, the value of 'n' was undefined. For function, there is nothing to execute. As these lines were already dealt with in memory creation phase.
Coming to line 6 i.e. var square2 = square(n), here functions are a bit different than any other language. A new execution context is created altogether. Again in this new execution context, in memory creation phase, we allocate memory to num and ans the two variables. And undefined is placed in them. Now, in code execution phase of this execution context, first 2 is assigned to num. Then var ans = num * num will store 4 in ans. After that, return ans returns the control of program back to where this function was invoked from.
When return keyword is encountered, It returns the control to the called line and also the function execution context is deleted. Same thing will be repeated for square4 and then after that is finished, the global execution context will be destroyed.
Javascript manages code execution context creation and deletion with the the help of Call Stack.
Call Stack is a mechanism to keep track of its place in script that calls multiple function.
Call Stack maintains the order of execution of execution contexts. It is also known as Program Stack, Control Stack, Runtime stack, Machine Stack, Execution context stack.
Episode 3 : Hoisting in JavaScript (variables & functions)
=============================================================
Let's observe the below code and it's explaination:
getName(); // Namaste Javascript
console.log(x); // undefined
var x = 7;
function getName() {
console.log("Namaste Javascript");
}
It should have been an outright error in many other languages, as it is not possible to even access something which is not even created (defined) yet But in JS, We know that in memory creation phase it assigns undefined and puts the content of function to function's memory. And in execution, it then executes whatever is asked. Here, as execution goes line by line and not after compiling, it could only print undefined and nothing else. This phenomenon, is not an error. However, if we remove var x = 7; then it gives error. Uncaught ReferenceError: x is not defined
Hoisting is a concept which enables us to extract values of variables and functions even before initialising/assigning value without getting error and this is happening due to the 1st phase (memory creation phase) of the Execution Context.
So in previous lecture, we learnt that execution context gets created in two phase, so even before code execution, memory is created so in case of variable, it will be initialized as undefined while in case of function the whole function code is placed in the memory. Example:
getName(); // Namaste JavaScript
console.log(x); // Uncaught Reference: x is not defined.
console.log(getName); // f getName(){ console.log("Namaste JavaScript); }
function getName() {
console.log("Namaste JavaScript");
}
Now let's observe a different example and try to understand the output.
getName(); // Uncaught TypeError: getName is not a function
console.log(getName);
var getName = function () {
console.log("Namaste JavaScript");
};
// The code won't execute as the first line itself throws an TypeError.
Episode 4 : Functions and Variable Environments
=================================================
var x = 1;
a();
b(); // we are calling the functions before defining them. This will work properly, as seen in Hoisting.
console.log(x);
function a() {
var x = 10; // local scope because of separate execution context
console.log(x);
}
function b() {
var x = 100;
console.log(x);
}
Outputs:
10
100
1
Code Flow in terms of Execution Context
The Global Execution Context (GEC) is created (the big box with Memory and Code subparts). Also GEC is pushed into Call Stack.
Call Stack : GEC
In first phase of GEC (memory phase), variable x:undefined and a and b have their entire function code as value initialized
In second phase of GEC (execution phase), when the function is called, a new local Execution Context is created. After x = 1 assigned to GEC x, a() is called. So local EC for a is made inside code part of GEC.
Call Stack: [GEC, a()]
For local EC, a totally different x variable assigned undefined(x inside a()) in phase 1, and in phase 2 it is assigned 10 and printed in console log. After printing, no more commands to run, so a() local EC is removed from both GEC and from Call stack.
Call Stack: GEC
Cursor goes back to b() function call. Same steps repeat.
Call Stack :[GEC, b()] -> GEC (after printing yet another totally different x value as 100 in console log)
Finally GEC is deleted and also removed from call stack. Program ends.
Episode 5 : Shortest JS Program, window & this keyword
=========================================================
The shortest JS program is empty file. Because even then, JS engine does a lot of things. As always, even in this case, it creates the GEC which has memory space and the execution context.
JS engine creates something known as 'window'. It is an object, which is created in the global space. It contains lots of functions and variables. These functions and variables can be accessed from anywhere in the program. JS engine also creates a this keyword, which points to the window object at the global level. So, in summary, along with GEC, a global object (window) and a this variable are created.
In different engines, the name of global object changes. Window in browsers, but in nodeJS it is called something else. At global level, this === window.
If we create any variable in the global scope, then the variables get attached to the global object.
var x = 10;
console.log(x); // 10
console.log(this.x); // 10
console.log(window.x); // 10
Episode 6 : undefined vs not defined in JS
============================================
In first phase (memory allocation) JS assigns each variable a placeholder called undefined.
undefined is when memory is allocated for the variable, but no value is assigned yet.
If an object/variable is not even declared/found in memory allocation phase, and tried to access it then it is Not defined.
Not Defined !== Undefined
When variable is declared but not assigned value, its current value is undefined. But when the variable itself is not declared but called in code, then it is not defined.
console.log(x); // undefined
var x = 25;
console.log(x); // 25
console.log(a); // Uncaught ReferenceError: a is not defined
JS is a loosely typed / weakly typed language. It doesn't attach variables to any datatype. We can say var a = 5, and then change the value to boolean a = true or string a = 'hello' later on.
Never assign undefined to a variable manually. Let it happen on it's own accord.
Episode 7 : The Scope Chain, Scope & Lexical Environment
===========================================================
Scope in Javascript is directly related to Lexical Environment.
Let's observe the below examples:
// CASE 1
function a() {
console.log(b); // 10
// Instead of printing undefined it prints 10, So somehow this a function could access the variable b outside the function scope.
}
var b = 10;
a();
// CASE 2
function a() {
c();
function c() {
console.log(b); // 10
}
}
var b = 10;
a();
// CASE 3
function a() {
c();
function c() {
var b = 100;
console.log(b); // 100
}
}
var b = 10;
a();
// CASE 4
function a() {
var b = 10;
c();
function c() {
console.log(b); // 10
}
}
a();
console.log(b); // Error, Not Defined
Let's try to understand the output in each of the cases above.
- In case 1: function a is able to access variable b from Global scope.
- In case 2: 10 is printed. It means that within nested function too, the global scope variable can be accessed.
- In case 3: 100 is printed meaning local variable of the same name took precedence over a global variable.
- In case 4: A function can access a global variable, but the global execution context can't access any local variable.
To summarize the above points in terms of execution context:
call_stack = [GEC, a(), c()]
Now lets also assign the memory sections of each execution context in call_stack.
c() = [[lexical environment pointer pointing to a()]]
a() = [b:10, c:{}, [lexical environment pointer pointing to GEC]]
GEC = [a:{},[lexical_environment pointer pointing to null]]
So, Lexical Environment = local memory + lexical env of its parent. Hence, Lexical Environement is the local memory along with the lexical environment of its parent
Lexical: In hierarchy, In order
Whenever an Execution Context is created, a Lexical environment(LE) is also created and is referenced in the local Execution Context(in memory space).
The process of going one by one to parent and checking for values is called scope chain or Lexcial environment chain.
function a() {
function c() {
// logic here
}
c(); // c is lexically inside a
} // a is lexically inside global execution
Lexical or Static scope refers to the accessibility of variables, functions and object based on physical location in source code.
Global {
Outer {
Inner
}
}
// Inner is surrounded by lexical scope of Outer
TLDR; An inner function can access variables which are in outer functions even if inner function is nested deep. In any other case, a function can't access variables not in its scope.
Episode 8 : let & const in JS, Temporal Dead Zone
====================================================
let and const declarations are hoisted. But its different from var.
console.log(a); // ReferenceError: Cannot access 'a' before initialization
console.log(b); // prints undefined as expected
let a = 10;
console.log(a); // 10
var b = 15;
console.log(window.a); // undefined
console.log(window.b); // 15
It looks like let isn't hoisted, but it is, let's understand.
Both a and b are actually initialized as undefined in hoisting stage. But var b is inside the storage space of GLOBAL, and a is in a separate memory object called script, where it can be accessed only after assigning some value to it first ie. one can access 'a' only if it is assigned. Thus, it throws error.
Temporal Dead Zone : Time since when the let variable was hoisted until it is initialized some value.
So any line till before "let a = 10" is the TDZ for a. Since a is not accessible on global, its not accessible in window/this also. window.b or this.b -> 15; But window.a or this.a -> undefined, just like window.x -> undefined (x isn't declared anywhere).
Reference Error are thrown when variables are in temporal dead zone.
Syntax Error doesn't even let us run single line of code.
let a = 10;
let a = 100; //this code is rejected upfront as SyntaxError. (duplicate declaration)
------------------
let a = 10;
var a = 100; // this code also rejected upfront as SyntaxError. (can't use same name in same scope)
Let is a stricter version of var. Now, const is even more stricter than let.
let a;
a = 10;
console.log(a) // 10. Note declaration and assigning of a is in different lines.
------------------
const b;
b = 10;
console.log(b); // SyntaxError: Missing initializer in const declaration. (This type of declaration won't work with const. const b = 10 only will work)
------------------
const b = 100;
b = 1000; //this gives us TypeError: Assignment to constant variable.
Types of Error: Syntax, Reference, and Type.
-
Uncaught ReferenceError: x is not defined— This Error signifies that x has never been in the scope of the program. This literally means that x was never defined/declared and is being tried to be accesed. -
Uncaught ReferenceError: cannot access 'a' before initialization— This Error signifies that 'a' cannot be accessed because it is declared as 'let' and since it is not assigned a value, it is its Temporal Dead Zone. Thus, this error occurs. -
Uncaught SyntaxError: Identifier 'a' has already been declared— This Error signifies that we are redeclaring a variable that is 'let' declared. No execution will take place. -
Uncaught SyntaxError: Missing initializer in const declaration— This Error signifies that we haven't initialized or assigned value to a const declaration. -
Uncaught TypeError: Assignment to constant variable— This Error signifies that we are reassigning to a const variable.
Some good practices
- Try using const wherever possible.
- If not, use let, Avoid var.
- Declare and initialize all variables with let to the top to avoid errors to shrink temporal dead zone window to zero.
Episode 9 : Block Scope & Shadowing in JS
============================================
What is a Block?
Block aka compound statement is used to group JS statements together into 1 group. We group them within {...}
{
var a = 10;
let b = 20;
const c = 30;
// Here let and const are hoisted in Block scope,
// While, var is hoisted in Global scope.
}
Block Scope and its accessibility example:
{
var a = 10;
let b = 20;
const c = 30;
}
console.log(a); // 10
console.log(b); // Uncaught ReferenceError: b is not defined
Reason? In the BLOCK SCOPE; we get b and c inside it initialized as undefined as a part of hoisting (in a seperate memory space called block). While, a is stored inside a GLOBAL scope. Thus, we say, let and const are BLOCK SCOPED. They are stored in a separate mem space which is reserved for this block. Also, they can't be accessed outside this block. But var a can be accessed anywhere as it is in global scope. Thus, we can't access them outside the Block.
What is Shadowing?
var a = 100;
{
var a = 10; // same name as global var
let b = 20;
const c = 30;
console.log(a); // 10
console.log(b); // 20
console.log(c); // 30
}
console.log(a); // 10, instead of the 100 we were expecting. So block "a" modified val of global "a" as well. In console, only b and c are in block space. a initially is in global space(a = 100), and when a = 10 line is run, a is not created in block space, but replaces 100 with 10 in global space itself.
So, If one has same named variable outside the block, the variable inside the block shadows the outside variable. This happens only for var.
Let's observe the behaviour in case of let and const and understand it's reason.
let b = 100;
{
var a = 10;
let b = 20;
const c = 30;
console.log(b); // 20
}
console.log(b); // 100, Both b's are in separate spaces (one in Block(20) and one in Script(another arbitrary mem space)(100)). Same is also true for const declarations.
Same logic is true even for functions:
const c = 100;
function x() {
const c = 10;
console.log(c); // 10
}
x();
console.log(c); // 100
What is Illegal Shadowing?
let a = 20;
{
var a = 20;
}
// Uncaught SyntaxError: Identifier 'a' has already been declared
We cannot shadow let with var. But it is valid to shadow a let using a let. However, we can shadow var with let.
All scope rules that work in function are same in arrow functions too. Since var is function scoped, it is not a problem with the code below.
let a = 20;
function x() {
var a = 20;
}
Episode 10 : Closures in JS
==============================
Function bundled along with it's lexical scope is closure.
JavaScript has a lexcial scope environment. If a function needs to access a variable, it first goes to its local memory. When it does not find it there, it goes to the memory of its lexical parent. See Below code, Over here function y along with its lexical scope i.e. (function x) would be called a closure.
function x() {
var a = 7;
function y() {
console.log(a);
}
return y;
}
var z = x();
console.log(z); // value of z is entire code of function y.
In above code, When y is returned, not only is the function returned but the entire closure (fun y + its lexical scope) is returned and put inside z. So when z is used somewhere else in program, it still remembers var a inside x().
Another Example:
function z() {
var b = 900;
function x() {
var a = 7;
function y() {
console.log(a, b);
}
y();
}
x();
}
z(); // 7 900
Thus In simple words, we can say: A closure is a function that has access to its outer function scope even after the function has returned. Meaning, A closure can remember and access variables and arguments reference of its outer function even after the function has returned.
Advantages of Closure:
Module Design Pattern: The module design pattern allows us to encapsulate related functionality into a single module or file. It helps organize code, prevent global namespace pollution, and promotes reusability.
// auth.js
const authModule = (function () {
let loggedInUser = null;
function login(username, password) {
// Authenticate user logic...
loggedInUser = username;
}
function logout() {
loggedInUser = null;
}
function getUserInfo() {
return loggedInUser;
}
return {
login,
logout,
getUserInfo,
};
})();
// Usage
authModule.login("john_doe", "secret");
console.log(authModule.getUserInfo()); // 'john_doe'
Currying: Currying is a technique where a function that takes multiple arguments is transformed into a series of functions that take one argument each. It enables partial function application and enhances code flexibility.
const calculateTotalPrice = (taxRate) => (price) =>
price + price * (taxRate / 100);
const calculateSalesTax = calculateTotalPrice(8); // 8% sales tax
const totalPrice = calculateSalesTax(100); // Price with tax
console.log(totalPrice); // 108
Memoization: Memoization optimizes expensive function calls by caching their results. It's useful for recursive or repetitive computations.
function fibonacci(n, memo = {}) {
if (n in memo) return memo[n];
if (n <= 1) return n;
memo[n] = fibonacci(n - 1, memo) + fibonacci(n - 2, memo);
return memo[n];
}
console.log(fibonacci(10)); // 55
Data Hiding and Encapsulation: Encapsulation hides the internal details of an object and exposes only necessary methods and properties. It improves code maintainability and security.
class Person {
#name; // Private field
constructor(name) {
this.#name = name;
}
getName() {
return this.#name;
}
}
const person = new Person("Alice");
console.log(person.getName()); // 'Alice'
// console.log(person.#name); // Error: Private field '#name' must be declared in an enclosing class
setTimeouts: setTimeout allows scheduling a function to run after a specified delay. It's commonly used for asynchronous tasks, animations, and event handling.
function showMessage(message, delay) {
setTimeout(() => {
console.log(message);
}, delay);
}
showMessage("Hello, world!", 2000); // Display after 2 seconds
Disadvantages of Closure:
- Over consumption of memory
- Memory Leak
- Freeze browser
Additional Notes: Currying, properly defined
The notes above showed one currying example (calculateTotalPrice) but currying deserves a formal definition since it's a favorite standalone interview question, separate from "what is a closure."
Currying is a technique of transforming a function that takes multiple arguments into a sequence of functions that each take a single argument (or a smaller group of arguments), returning a new function at each step until all arguments have been supplied.
// Normal function
function sum(a, b, c) {
return a + b + c;
}
sum(1, 2, 3); // 6
// Curried version (manual)
function curriedSum(a) {
return function (b) {
return function (c) {
return a + b + c;
};
};
}
curriedSum(1)(2)(3); // 6
// Same thing, with arrow functions (much shorter)
const curriedSumArrow = (a) => (b) => (c) => a + b + c;
curriedSumArrow(1)(2)(3); // 6
Why does this work? Each inner function is a closure — it remembers the arguments captured by the outer functions, even after those outer functions have already returned. That's the direct link between currying and closures.
A generic curry polyfill (asked often in interviews)
Real-world code doesn't want to hand-write nested functions for every case — this polyfill converts any function into its curried form automatically:
function curry(fn) {
return function curried(...args) {
// If enough arguments have been collected, call the original function
if (args.length >= fn.length) {
return fn.apply(this, args);
}
// Otherwise, return a function that collects more arguments
return function (...nextArgs) {
return curried.apply(this, [...args, ...nextArgs]);
};
};
}
function add3(a, b, c) {
return a + b + c;
}
const curriedAdd3 = curry(add3);
console.log(curriedAdd3(1, 2, 3)); // 6
console.log(curriedAdd3(1)(2, 3)); // 6
console.log(curriedAdd3(1)(2)(3)); // 6
console.log(curriedAdd3(1, 2)(3)); // 6
// All of these work, because curry() adapts to however many args are passed at each step.
fn.length here is what makes this generic — it reads how many parameters the original function declared, so curry() knows when it has "enough" arguments to actually invoke it.
Why curry in practice? Mainly for partial application — pre-filling some arguments of a function to create a more specific, reusable version of it (e.g. the calculateTotalPrice(taxRate) example above pre-fills the tax rate once, then the resulting function can be reused for any price).
Episode 11 : setTimeout + Closures Interview Question
=========================================================
Time, tide and Javascript wait for none.
function x() {
var i = 1;
setTimeout(function () {
console.log(i);
}, 3000);
console.log("Namaste Javascript");
}
x();
// Output:
// Namaste Javascript
// 1 // after waiting 3 seconds
We expect JS to wait 3 sec, print 1 and then go down and print the string. But JS prints string immediately, waits 3 sec and then prints 1.
The function inside setTimeout forms a closure (remembers reference to i). So wherever function goes it carries this ref along with it.
setTimeout takes this callback function & attaches timer of 3000ms and stores it. Goes to next line without waiting and prints string.
After 3000ms runs out, JS takes function, puts it into call stack and runs it.
Q: Print 1 after 1 sec, 2 after 2 sec till 5 : Tricky interview question
We assume this has a simple approach as below:
function x() {
for (var i = 1; i <= 5; i++) {
setTimeout(function () {
console.log(i);
}, i * 1000);
}
console.log("Namaste Javascript");
}
x();
// Output:
// Namaste Javascript
// 6
// 6
// 6
// 6
// 6
Reason? This happens because of closures. When setTimeout stores the function somewhere and attaches timer to it, the function remembers its reference to i, not value of i. All 5 copies of function point to same reference of i. JS stores these 5 functions, prints string and then comes back to the functions. By then the timer has run fully. And due to looping, the i value became 6. And when the callback fun runs the variable i = 6. So same 6 is printed in each log.
To avoid this, we can use let instead of var as let has Block scope. For each iteration, the i is a new variable altogether(new copy of i). Everytime setTimeout is run, the inside function forms closure with new variable i.
But what if interviewer ask us to implement using var?
function x() {
for (var i = 1; i <= 5; i++) {
function close(i) {
setTimeout(function () {
console.log(i);
}, i * 1000);
// put the setT function inside new function close()
}
close(i); // everytime you call close(i) it creates new copy of i. Only this time, it is with var itself!
}
console.log("Namaste Javascript");
}
x();
Episode 12 : Famous Interview Questions ft. Closures
=======================================================
Q1: What is Closure in Javascript?
Ans: A function along with reference to its outer environment together forms a closure. Or in other words, A Closure is a combination of a function and its lexical scope bundled together.
function outer() {
var a = 10;
function inner() {
console.log(a);
} // inner forms a closure with outer
return inner;
}
outer()(); // 10 // over here first `()` will return inner function and then using second `()` to call inner function
Q2: Will the below code still forms a closure?
function outer() {
function inner() {
console.log(a);
}
var a = 10;
return inner;
}
outer()(); // 10
Ans: Yes, because inner function forms a closure with its outer environment so sequence doesn't matter.
Q3: Changing var to let, will it make any difference?
function outer() {
let a = 10;
function inner() {
console.log(a);
}
return inner;
}
outer()(); // 10
Ans: It will still behave the same way.
Q4: Will inner function have the access to outer function argument?
function outer(str) {
let a = 10;
function inner() {
console.log(a, str);
}
return inner;
}
outer("Hello There")(); // 10 "Hello There"
Ans: Inner function will now form closure and will have access to both a and str.
Q5: In below code, will inner form closure with outest?
function outest() {
var c = 20;
function outer(str) {
let a = 10;
function inner() {
console.log(a, c, str);
}
return inner;
}
return outer;
}
outest()("Hello There")(); // 10 20 "Hello There"
Ans: Yes, inner will have access to all its outer environment.
Q6: Output of below code and explaination?
function outest() {
var c = 20;
function outer(str) {
let a = 10;
function inner() {
console.log(a, c, str);
}
return inner;
}
return outer;
}
let a = 100;
outest()("Hello There")(); // 10 20 "Hello There"
Ans: Still the same output, the inner function will have reference to inner a, so conflicting name won't matter here. If it wouldn't have find a inside outer function then it would have went more outer to find a and thus have printed 100. So, it try to resolve variable in scope chain and if a wouldn't have been found it would have given reference error.
Q7: Advantage of Closure?
- Module Design Pattern
- Currying
- Memoize
- Data hiding and encapsulation
- setTimeouts etc.
Q8: Discuss more on Data hiding and encapsulation?
// without closures
var count = 0;
function increment(){
count++;
}
// in the above code, anyone can access count and change it.
------------------------------------------------------------------
// (with closures) -> put everything into a function
function counter() {
var count = 0;
function increment(){
count++;
}
}
console.log(count); // this will give referenceError as count can't be accessed. So now we are able to achieve hiding of data
------------------------------------------------------------------
//(increment with function using closure) true function
function counter() {
var count = 0;
return function increment(){
count++;
console.log(count);
}
}
var counter1 = counter(); //counter function has closure with count var.
counter1(); // increments counter
var counter2 = counter();
counter2(); // here counter2 is whole new copy of counter function and it wont impack the output of counter1
*************************
// Above code is not good and scalable for say, when you plan to implement decrement counter at a later stage.
// To address this issue, we use constructors
// Adding decrement counter and refactoring code:
function Counter() {
//constructor function. Good coding would be to capitalize first letter of constructor function.
var count = 0;
this.incrementCounter = function() { //anonymous function
count++;
console.log(count);
}
this.decrementCounter = function() {
count--;
console.log(count);
}
}
var counter1 = new Counter(); // new keyword for constructor fun
counter1.incrementCounter();
counter1.incrementCounter();
counter1.decrementCounter();
// returns 1 2 1
Q9: Disadvantage of closure?
Ans: Overconsumption of memory when using closure as everytime as those closed over variables are not garbage collected till program expires. So when creating many closures, more memory is accumulated and this can create memory leaks if not handled.
Garbage collector : Program in JS engine or browser that frees up unused memory. In highlevel languages like C++ or JAVA, garbage collection is left to the programmer, but in JS engine its done implicitly.
function a() {
var x = 0;
return function b() {
console.log(x);
};
}
var y = a(); // y is a copy of b()
y();
// Once a() is called, its element x should be garbage collected ideally. But fun b has closure over var x. So mem of x cannot be freed. Like this if more closures formed, it becomes an issue. To tacke this, JS engines like v8 and Chrome have smart garbage collection mechanisms. Say we have var x = 0, z = 10 in above code. When console log happens, x is printed as 0 but z is removed automatically.
Episode 13 : First Class Functions ft. Anonymous Functions
=============================================================
Functions are heart of Javascript.
Q: What is Function statement?
Below way of creating function are function statement.
function a() {
console.log("Hello");
}
a(); // Hello
Q: What is Function Expression?
Assigning a function to a variable. Function acts like a value.
var b = function () {
console.log("Hello");
};
b();
Q: Difference between function statement and expression
The major difference between these two lies in Hoisting.
a(); // "Hello A"
b(); // TypeError
function a() {
console.log("Hello A");
}
var b = function () {
console.log("Hello B");
};
// Why? During mem creation phase a is created in memory and function assigned to a. But b is created like a variable (b:undefined) and until code reaches the function() part, it is still undefined. So it cannot be called.
Q: What is Function Declaration?
Other name for function statement.
Q: What is Anonymous Function?
A function without a name.
function () {
}// this is going to throw Syntax Error - Function Statement requires function name.
They don't have their own identity. So an anonymous function without code inside it results in an error. Anonymous functions are used when functions are used as values eg. the code sample for function expression above.
Q: What is Named Function Expression?
Same as Function Expression but function has a name instead of being anonymous.
var b = function xyz() {
console.log("b called");
};
b(); // "b called"
xyz(); // Throws ReferenceError:xyz is not defined.
// xyz function is not created in global scope. So it can't be called.
Q: Parameters vs Arguments?
var b = function (param1, param2) {
// labels/identifiers are parameters
console.log("b called");
};
b(arg1, arg2); // arguments - values passed inside function call
Q: What is First Class Function aka First Class Citizens?
We can pass functions inside a function as arguments and /or return a function(HOF). These ability are altogether known as First class function. It is programming concept available in some other languages too.
var b = function (param1) {
console.log(param1); // prints " f() {} "
};
b(function () {});
// Other way of doing the same thing:
var b = function (param1) {
console.log(param1);
};
function xyz() {}
b(xyz); // same thing as prev code
// we can return a function from a function:
var b = function (param1) {
return function () {};
};
console.log(b()); //we log the entire fun within b.
Episode 14 : Callback Functions in JS ft. Event Listeners
============================================================
Callback Functions
Functions are first class citizens ie. take a function A and pass it to another function B. Here, A is a callback function. So basically I am giving access to function B to call function A. This callback function gives us the access to whole Asynchronous world in Synchronous world.
setTimeout(function () {
console.log("Timer");
}, 1000); // first argument is callback function and second is timer.
JS is a synchronous and single threaded language. But due to callbacks, we can do async things in JS.
setTimeout(function () {
console.log("timer");
}, 5000);
function x(y) {
console.log("x");
y();
}
x(function y() {
console.log("y");
});
// x y timer
In the call stack, first x and y are present. After code execution, they go away and stack is empty. Then after 5 seconds (from beginning) anonymous suddenly appear up in stack ie. setTimeout.
All 3 functions are executed through call stack. If any operation blocks the call stack, its called blocking the main thread.
Say if x() takes 30 sec to run, then JS has to wait for it to finish as it has only 1 call stack/1 main thread. Never block main thread.
Always use async for functions that take time eg. setTimeout.
// Another Example of callback
function printStr(str, cb) {
setTimeout(
() => {
console.log(str);
cb();
},
Math.floor(Math.random() * 100) + 1,
);
}
function printAll() {
printStr("A", () => {
printStr("B", () => {
printStr("C", () => {});
});
});
}
printAll(); // A B C // in order
Event Listener
We will create a button in html and attach event to it.
<!-- index.html -->
<button id="clickMe">Click Me!</button>
// in index.js
document.getElementById("clickMe").addEventListener("click", function xyz() {
//when event click occurs, this callback function (xyz) is called into callstack
console.log("Button clicked");
});
Lets implement a increment counter button. Using global variable (not good as anyone can change it):
let count = 0;
document
.getElementById("clickMe")
.addEventListener("click", function xyz() {
console.log("Button clicked", ++count);
});
Use closures for data abstraction:
function attachEventList() {
//creating new function for closure
let count = 0;
document
.getElementById("clickMe")
.addEventListener("click", function xyz() {
console.log("Button clicked", ++count); //now callback function forms closure with outer scope(count)
});
}
attachEventList();
Garbage Collection and removeEventListeners
Event listeners are heavy as they form closures. So even when call stack is empty, EventListener won't free up memory allocated to count as it doesn't know when it may need count again. So we remove event listeners when we don't need them (garbage collected) onClick, onHover, onScroll all in a page can slow it down heavily.
Episode 15 : Asynchronous JavaScript & EVENT LOOP from scratch
==================================================================
Note: Call stack will execeute any execeution context which enters it. Time, tide and JS waits for none. TLDR; Call stack has no timer.
Browser has JS Engine which has Call Stack which has Global execution context, local execution context etc. But browser has many other superpowers - Local storage space, Timer, place to enter URL, Bluetooth access, Geolocation access and so on.
Now JS needs some way to connect the callstack with all these superpowers. This is done using Web APIs.
WebAPIs
None of the below are part of Javascript! These are extra superpowers that browser has. Browser gives access to JS callstack to use these powers.
setTimeout(), DOM APIs, fetch(), localstorage, console (yes, even console.log is not JS!!), location and so many more.
-
setTimeout(): Timer function - DOM APIs : eg.
Document.xxxx; Used to access HTML DOM tree. (Document Object Manipulation) -
fetch(): Used to make connection with external servers eg. Netflix servers etc.
We get all these inside call stack through global object ie. window. Use window keyword like : window.setTimeout(), window.localstorage, window.console.log() to log something inside console. As window is global obj, and all the above functions are present in global object, we don't explicity write window but it is implied.
Let's undertand the below code and its explaination:
console.log("start");
setTimeout(function cb() {
console.log("timer");
}, 5000);
console.log("end");
// start end timer
First a GEC is created and put inside call stack. console.log("Start") calls the console web api (through window) which in turn actually modifies values in console. setTimeout(function cb() {...}, 5000) calls the setTimeout web api which gives access to timer feature. It stores the callback cb() and starts timer. console.log("End") calls console api and logs in console window. After this GEC pops from call stack.
While all this is happening, the timer is constantly ticking. After it becomes 0, the callback cb() has to run. Now we need this cb to go into call stack. Only then will it be executed. For this we need event loop and Callback queue.
Event Loops and Callback Queue
Q: How after 5 secs timer is console? cb() cannot simply directly go to callstack to be execeuted. It goes through the callback queue when timer expires. Event loop keep checking the callback queue, and see if it has any element to puts it into call stack. It is like a gate keeper. Once cb() is in callback queue, eventloop pushes it to callstack to run. Console API is used and log printed.
Another example:
console.log("Start");
document.getElementById("btn").addEventListener("click", function cb() {
// cb() registered inside webapi environment and event(click) attached to it. i.e. REGISTERING CALLBACK AND ATTACHING EVENT TO IT.
console.log("Callback");
});
console.log("End"); // calls console api and logs in console window. After this GEC get removed from call stack.
// In above code, even after console prints "Start" and "End" and pops GEC out, the eventListener stays in webapi env(with hope that user may click it some day) until explicitly removed, or the browser is closed.
Eventloop has just one job to keep checking callback queue and if found something push it to call stack and delete from callback queue.
Q: Need of callback queue? Ans: Suppose user clciks button x6 times. So 6 cb() are put inside callback queue. Event loop sees if call stack is empty/has space and whether callback queue is not empty(6 elements here). Elements of callback queue popped off, put in callstack, executed and then popped off from call stack.
Behaviour of fetch (Microtask Queue?)
console.log("Start");
setTimeout(function cbT() {
console.log("CB Timeout");
}, 5000);
fetch("https://api.netflix.com").then(function cbF() {
console.log("CB Netflix");
}); // take 2 seconds to bring response
// millions lines of code
console.log("End");
Code Explaination: Same steps for everything before fetch() in above code. fetch registers cbF into webapi environment along with existing cbT. cbT is waiting for 5000ms to end so that it can be put inside callback queue. cbF is waiting for data to be returned from Netflix servers gonna take 2 seconds. After this millions of lines of code is running, by the time millions line of code will execute, 5 seconds has finished and now the timer has expired and response from Netflix server is ready. Data back from cbF ready to be executed gets stored into something called a Microtask Queue. Also after expiration of timer, cbT is ready to execute in Callback Queue. Microtask Queue is exactly same as Callback Queue, but it has higher priority. Functions in Microtask Queue are executed earlier than Callback Queue. In console, first Start and End are printed in console. First cbF goes in callstack and "CB Netflix" is printed. cbF popped from callstack. Next cbT is removed from callback Queue, put in Call Stack, "CB Timeout" is printed, and cbT removed from callstack.
What enters the Microtask Queue?
All the callback functions that come through promises go in microtask Queue. Mutation Observer : Keeps on checking whether there is mutation in DOM tree or not, and if there, then it execeutes some callback function. Callback functions that come through promises and mutation observer go inside Microtask Queue. All the rest goes inside Callback Queue aka. Task Queue. If the task in microtask Queue keeps creating new tasks in the queue, element in callback queue never gets chance to be run. This is called starvation.
Some Important Questions
- When does the event loop actually start? - Event loop, as the name suggests, is a single-thread, loop that is almost infinite. It's always running and doing its job.
- Are only asynchronous web api callbacks are registered in web api environment? - YES, the synchronous callback functions like what we pass inside map, filter and reduce aren't registered in the Web API environment. It's just those async callback functions which go through all this.
- Does the web API environment stores only the callback function and pushes the same callback to queue/microtask queue? - Yes, the callback functions are stored, and a reference is scheduled in the queues. Moreover, in the case of event listeners(for example click handlers), the original callbacks stay in the web API environment forever, that's why it's adviced to explicitly remove the listeners when not in use so that the garbage collector does its job.
- How does it matter if we delay for setTimeout would be 0ms. Then callback will move to queue without any wait? - No, there are trust issues with setTimeout(). The callback function needs to wait until the Call Stack is empty. So the 0 ms callback might have to wait for 100ms also if the stack is busy.
Additional Notes: one rule and one runtime difference these notes don't spell out
Rule — microtasks always fully drain before the next task runs. It's not just "microtasks jump the queue once" — after every single task (a setTimeout callback, a click handler, etc.) finishes, the event loop drains the entire microtask queue — including any new microtasks that got added while draining it — before it's allowed to pick up the next task from the callback queue. This is why a promise .then() chain that keeps returning new promises can, in theory, starve the callback queue completely (the "starvation" mentioned above).
This browser diagram is for the browser — Node.js works differently. Everything above (Web APIs, window, DOM APIs) describes the browser's JS runtime. Node.js has no window or DOM — instead of Web APIs it uses libuv, a C++ library that gives Node its own thread pool and its own event loop implementation, which runs in distinct phases (timers → pending callbacks → poll → check → close callbacks), each with its own queue. The core idea — JS is single-threaded and the event loop hands async work back to the call stack when it's ready — is the same in both, but the exact queue mechanics differ enough that "the event loop" in a Node interview answer isn't identical to "the event loop" in a browser interview answer.
Episode 16 : JS Engine Exposed, Google's V8 Architecture
============================================================
JS runs literally everywhere from smart watch to robots to browsers because of Javascript Runtime Environment (JRE).
JRE is like a big container which has everything which are required to run Javascript code.
JRE consists of a JS Engine (heart of JRE), set of APIs to connect with outside environment, event loop, Callback queue, Microtask queue etc.
Browser can execute javascript code because it has the Javascript Runtime Environment.
ECMAScript is a governing body of JS. It has set of rules which are followed by all JS engines like Chakra(Internet Explorer), V8 Engine (Edge) Spidermonkey(Firefox)(first javascript engine created by JS creator himself), v8(Chrome).
Javascript Engine is not a machine. Its software written in low level languages (eg. C++) that takes in hi-level code in JS and spits out low level machine code.
Code inside Javascript Engine passes through 3 steps : Parsing, Compilation and Execution.
- Parsing - Code is broken down into tokens. In "let a = 7" -> let, a, =, 7 are all tokens. Also we have a syntax parser that takes code and converts it into an AST (Abstract Syntax Tree) which is a JSON with all key values like type, start, end, body etc (looks like package.json but for a line of code in JS. Kinda unimportant)(Check out astexplorer.net -> converts line of code into AST).
- Compilation - JS has something called Just-in-time(JIT) Compilation - uses both interpreter & compiler. Also compilation and execution both go hand in hand. The AST from previous step goes to interpreter which converts hi-level code to byte code and moves to execeution. While interpreting, compiler also works hand in hand to compile and form optimized code during runtime. Does JavaScript really Compiles? The answer is a loud YES. JS used to be only interpreter in old times, but now has both to compile and interpreter code and this make JS a JIT compiled language, its like best of both world.
- Execution - Needs 2 components ie. Memory heap(place where all memory is stored) and Call Stack(same call stack from prev episodes). There is also a garbage collector. It uses an algo called Mark and Sweep.
Companies use different JS engines and each try to make theirs the best. v8 of Google has Interpreter called Ignition, a compiler called Turbo Fan and garbage collector called Orinoco.
Additional Notes: why V8 is actually fast (Hidden Classes & Inline Caching)
The steps above (Parsing → Compilation → Execution) explain the pipeline, but not why it's fast despite JS being dynamically typed. Two V8-specific optimizations are worth knowing for interviews:
- Hidden Classes: JS objects don't have fixed structures like classes in C++/Java — you can add or remove properties at any time. This should make property lookups slow. To fix this, V8 secretly creates "hidden classes" behind the scenes — if two objects are created with properties added in the exact same order, V8 treats them as sharing the same hidden class internally, so it can look up their properties by a fast, fixed memory offset instead of a slow dictionary lookup.
function Point(x, y) {
this.x = x;
this.y = y;
}
const p1 = new Point(1, 2); // gets hidden class "C0" -> adds x -> "C1" -> adds y -> "C2"
const p2 = new Point(3, 4); // follows the exact same path, so p2 also ends up on hidden class "C2"
// Because p1 and p2 share a hidden class, V8 can access p1.x and p2.x equally fast.
const p3 = new Point(5, 6);
p3.z = 10; // adding a property in a DIFFERENT order/shape than p1/p2 creates a NEW hidden class for p3
// This is why adding properties in a consistent order/shape matters for performance-sensitive code.
- Inline Caching: V8 remembers ("caches") the result of a previous property lookup at a specific line of code, keyed to the hidden class it saw last time. So the next time that same line runs with an object of the same hidden class, V8 skips the lookup entirely and reuses the cached location. This is why keeping object shapes consistent (same properties, same order) across many objects of the "same kind" measurably speeds up hot code paths.
Episode 17 : Trust issues with setTimeout()
==============================================
setTimeout with timer of 5 secs sometimes does not exactly guarantees that the callback function will execute exactly after 5s.
Let's observe the below code and it's explaination:
console.log("Start");
setTimeout(function cb() {
console.log("Callback");
}, 5000);
console.log("End");
// Millions of lines of code to execute
// o/p: Over here setTimeout exactly doesn't guarantee that the callback function will be called exactly after 5s. Maybe 6,7 or even 10! It all depends on callstack. Why?
Reason?
First GEC is created and pushed in callstack. Start is printed in console. When setTimeout is seen, callback function is registered into webapi's env. And timer is attached to it and started. callback waits for its turn to be execeuted once timer expires. But JS waits for none. Goes to next line. End is printed in console. After "End", we have 1 million lines of code that takes 10 sec(say) to finish execution. So GEC won't pop out of stack. It runs all the code for 10 sec.
But in the background, the timer runs for 5s. While callstack runs the 1M line of code, this timer has already expired and callback fun has been pushed to Callback queue and waiting to pushed to callstack to get executed. Event loop keeps checking if callstack is empty or not. But here GEC is still in stack so cb can't be popped from callback Queue and pushed to CallStack. Though setTimeout is only for 5s, it waits for 10s until callstack is empty before it can execute (When GEC popped after 10sec, callstack() is pushed into call stack and immediately executed (Whatever is pushed to callstack is executed instantly).
This is called as the Concurrency model of JS. This is the logic behind setTimeout's trust issues.
The First rule of JavaScript: Do not block the main thread (as JS is a single threaded(only 1 callstack) language).
setTimeout guarantees that it will take at least the given timer to execute the code.
JS is a synchronous single threaded language. With just 1 thread it runs all pieces of code. It becomes kind of an interpreter language, and runs code very fast inside browser (no need to wait for code to be compiled) (JIT - Just in time compilation). And there are still ways to do async operations as well.
What if timeout = 0sec?
console.log("Start");
setTimeout(function cb() {
console.log("Callback");
}, 0);
console.log("End");
// Even though timer = 0s, the cb() has to go through the queue. Registers calback in webapi's env , moves to callback queue, and execute once callstack is empty.
// O/p - Start End Callback
// This method of putting timer = 0, can be used to defer a less imp function by a little so the more important function(here printing "End") can take place
Episode 18 : Higher-Order Functions ft. Functional Programming
==================================================================
Q: What is Higher Order Function?
Ans: A Higher-order functions are regular functions that take other functions as arguments or return functions as their results.
function x() {
console.log("Hi)";
};
function y(x) {
x();
};
y(); // Hi
// y is a higher order function
// x is a callback function
Let's try to understand how we should approach solution in interview. I have an array of radius and I have to calculate area using these radius and store in an array.
First Approach:
const radius = [1, 2, 3, 4];
const calculateArea = function (radius) {
const output = [];
for (let i = 0; i < radius.length; i++) {
output.push(Math.PI * radius[i] * radius[i]);
}
return output;
};
console.log(calculateArea(radius));
The above solution works perfectly fine but what if we have now requirement to calculate array of circumference. Code now be like:
const radius = [1, 2, 3, 4];
const calculateCircumference = function (radius) {
const output = [];
for (let i = 0; i < radius.length; i++) {
output.push(2 * Math.PI * radius[i]);
}
return output;
};
console.log(calculateCircumference(radius));
But over here we are violating some principle like DRY Principle, now lets observe the better approach.
const radiusArr = [1, 2, 3, 4];
// logic to calculate area
const area = function (radius) {
return Math.PI * radius * radius;
}
// logic to calculate circumference
const circumference = function (radius) {
return 2 * Math.PI * radius;
}
const calculate = function(radiusArr, operation) {
const output = [];
for (let i = 0; i < radiusArr.length; i++) {
output.push(operation(radiusArr[i]));
}
return output;
}
console.log(calculate(radiusArr, area));
console.log(calculate(radiusArr, circumference));
// Over here calculate is HOF
// Over here we have extracted logic into separate functions. This is the beauty of functional programming.
Polyfill of map
// Over here calculate is nothing but polyfill of map function
// console.log(radiusArr.map(area)) == console.log(calculate(radiusArr, area));
***************************************************
Lets convert above calculate function as map function and try to use. So,
Array.prototype.calculate = function(operation) {
const output = [];
for (let i = 0; i < this.length; i++) {
output.push(operation(this[i]));
}
return output;
}
console.log(radiusArr.calculate(area))
Episode 19 : map, filter & reduce
====================================
map, filter & reducer are Higher Order Functions.
Map function
It is basically used to transform a array. The map() method creates a new array with the results of calling a function for every array element.
const output = arr.map(function) // this function tells map that what transformation I want on each element of array
const arr = [5, 1, 3, 2, 6];
// Task 1: Double the array element: [10, 2, 6, 4, 12]
function double(x) {
return x * 2;
}
const doubleArr = arr.map(double); // Internally map will run double function for each element of array and create a new array and returns it.
console.log(doubleArr); // [10, 2, 6, 4, 12]
// Task 2: Triple the array element
const arr = [5, 1, 3, 2, 6];
// Transformation logic
function triple(x) {
return x * 3;
}
const tripleArr = arr.map(triple);
console.log(tripleArr); // [15, 3, 9, 6, 18]
// Task 3: Convert array elements to binary
const arr = [5, 1, 3, 2, 6];
// Transformation logic:
function binary(x) {
return x.toString(2);
}
const binaryArr = arr.map(binary);
// The above code can be rewritten as :
const binaryArr = arr.map(function binary(x) {
return x.toString(2);
}
// OR -> Arrow function
const binaryArr = arr.map((x) => x.toString(2));
So basically map function is mapping each and every value and transforming it based on given condition.
Filter function
Filter function is basically used to filter the value inside an array. The arr.filter() method is used to create a new array from a given array consisting of only those elements from the given array which satisfy a condition set by the argument method.
const array = [5, 1, 3, 2, 6];
// filter odd values
function isOdd(x) {
return x % 2;
}
const oddArr = array.filter(isOdd); // [5,1,3]
// Other way of writing the above:
const oddArr = arr.filter((x) => x % 2);
Filter function creates an array and store only those values which evaluates to true.
Reduce function
It is a function which take all the values of array and gives a single output of it. It reduces the array to give a single output.
const array = [5, 1, 3, 2, 6];
// Calculate sum of elements of array - Non functional programming way
function findSum(arr) {
let sum = 0;
for (let i = 0; i < arr.length; i++) {
sum = sum + arr[i];
}
return sum;
}
console.log(findSum(array)); // 17
// reduce function way
const sumOfElem = arr.reduce(function (accumulator, current) {
// current represent the value of array
// accumulator is used the result from element of array.
// In comparison to previous code snippet, sum variable is accumulator and arr[i] is current
accumulator = accumulator + current;
return accumulator;
}, 0); //In above example sum was initialized with 0, so over here accumulator also needs to be initialized, so the second argument to reduce function represent the initialization value.
console.log(sumOfElem); // 17
// find max inside array: Non functional programming way:
const array = [5, 1, 3, 2, 6];
function findMax(arr) {
let max = 0;
for(let i = 0; i < arr.length; i++ {
if (arr[i] > max) {
max = arr[i]
}
}
return max;
}
console.log(findMax(array)); // 6
// using reduce
const output = arr.reduce((acc, current) => {
if (current > acc ) {
acc = current;
}
return acc;
}, 0);
console.log(output); // 6
// acc is just a label which represent the accumulated value till now,
// so we can also label it as max in this case
const output = arr.reduce((max, current) => {
if (current > max) {
max= current;
}
return max;
}, 0);
console.log(output); // 6
Tricky MAP
const users = [
{ firstName: "Manpreet", lastName: "Singh", age: 23 },
{ firstName: "Ashish", lastName: "Kumar", age: 29 },
{ firstName: "Ankit", lastName: "Roy", age: 29 },
{ firstName: "Pranav", lastName: "Mukherjee", age: 50 },
];
// Get array of full name : ["Manpreet Singh", "Ashish Kumar", ...]
const fullNameArr = users.map((user) => user.firstName + " " + user.lastName);
console.log(fullNameArr); // ["Manpreet Singh", "Ashish Kumar", ...]
----------------------------------------------------------
// Get the count/report of how many unique people with unique age are there
// like: {29 : 2, 75 : 1, 50 : 1}
// We should use reduce, why? we want to deduce some information from the array. Basically we want to get a single object as output
const report = users.reduce((acc, curr) => {
if(acc[curr.age]) {
acc[curr.age] = ++ acc[curr.age] ;
} else {
acc[curr.age] = 1;
}
return acc; //to every time return update object
}, {})
console.log(report) // {29 : 2, 75 : 1, 50 : 1}
Function Chaining
// First name of all people whose age is less than 30
const users = [
{ firstName: "Manpreet", lastName: "Singh", age: 23 },
{ firstName: "Ashish", lastName: "Kumar", age: 29 },
{ firstName: "Ankit", lastName: "Roy", age: 29 },
{ firstName: "Pranav", lastName: "Mukherjee", age: 50 },
];
// function chaining
const output = users
.filter((user) => user.age < 30)
.map((user) => user.firstName);
console.log(output); // ["Manpreet", "Ashish", "Ankit"]
// Homework challenge: Implement the same logic using reduce
const output = users.reduce((acc, curr) => {
if (curr.age < 30) {
acc.push(curr.firstName);
}
return acc;
}, []);
console.log(output); // ["Manpreet", "Ashish", "Ankit"]
Episode 20 : Callback
========================
There are 2 Parts of Callback:
- Good Part of callback - Callback are super important while writing asynchronous code in JS
- Bad Part of Callback - Using callback we can face issue: Callback Hell, Inversion of control
Understanding of Bad part of callback is super important to learn Promise in next lecture.
JavaScript is synchronous, single threaded language. It can Just do one thing at a time, it has just one call-stack and it can execute one thing at a time. Whatever code we give to Javascript will be quickly executed by Javascript engine, it does not wait.
console.log("Namaste");
console.log("JavaScript");
console.log("Season 2");
// Namaste
// JavaScript
// Season 2
// It is quickly printing because Time, tide & Javascript waits for none.
But what if we have to delay execution of any line, we could utilize callback, How?
console.log("Namaste");
setTimeout(function () {
console.log("JavaScript");
}, 5000);
console.log("Season 2");
// Namaste
// Season 2
// JavaScript
// Here we are delaying the execution using callback approach of setTimeout.
e-Commerce web app situation
Assume a scenario of e-Commerce web, where one user is placing order, he has added items like, shoes, pants and kurta in cart and now he is placing order. So in backend the situation could look something like this.
const cart = ["shoes", "pants", "kurta"];
// Two steps to place a order
// 1. Create a Order
// 2. Proceed to Payment
// It could look something like this:
api.createOrder();
api.proceedToPayment();
Assumption, once order is created then only we can proceed to payment, so there is a dependency. So How to manage this dependency. Callback can come as rescue, How?
api.createOrder(cart, function () {
api.proceedToPayment();
});
// Over here createOrder api is first creating a order then it is responsible to call api.proceedToPayment() as part of callback approach.
To make it a bit complicated, what if after payment is done, you have to show Order summary by calling api.showOrderSummary() and now it has dependency on api.proceedToPayment() Now my code should look something like this:
api.createOrder(cart, function () {
api.proceedToPayment(function () {
api.showOrderSummary();
});
});
Now what if we have to update the wallet, now this will have a dependency over showOrderSummary:
api.createOrder(cart, function () {
api.proceedToPayment(function () {
api.showOrderSummary(function () {
api.updateWallet();
});
});
});
// Callback Hell
When we have a large codebase and multiple apis and have dependency on each other, then we fall into callback hell. These codes are tough to maintain. These callback hell structure is also known as Pyramid of Doom.
Till this point we are comfortable with concept of callback hell but now lets discuss about Inversion of Control. It is very important to understand in order to get comfortable around the concept of promise.
Inversion of control is like that you lose the control of code when we are using callback.
api.createOrder(cart, function () {
api.proceedToPayment();
});
// So over here, we are creating a order and then we are blindly trusting createOrder to call proceedToPayment.
// It is risky, as proceedToPayment is important part of code and we are blindly trusting createOrder to call it and handle it.
// When we pass a function as a callback, basically we are dependant on our parent function that it is his responsibility to run that function. This is called inversion of control because we are dependant on that function. What if parent function stopped working, what if it was developed by another programmer or callback runs two times or never run at all.
Async programming in JavaScript exists because callback exits.
more at http://callbackhell.com/
Episode 21 : Promises
========================
Promises are used to handle async operations in JavaScript.
We will discuss with code example that how things used to work before Promises and then how it works after Promises.
Suppose, taking an example of E-Commerce:
const cart = ["shoes", "pants", "kurta"];
// Below two functions are asynchronous and dependent on each other
const orderId = createOrder(cart);
proceedToPayment(orderId);
// with Callback (Before Promise)
// Below here, it is the responsibility of createOrder function to first create the order then call the callback function
createOrder(cart, function () {
proceedToPayment(orderId);
});
// Above there is the issue of Inversion of Control
Q: How to fix the above issue? A: Using Promise.
Now, we will make createOrder function return a promise and we will capture that promise into a variable.
Promise is nothing but we can assume it to be empty object with some data value in it, and this data value will hold whatever this createOrder function will return.
Since createOrder function is an async function and we don't know how much time will it take to finish execution.
So the moment createOrder will get executed, it will return you a undefined value. Let's say after 5 secs execution finished so now orderId is ready so, it will fill the undefined value with the orderId.
In short, When createOrder get executed, it immediately returns a promise object with undefined value. then javascript will continue to execute with other lines of code. After sometime when createOrder has finished execution and orderId is ready then that will automatically be assigned to our returned promise which was earlier undefined.
Q: Question is how we will get to know response is ready? A: So, we will attach a callback function to the promise object using then to get triggered automatically when result is ready.
const cart = ["shoes", "pants", "kurta"];
const promiseRef = createOrder(cart);
// this promiseRef has access to then
// {data: undefined}
// Initially it will be undefined so below code won't trigger
// After some time, when execution has finished and promiseRef has the data then automatically the below line will get triggered.
promiseRef.then(function () {
proceedToPayment(orderId);
});
Q: How it is better than callback approach? In Earlier solution we used to pass the function and then used to trust the function to execute the callback. But with promise, we are attaching a callback function to a promiseObject.
There is difference between these words, passing a function and attaching a function.
Promise guarantee, it will callback the attached function once it has the fulfilled data. And it will call it only once. Just once.
Earlier we talked about promise are object with empty data but that's not entirely true, Promise are much more than that.
Now let's understand and see a real promise object. fetch is a web-api which is utilized to make api call and it returns a promise. We will be calling public github api to fetch data https://api.github.com/users/Manpreets59
// We will be calling public github api to fetch data
const URL = "https://api.github.com/users/Manpreets59";
const user = fetch(URL);
// User above will be a promise.
console.log(user); // Promise {<Pending>}
/** OBSERVATIONS:
* If we will deep dive and see, this promise object has 3 things
* prototype, promiseState & promiseResult
* & this promiseResult is the same data which we talked earlier as data
* & initially promiseResult is undefined
*
* promiseResult will store data returned from API call
* promiseState will tell in which state the promise is currently, initially it will be in pending state and later it will become fulfilled
*/
/**
* When above line is executed, fetch makes API call and return a promise instantly which is in Pending state and Javascript doesn't wait to get it fulfilled
* And in next line it console out the pending promise.
* NOTE: chrome browser has some in-consistency, the moment console happens it shows in pending state but if you will expand that it will show fulfilled because chrome updated the log when promise get fulfilled.
* Once fulfilled data is there in promiseResult and it is inside body in ReadableStream format and there is a way to extract data.
*/
Now we can attach callback to above response? Using .then
const URL = "https://api.github.com/users/Manpreets59";
const user = fetch(URL);
user.then(function (data) {
console.log(data);
});
// And this is how Promise is used.
// It guarantees that it could be resolved only once, either it could be success or failure
/*
A Promise is in one of these states:
pending: initial state, neither fulfilled nor rejected.
fulfilled: meaning that the operation was completed successfully.
rejected: meaning that the operation failed.
*/
Promise Object are immutable. Once promise is fulfilled and we have data we can pass here and there and we don't have to worry that someone can mutate that data. So over above we can't directly mutate user promise object, we will have to use .then
Interview Guide
What is Promise? Promise object is a placeholder for certain period of time until we receive value from asynchronous operation. A container for a future value. A Promise is an object representing the eventual completion or failure of an asynchronous operation.
We are now done solving one issue of callback i.e. Inversion of Control. But there is one more issue, callback hell...
// Callback Hell Example
createOrder(cart, function (orderId) {
proceedToPayment(orderId, function (paymentInf) {
showOrderSummary(paymentInf, function (balance) {
updateWalletBalance(balance);
});
});
});
// And now above code is expanding horizontally and this is called pyramid of doom.
// Callback hell is ugly and hard to maintain.
// Promise fixes this issue too using Promise Chaining
// Example Below is a Promise Chaining
createOrder(cart)
.then(function (orderId) {
proceedToPayment(orderId);
})
.then(function (paymentInf) {
showOrderSummary(paymentInf);
})
.then(function (balance) {
updateWalletBalance(balance);
});
// Common PitFall
// We forget to return promise in Promise Chaining
// The idea is promise/data returned from one .then become data for next .then
// So,
createOrder(cart)
.then(function (orderId) {
return proceedToPayment(orderId);
})
.then(function (paymentInf) {
return showOrderSummary(paymentInf);
})
.then(function (balance) {
return updateWalletBalance(balance);
});
// To improve readability you can use arrow function instead of regular function
Episode 22 : Creating a Promise, Chaining & Error Handling
=============================================================
const cart = ["shoes", "pants", "kurta"];
// Consumer part of promise
const promise = createOrder(cart); // orderId
// Our expectation is above function is going to return me a promise.
promise.then(function (orderId) {
proceedToPayment(orderId);
});
// Above snippet we have observed in our previous lecture itself.
// Now we will see, how createOrder is implemented so that it is returning a promise
// In short we will see, "How we can create Promise" and then return it.
// Producer part of Promise
function createOrder(cart) {
// JS provides a Promise constructor through which we can create promise
// It accepts a callback function with two parameter resolve & reject
const promise = new Promise(function (resolve, reject) {
// What is this resolve and reject?
// These are function which are passed by javascript to us in order to handle success and failure of function call.
// Now we will write logic to createOrder
/** Mock logic steps
* 1. validateCart
* 2. Insert in DB and get an orderId
*/
// We are assuming in real world scenario, validateCart would be defined
if (!validateCart(cart)) {
// If cart not valid, reject the promise
const err = new Error("Cart is not Valid");
reject(err);
}
const orderId = "12345"; // We got this id by calling to db (Assumption)
if (orderId) {
// Success scenario
resolve(orderId);
}
});
return promise;
}
Over above, if your validateCart is returning true, so the above promise will be resolved (success):
const cart = ["shoes", "pants", "kurta"];
const promise = createOrder(cart); // orderId
// What will be printed in below line?
// It prints Promise {<pending>}, but why?
// Because above createOrder is going to take sometime to get resolved, so pending state. But once the promise is resolved, .then would be executed for callback.
console.log(promise);
promise.then(function (orderId) {
proceedToPayment(orderId);
});
function createOrder(cart) {
const promise = new Promise(function (resolve, reject) {
if (!validateCart(cart)) {
const err = new Error("Cart is not Valid");
reject(err);
}
const orderId = "12345";
if (orderId) {
resolve(orderId);
}
});
return promise;
}
Now let's see if there was some error and we are rejecting the promise, how we could catch that? -> Using .catch
const cart = ["shoes", "pants", "kurta"];
const promise = createOrder(cart); // orderId
// Here we are consuming Promise and will try to catch promise error
promise
.then(function (orderId) {
// success aka resolved promise handling
proceedToPayment(orderId);
})
.catch(function (err) {
// failure aka reject handling
console.log(err);
});
// Here we are creating Promise
function createOrder(cart) {
const promise = new Promise(function (resolve, reject) {
// Assume below validateCart return false then the promise will be rejected
// And then our browser is going to throw the error.
if (!validateCart(cart)) {
const err = new Error("Cart is not Valid");
reject(err);
}
const orderId = "12345";
if (orderId) {
resolve(orderId);
}
});
return promise;
}
Now, Let's understand the concept of Promise Chaining -> for this we will assume after createOrder we have to invoke proceedToPayment -> In promise chaining, whatever is returned from first .then become data for next .then and so on... -> At any point of promise chaining, if promise is rejected, the execution will fallback to .catch and others promise won't run.
const cart = ["shoes", "pants", "kurta"];
createOrder(cart)
.then(function (orderId) {
// success aka resolved promise handling
// we have return data or promise so that we can keep chaining the promises, here we are returning data
console.log(orderId);
return orderId;
})
.then(function (orderId) {
// Promise chaining
// we will make sure that proceedToPayment returns a promise too
return proceedToPayment(orderId);
})
.then(function (paymentInfo) {
// from above, proceedToPayment is returning a promise so we can consume using .then
console.log(paymentInfo);
})
.catch(function (err) {
// failure aka reject handling
console.log(err);
});
// Here we are creating Promise
function createOrder(cart) {
const promise = new Promise(function (resolve, reject) {
// Assume below validateCart return false then the promise will be rejected
// And then our browser is going to throw the error.
if (!validateCart(cart)) {
const err = new Error("Cart is not Valid");
reject(err);
}
const orderId = "12345";
if (orderId) {
resolve(orderId);
}
});
return promise;
}
function proceedToPayment(cart) {
return new Promise(function (resolve, reject) {
// For time being, we are simply resolving promise
resolve("Payment Successful");
});
}
Q: What if we want to continue execution even if any of my promise is failing, how to achieve this? -> By placing the .catch block at some level after which we are not concerned with failure. -> There could be multiple .catch too. Eg:
createOrder(cart)
.then(function (orderId) {
// success aka resolved promise handling
// we have return data or promise so that we can keep chaining the promises, here we are returning data
console.log(orderId);
return orderId;
})
.catch(function (err) {
// Whatever fails below it, catch wont care
// this block is responsible for code block above it.
console.log(err);
});
.then(function (orderId) {
// Promise chaining
// we will make sure that proceedToPayment returns a promise too
return proceedToPayment(orderId);
})
.then(function (paymentInfo) {
// from above, proceedToPayment is returning a promise so we can consume using .then
console.log(paymentInfo);
})
Episode 23 : async await
===========================
Topics Covered: What is async? What is await? How async await works behind the scenes? Example of using async/await. Error Handling. Async await vs Promise.then/.catch
Q: What is async?
A: Async is a keyword that is used before a function to create a async function.
Q: What is async function and how it is different from normal function?
// async function always returns a promise, even if I return a simple string from below function, async keyword will wrap it under Promise and then return.
async function getData() {
return "Namaste JavaScript";
}
const dataPromise = getData();
console.log(dataPromise); // Promise {<fulfilled>: 'Namaste JavaScript'}
//How to extract data from above promise? One way is using promise .then
dataPromise.then((res) => console.log(res)); // Namaste JavaScript
Another example where async function is returning a Promise:
const p = new Promise((resolve, reject) => {
resolve("Promise resolved value!!");
});
async function getData() {
return p;
}
// In above case, since we are already returning a promise async function would simply return that instead of wrapping with a new Promise.
const dataPromise = getData();
console.log(dataPromise); // Promise {<fulfilled>: 'Promise resolved value!!'}
dataPromise.then((res) => console.log(res)); // Promise resolved value!!
Q: How we can use await along with async function?
A: async and await combo is used to handle promises. But Question is how we used to handle promises earlier and why we even need async/await?
const p = new Promise((resolve, reject) => {
resolve("Promise resolved value!!");
});
function getData() {
p.then((res) => console.log(res));
}
getData(); // Promise resolved value!!
// Till now we have been using Promise.then/.catch to handle promise.
// Now let's see how async await can help us and how it is different
// The rule is we have to use keyword await in front of promise.
async function handlePromise() {
const val = await p;
console.log(val);
}
handlePromise(); // Promise resolved value!!
await is a keyword that can only be used inside a async function.
await function () {}; // Syntax error: await is only valid under async function.
Q: What makes async-await special?
A: Let's understand with one example where we will compare async-await way of resolving promise with older .then/.catch fashion. For that we will modify our promise p.
const p = new Promise((resolve, reject) => {
setTimeout(() => {
resolve("Promise resolved value!!");
}, 3000);
});
// Let's now compare with some modification:
// Promise.then/.catch way
function getData() {
// JS engine will not wait for promise to be resolved
p.then((res) => console.log(res));
console.log("Hello There!");
}
getData(); // First Hello There! would be printed and then after 3 secs 'Promise resolved value!!' will be printed.
// Above happened as Javascript wait for none, so it will register this promise and take this callback function and register separately then js will move on and execute the following console and later once promise is resolved, following console will be printed.
// Problem: Normally one used to get confused that JS will wait for promise to be resolved before executing following lines.
// async-wait way:
async function handlePromise() {
// JS Engine will waiting for promise to resolve.
const val = await p;
console.log("Hello There!");
console.log(val);
}
handlePromise(); // This time Hello There! won't be printed immediately instead after 3 secs Hello There! will be printed followed by 'Promise resolved value!!'
// So basically code was waiting at await line to get the promise resolve before moving on to next line.
// Above is the major difference between Promise.then/.catch vs async-await
Let's brainstorm more around async-await:
async function handlePromise() {
console.log("Hi");
const val = await p;
console.log("Hello There!");
console.log(val);
const val2 = await p;
console.log("Hello There! 2");
console.log(val2);
}
handlePromise();
// In above code example, will our program wait for 2 time or will it execute parallely.
// Hi printed instantly -> now code will wait for 3 secs -> After 3 secs both promises will be resolved so ('Hello There!' 'Promise resolved value!!' 'Hello There! 2' 'Promise resolved value!!') will get printed immediately.
// Let's create one promise and then resolve two different promise.
const p2 = new Promise((resolve, reject) => {
setTimeout(() => {
resolve("Promise resolved value by p2!!");
}, 2000);
});
async function handlePromise() {
console.log("Hi");
const val = await p;
console.log("Hello There!");
console.log(val);
const val2 = await p2;
console.log("Hello There! 2");
console.log(val2);
}
handlePromise();
// Hi printed instantly -> now code will wait for 3 secs -> After 3 secs both promises will be resolved so ('Hello There!' 'Promise resolved value!!' 'Hello There! 2' 'Promise resolved value by p2!!') will get printed immediately. So even though p2 was resolved after 2 secs it had to wait for p to get resolved
// Now let's reverse the order execution of promise and observe response.
async function handlePromise() {
console.log("Hi");
const val = await p2;
console.log("Hello There!");
console.log(val);
const val2 = await p;
console.log("Hello There! 2");
console.log(val2);
}
handlePromise();
// Hi printed instantly -> now code will wait for 2 secs -> After 2 secs ('Hello There!' 'Promise resolved value by p2!!') will get printed and in the subsequent second i.e. after 3 secs ('Hello There! 2' 'Promise resolved value!!') will get printed
Q: Question is Is program actually waiting or what is happening behind the scene?
A: As we know, Time, Tide and JS wait for none. And it's true. Over here it appears that JS engine is waiting but JS engine is not waiting over here. It has not occupied the call stack if that would have been the case our page may have got frozen. So JS engine is not waiting. So if it is not waiting then what it is doing behind the scene? Let's understand with below code snippet.
const p1 = new Promise((resolve, reject) => {
setTimeout(() => {
resolve("Promise resolved value by p1!!");
}, 5000);
});
const p2 = new Promise((resolve, reject) => {
setTimeout(() => {
resolve("Promise resolved value by p2!!");
}, 10000);
});
async function handlePromise() {
console.log("Hi");
debugger;
const val = await p1;
console.log("Hello There!");
debugger;
console.log(val);
const val2 = await p2;
console.log("Hello There! 2");
debugger;
console.log(val2);
}
handlePromise();
// When this function is executed, it will go line by line as JS is synchronous single threaded language. Lets observe what is happening under call-stack. Above you can see we have set the break-points.
// call stack flow -> handlePromise() is pushed -> It will log Hi to console -> Next it sees we have await where promise is suppose to be resolved -> So will it wait for promise to resolve and block call stack? No -> thus handlePromise() execution get suspended and moved out of call stack -> So when JS sees await keyword it suspend the execution of function till promise is resolved -> So p1 will get resolved after 5 secs so handlePromise() will be pushed to call-stack again after 5 secs. -> But this time it will start executing from where it had left. -> Now it will log 'Hello There!' and 'Promise resolved value by p1!!' -> then it will check whether p2 is resolved or not -> It will find since p2 will take 10 secs to resolve so the same above process will repeat -> execution will be suspended until promise is resolved.
// Thus JS is not waiting, call stack is not getting blocked.
// Moreover in above scenario what if p1 would be taking 10 secs and p2 5 secs -> even though p2 got resolved earlier but JS is synchronous single threaded language so it will first wait for p1 to be resolved and then will immediately execute all.
Q: Explain code output and it's reason
const p1 = new Promise((resolve, reject) => {
setTimeout(() => {
resolve("Promise resolved value by p1!!");
}, 10000);
});
const p2 = new Promise((resolve, reject) => {
setTimeout(() => {
resolve("Promise resolved value by p2!!");
}, 5000);
});
async function handlePromise() {
console.log("Hi");
debugger;
const val = await p1;
console.log("Hello There!");
debugger;
console.log(val);
const val2 = await p2;
console.log("Hello There! 2");
debugger;
console.log(val2);
}
handlePromise();
// Hi will be printed then because of p1 await, it will for 10seconds and print Promise resolved value by p1!! followed by Hello There! and then instantly Promise resolved value by p2!! followed by Hello There! 2
// Why p2 invoked instanly without waiting another 5 seconds?
A: Promises start executing immediately when they are created, not when they are awaited. p1's setTimeout(10s) starts right away p2's setTimeout(5s) also starts right away Both timers begin ticking as soon as the file is executed, before handlePromise() even hits the first await.
await does NOT start a promise. It only waits for an already-running promise.
Q: How to make p2 wait after p1
A:
function createP1() {
return new Promise((resolve) =>
setTimeout(() => resolve("p1 resolved"), 10000),
);
}
function createP2() {
return new Promise((resolve) =>
setTimeout(() => resolve("p2 resolved"), 5000),
);
}
async function handlePromise() {
console.log("Hi");
const val1 = await createP1();
console.log(val1);
const val2 = await createP2();
console.log(val2);
}
Real World example of async/await
async function handlePromise() {
// fetch() => Response Object which as body as Readable stream => Response.json() is also a promise which when resolved => value
const data = await fetch("https://api.github.com/users/Manpreets59");
const res = await data.json();
console.log(res);
}
handlePromise();
Error Handling
While we were using normal Promise we were using .catch to handle error, now in async-await we would be using try-catch block to handle error.
async function handlePromise() {
try {
const data = await fetch("https://api.github.com/users/Manpreets59");
const res = await data.json();
console.log(res);
} catch (err) {
console.log(err);
}
}
handlePromise();
// In above whenever any error will occur the execution will move to catch block. One could try above with bad url which will result in error.
// Other way of handling error:
handlePromise().catch((err) => console.log(err)); // this will work as handlePromise will return error promise in case of failure.
Async await vs Promise.then/.catch
What one should use? async-await is just a syntactic sugar around promise. Behind the scene async-await is just promise. So both are same, it's just async-await is new way of writing code. async-await solves few of the short-coming of Promise like Promise Chaining. async-await also increases the readability. So sort of it is always advisable to use async-await.
Episode 24 : Promise APIs (all, allSettled, race, any) + Interview Questions
================================================================================
4 Promise APIs which are majorly used:
- Promise.all()
- Promise.allSettled()
- Promise.race()
- Promise.any()
One simply doesn't use async/await without knowing promises!
Promise.all()
A promise is a placeholder for a value that's going to be available sometime later. The promise helps handle asynchronous operations. JavaScript provides a helper function Promise.all(promisesArrayOrIterable) to handle multiple promises at once, in parallel, and get the results in a single aggregate array.
Q: In what situation one could use above api? A: Suppose, you have to make parallel API call and get the result, how one can do? This is where Promise.all can be utilized. It is used to handle multiple promises together.
Promise.all([p1, p2, p3]) -> Lets assume we are making 3 API call to fetch data. Also assume p1 takes 3 seconds, p2 takes 1 second, p3 takes 2 seconds.
In first scenario let's assume all 3 promises are successful. So Promise.all will take 3secs and will give promise value of result like [val1, val2, val3]. It will wait for all of them to finish then it will collect the results and give array as output.
What if any of the promise gets rejected, for eg: Promise.all([p1, p2, p3]). But this time, p2 get rejected after 1 sec. Thus Promise.all will throw same error as p2 immediately as soon as error happened. It will not wait for other promise to either become success or failure. Moreover, p1 and p2 wont get cancelled as they are already triggered so it may result in success or failure depending upon their fate but Promise.all wont care. So its a situation of or/null.
To conclude, the Promise.all() waits for all the input promises to resolve and returns a new promise that resolves to an array containing the results of the input promises. If one of the input promises is rejected, the Promise.all() method immediately returns a promise that is rejected with an error of the first rejected promise.
Promise.allSettled()
Promise.allSettled() method that accepts a list of Promises and returns a new promise that resolves after all the input promises have settled, either resolved or rejected.
Promise.allSettled([p1, p2, p3]) -> Lets assume we are making 3 API call to fetch data. Also assume p1 takes 3 seconds, p2 takes 1 second, p3 takes 2 seconds.
In first scenario let's assume all 3 promises are successful. So Promise.allSettled will take 3secs and will give promise value of result like [val1, val2, val3]. It will wait for all of them to finish then it will collect the results and give array as output.
What if any of the promise gets rejected, for eg: Promise.all([p1, p2, p3]). But this time, p2 get rejected after 1 sec. Thus Promise.allSettled will still wait for all promises to get settled. So After 3 secs, it will be [val1, err, val3]
Promise.all() -> Fail Fast
Promise.allSettled() -> Will wait and provide accumulative result
Promise.race()
The Promise.race() static method accepts a list of promises as an iterable object and returns a new promise that fulfills or rejects as soon as there is one promise that fulfills or rejects, with the value or reason from that promise. The name of Promise.race() implies that all the promises race against each other with a single winner, either resolved or rejected.
Promise.race([p1, p2, p3]) -> Lets assume we are making 3 API call to fetch data. Also assume p1 takes 3 seconds, p2 takes 1 second, p3 takes 2 seconds. So as soon as first promise will resolve or reject, it will give the output.
So in Happy scenario, Promise.race will give (val2) as output after 1sec as p2 got resolved at the earliest. Whereas if it would have been failed Promise.race would have still given output after 1 sec but this time with error.
Promise.any()
The Promise.any() method accepts a list of Promise objects as an iterable object. If one of the promises in the iterable object is fulfilled, the Promise.any() returns a single promise that resolves to a value which is the result of the fulfilled promise.
Promise.any([p1, p2, p3]) -> Lets assume we are making 3 API call to fetch data. Also assume p1 takes 3 seconds, p2 takes 1 second, p3 takes 2 seconds. So as soon as first promise will be successful, it will give the output.
If in above situation what if p2 got rejected, nothing will happen as Promise.any seek for success, so the moment first success will happen that will become the result.
But what if all promises got failed, so the returned result will be aggregated error i.e. [err1, err2, err3].
Code Examples:
Promise.all()
// First Scenario
const p1 = new Promise((resolve, reject) => {
setTimeout(() => {
resolve("P1 Success");
}, 3000);
});
const p2 = new Promise((resolve, reject) => {
setTimeout(() => {
resolve("P2 Success");
}, 1000);
});
const p3 = new Promise((resolve, reject) => {
setTimeout(() => {
resolve("P3 Success");
}, 2000);
});
Promise.all([p1, p2, p3]).then((results) => {
console.log(results); // ['P1 Success', 'P2 Success', 'P3 Success'] -> took 3 secs
});
// Second Scenario
const p1 = new Promise((resolve, reject) => {
setTimeout(() => {
resolve("P1 Success");
}, 3000);
});
const p2 = new Promise((resolve, reject) => {
setTimeout(() => {
reject("P2 Fail");
}, 1000);
});
const p3 = new Promise((resolve, reject) => {
setTimeout(() => {
resolve("P3 Success");
}, 2000);
});
Promise.all([p1, p2, p3])
.then((results) => console.log(results))
.catch((err) => console.error(err)); // throws error after 1 sec i.e. 'P2 Fails'
Promise.allSettled()
This is the safest among all Promises API.
const p1 = new Promise((resolve, reject) => {
setTimeout(() => {
resolve("P1 Success");
}, 3000);
});
const p2 = new Promise((resolve, reject) => {
setTimeout(() => {
resolve("P2 Success");
}, 1000);
});
const p3 = new Promise((resolve, reject) => {
setTimeout(() => {
reject("P3 Fail");
}, 2000);
});
Promise.allSettled([p1, p2, p3])
.then((results) => console.log(results))
.catch((err) => console.error(err));
// Over here, it will wait for all promises to be either settled or rejected and then return,
/*
[
{status: 'fulfilled', value: 'P1 Success'},
{status: 'fulfilled', value: 'P2 Success'},
{status: 'rejected', reason: 'P3 Fail'}
]
*/
Promise.race()
// First Scenario
const p1 = new Promise((resolve, reject) => {
setTimeout(() => {
resolve("P1 Success");
}, 3000);
});
const p2 = new Promise((resolve, reject) => {
setTimeout(() => {
resolve("P2 Success");
}, 1000);
});
const p3 = new Promise((resolve, reject) => {
setTimeout(() => {
reject("P3 Fail");
}, 2000);
});
Promise.race([p1, p2, p3])
.then((results) => console.log(results))
.catch((err) => console.error(err));
// It will return as soon as first promise is resolved or rejected.
// In above example O/P: "P2 Success"
// Second Scenario
const p1 = new Promise((resolve, reject) => {
setTimeout(() => {
resolve("P1 Success");
}, 3000);
});
const p2 = new Promise((resolve, reject) => {
setTimeout(() => {
resolve("P2 Success");
}, 5000);
});
const p3 = new Promise((resolve, reject) => {
setTimeout(() => {
reject("P3 Fail");
}, 2000);
});
Promise.race([p1, p2, p3])
.then((results) => console.log(results))
.catch((err) => console.error(err));
//After 2 secs O/P: "P3 Fail"
Notes: Once promise is settled, it means -> got the result. Moreover, settled is broadly divided into two categories: resolve, success, fulfilled; reject, failure, rejected.
Promise.any()
// First Scenario
const p1 = new Promise((resolve, reject) => {
setTimeout(() => {
resolve("P1 Success");
}, 3000);
});
const p2 = new Promise((resolve, reject) => {
setTimeout(() => {
resolve("P2 Success");
}, 5000);
});
const p3 = new Promise((resolve, reject) => {
setTimeout(() => {
reject("P3 Fail");
}, 2000);
});
Promise.any([p1, p2, p3])
.then((results) => console.log(results))
.catch((err) => console.error(err));
// It will wait for first settled success
// In above, p3 will settled first, but since it is rejected, so it will wait further so at 3rd second it will print "P1 Success"
// Second Scenario
const p1 = new Promise((resolve, reject) => {
setTimeout(() => {
reject("P1 Fail");
}, 3000);
});
const p2 = new Promise((resolve, reject) => {
setTimeout(() => {
resolve("P2 Success");
}, 5000);
});
const p3 = new Promise((resolve, reject) => {
setTimeout(() => {
reject("P3 Fail");
}, 2000);
});
Promise.any([p1, p2, p3])
.then((results) => console.log(results))
.catch((err) => console.error(err));
// After 5 secs: 'P2 Success'
// Third Scenario
const p1 = new Promise((resolve, reject) => {
setTimeout(() => {
reject("P1 Fail");
}, 3000);
});
const p2 = new Promise((resolve, reject) => {
setTimeout(() => {
reject("P2 Fail");
}, 5000);
});
const p3 = new Promise((resolve, reject) => {
setTimeout(() => {
reject("P3 Fail");
}, 2000);
});
Promise.any([p1, p2, p3])
.then((results) => console.log(results))
.catch((err) => {
console.error(err);
console.error(err.errors); // ['P1 Fail', 'P2 Fail', 'P3 Fail']
});
// Since all are rejected, so it will give "aggregate error" as output
// AggregateError: All promises were rejected
// To get AggregateError array you need to write "err.errors"
Summary
There are 6 static methods of Promise class:
-
Promise.all(promises)– waits for all promises to resolve and returns an array of their results. If any of the given promises rejects, it becomes the error of Promise.all, and all other results are ignored. -
Promise.allSettled(promises)(recently added method) – waits for all promises to settle and returns their results as an array of objects with: status: "fulfilled" or "rejected", value (if fulfilled) or reason (if rejected). -
Promise.race(promises)– waits for the first promise to settle, and its result/error becomes the outcome. -
Promise.any(promises)(recently added method) – waits for the first promise to fulfill, and its result becomes the outcome. If all of the given promises are rejected, AggregateError becomes the error of Promise.any. -
Promise.resolve(value)– makes a resolved promise with the given value. -
Promise.reject(error)– makes a rejected promise with the given error.
Of all these, Promise.all is probably the most common in practice.
Episode 25 : this keyword in JavaScript
==========================================
In JavaScript, the this keyword refers to an object, which object depends on how this is being invoked (used or called).
this in global space
Anything defined globally is said to be in a global space.
console.log(this); // refers to global object i.e. window in case of browser
// global object differs based on runtime environment,
this inside a function
function x() {
// the below value depends on strict/non-strict mode
console.log(this);
// in strict mode - undefined
// in non-strict mode - refers to global window object
}
x();
Notes: On the first go feels like this keyword in global space and inside function behaves same but in reality it's different. The moment you make JS run in strict mode by using: "use strict" at the top, this keyword inside function returns undefined whereas global space will still refers to global window object.
this substitution -> According to this substitution, if the value of this keyword is null/undefined, it will be replaced by globalObject only in non-strict mode. This is the reason why this refers to global window object inside function in non-strict mode.
So to summarize, the value of this keyword inside function is undefined, but because of this substitution in non-strict mode this keyword refers to globalWindowObject and in strict mode it will still be undefined.
this keyword value depends on how the function is called. For eg: In strict mode:
x(); // undefined
window.x(); // global window object
this inside a object's method
// x key below is a method as per terminology
const obj = {
a: 10,
x: function () {
console.log(this); // {a: 10, x: f()}
console.log(this.a); // 10
},
};
obj.x(); // value of this is referring to current object i.e. obj
call, apply & bind methods
const student = {
name: "Manpreet",
printName: function () {
console.log(this.name);
},
};
student.printName(); // Manpreet
const student2 = {
name: "Kajal",
};
student2.printName(); // throw error
// how to re-use printName method from student object
student.printName.call(student2); // Kajal
// Above call method is taking the value of this keyword
// So, Inside printName method value of this is now student2 object
// So, call, bind and apply is used to set the value of this keyword.
this inside arrow function
Arrow function doesn't have their own this value, they take the value from enclosing lexical context.
const obj = {
a: 10,
x: () => {
console.log(this); // window object
// Above the value of this won't be obj anymore instead it will be enclosing lexical context i.e. window object in current scenario.
},
};
obj.x();
const obj2 = {
a: 10,
x: function () {
const y = () => {
console.log(this);
// Above the value of this will be obj2 as function y's enclosing lexical context is function x.
};
y();
},
};
obj2.x();
this inside DOM
It refers to HTML element.
<button onclick="alert(this)">Click Me</button>
<!-- [object HTMLButtonElement] Button element -->
Episode 26 : Prototype, Prototypal Inheritance & this inside classes
=======================================================================
Every object in JS has a hidden internal link to another object called its prototype. This is the backbone of how JS does inheritance — JS does not have "classes" in the classical (Java/C++) sense, it uses prototypal inheritance instead. class in JS (covered later in this episode) is just syntax sugar over this same mechanism.
__proto__ vs prototype — the most confused pair in JS
function Person(name) {
this.name = name;
}
Person.prototype.getName = function () {
return this.name;
};
const p1 = new Person("Manpreet");
console.log(p1.getName()); // "Manpreet"
-
Person.prototypeis an object that exists on every function. It's where you attach methods you want every instance to share (instead of copying the method into every object, which would waste memory). -
p1.__proto__is the actual hidden link thatp1uses to look up things not found on itself. When you writenew Person(...), JS setsp1.__proto__ = Person.prototype. - So:
prototypeis the blueprint living on the constructor function.__proto__is the reference every instance holds back to that blueprint.
console.log(p1.__proto__ === Person.prototype); // true
console.log(Person.prototype.constructor === Person); // true
The Prototype Chain
When you access p1.getName(), JS does this lookup:
- Does
p1itself have agetNameproperty? No. - Look at
p1.__proto__(i.e.Person.prototype). Found it! Use it.
If it wasn't found there either, JS would go one level up: Person.prototype.__proto__, which is Object.prototype (the root of almost every prototype chain), and finally Object.prototype.__proto__ is null — the chain ends here. This is why you can call .toString() or .hasOwnProperty() on literally any object — those live on Object.prototype.
console.log(p1.__proto__.__proto__ === Object.prototype); // true
console.log(p1.__proto__.__proto__.__proto__); // null
Object.create()
This is the purest way to see prototypal inheritance without constructor functions at all:
const animal = {
speak() {
console.log(`${this.name} makes a noise.`);
},
};
const dog = Object.create(animal); // dog.__proto__ = animal
dog.name = "Rex";
dog.speak(); // "Rex makes a noise."
Prototypal Inheritance between constructor functions
function Animal(name) {
this.name = name;
}
Animal.prototype.speak = function () {
console.log(`${this.name} makes a noise.`);
};
function Dog(name) {
Animal.call(this, name); // borrow constructor logic
}
Dog.prototype = Object.create(Animal.prototype); // set up chain
Dog.prototype.constructor = Dog; // fix constructor reference
Dog.prototype.bark = function () {
console.log(`${this.name} barks.`);
};
const rex = new Dog("Rex");
rex.speak(); // "Rex makes a noise." (inherited)
rex.bark(); // "Rex barks."
class — syntactic sugar, not a new model
class Animal {
constructor(name) {
this.name = name;
}
speak() {
console.log(`${this.name} makes a noise.`);
}
}
class Dog extends Animal {
constructor(name, breed) {
super(name); // must call before using `this`
this.breed = breed;
}
bark() {
console.log(`${this.name} barks.`);
}
}
const rex = new Dog("Rex", "Labrador");
rex.speak(); // inherited via prototype chain, same as before
rex.bark();
Under the hood this compiles down to exactly the constructor-function + Object.create pattern above. class just hides the plumbing — that's why interviewers love asking "is JS class-based or prototype-based?" (Answer: prototype-based; class is sugar.)
this inside class methods
Same rules as regular object methods — this refers to whatever called the method. The gotcha: if you pass a class method as a bare callback (e.g. to setTimeout or an event handler), it loses its this binding because it's no longer being called "as obj.method()":
class Counter {
count = 0;
increment() {
this.count++;
console.log(this.count);
}
}
const c = new Counter();
const fn = c.increment;
fn(); // TypeError / undefined count — `this` is lost
// Fix with .bind, or an arrow-function class field:
class Counter2 {
count = 0;
increment = () => {
this.count++; // arrow captures `this` from enclosing class instance
console.log(this.count);
};
}
Object.freeze / Object.seal
const obj = { a: 1 };
Object.freeze(obj); // can't add, remove, or modify properties
obj.a = 100; // silently fails (throws in strict mode)
console.log(obj.a); // 1
const obj2 = { b: 1 };
Object.seal(obj2); // can modify existing props, can't add/remove
obj2.b = 100; // works
obj2.c = 5; // silently fails
Common interview questions
- Difference between
__proto__andprototype? (answered above) - Is JS single-inheritance or multiple-inheritance? Single, via the prototype chain (mixins are used to fake multiple inheritance).
- What does
newactually do? Creates a new empty object → sets its__proto__to the constructor's.prototype→ calls the constructor withthisbound to the new object → returns the object (unless the constructor explicitly returns another object).
Episode 27 : DOM Events — Bubbling, Capturing, Delegation, Debounce & Throttle
================================================================================
Event Bubbling
When an event fires on an element, it doesn't just run on that element — it "bubbles" upward through every ancestor, all the way to document.
<div id="grandparent">
<div id="parent">
<button id="child">Click Me</button>
</div>
</div>
document.getElementById("grandparent").addEventListener("click", () => console.log("grandparent"));
document.getElementById("parent").addEventListener("click", () => console.log("parent"));
document.getElementById("child").addEventListener("click", () => console.log("child"));
// Clicking the button logs:
// child
// parent
// grandparent
Event Capturing
The reverse phase — before bubbling happens, the event actually travels down from document to the target first (the "capturing phase"). By default listeners run in the bubbling phase; pass { capture: true } (or true) to run in the capturing phase instead:
document.getElementById("grandparent").addEventListener(
"click",
() => console.log("grandparent - capturing"),
{ capture: true },
);
// This now fires FIRST, before the bubbling-phase listeners above.
So the full order for a click is: capturing (top → target) → target → bubbling (target → top).
stopPropagation()
Stops the event from continuing to bubble (or capture) further:
document.getElementById("child").addEventListener("click", (e) => {
e.stopPropagation();
console.log("child only — parent/grandparent won't fire");
});
Event Delegation
Instead of attaching a listener to every single child element (expensive, and doesn't work for elements added later), attach one listener to a common parent and use bubbling + e.target to figure out which child was actually clicked.
<ul id="todo-list">
<li>Buy milk</li>
<li>Walk dog</li>
<li>Write code</li>
</ul>
document.getElementById("todo-list").addEventListener("click", (e) => {
if (e.target.tagName === "LI") {
console.log("Clicked:", e.target.textContent);
}
});
// Works even for <li> elements added to the list AFTER this listener was attached.
This is why delegation is the standard pattern for dynamic lists, tables, and any UI where children are added/removed at runtime.
Debounce
Debounce means: "only run the function once the user has stopped triggering the event for X ms." Classic use case: a search box's autocomplete — you don't want to fire an API call on every keystroke, only once the user pauses typing.
function debounce(fn, delay) {
let timer;
return function (...args) {
clearTimeout(timer); // cancel the previous pending call
timer = setTimeout(() => fn.apply(this, args), delay);
};
}
const handleSearch = debounce((e) => {
console.log("Searching for:", e.target.value);
}, 300);
searchInput.addEventListener("input", handleSearch);
// Typing "hello" fast only triggers ONE console.log, 300ms after the last keystroke.
Throttle
Throttle means: "run the function at most once every X ms, no matter how many times the event fires." Classic use case: a scroll handler or window-resize handler, where you want regular updates but not on every single pixel of scroll.
function throttle(fn, limit) {
let inThrottle = false;
return function (...args) {
if (!inThrottle) {
fn.apply(this, args);
inThrottle = true;
setTimeout(() => (inThrottle = false), limit);
}
};
}
const handleScroll = throttle(() => {
console.log("Scroll position:", window.scrollY);
}, 200);
window.addEventListener("scroll", handleScroll);
// Fires at most once every 200ms while scrolling continuously.
Debounce vs Throttle — the one-liner interviewers want to hear
Debounce waits for a pause in activity before firing (fires once, at the end). Throttle fires at a regular cadence regardless of activity (fires repeatedly, evenly spaced).
Episode 28 : Modern JS Syntax — Destructuring, Spread/Rest, Optional Chaining, Modules
========================================================================================
Array & Object Destructuring
Pulling values out of arrays/objects into standalone variables, in one line.
// Array destructuring
const arr = [1, 2, 3];
const [a, b, c] = arr;
console.log(a, b, c); // 1 2 3
const [first, , third] = arr; // skip an element
console.log(first, third); // 1 3
// Object destructuring
const user = { name: "Manpreet", age: 25, city: "Delhi" };
const { name, age } = user;
console.log(name, age); // Manpreet 25
// Renaming while destructuring
const { name: userName } = user;
console.log(userName); // Manpreet
// Default values
const { country = "India" } = user;
console.log(country); // India (fallback since `country` doesn't exist on user)
// Nested destructuring
const person = { info: { name: "Manpreet", address: { city: "Delhi" } } };
const { info: { address: { city } } } = person;
console.log(city); // Delhi
Spread Operator (...) — expanding
Used to "spread out" the elements of an array/object.
const arr1 = [1, 2, 3];
const arr2 = [...arr1, 4, 5]; // [1, 2, 3, 4, 5]
const obj1 = { a: 1, b: 2 };
const obj2 = { ...obj1, c: 3 }; // { a: 1, b: 2, c: 3 }
// Common use: shallow-clone + override (never mutate original state directly)
const updatedUser = { ...user, age: 26 }; // copies user, overrides age
// Spread into function calls
function sum(a, b, c) {
return a + b + c;
}
console.log(sum(...arr1)); // 6
Rest Operator (...) — collecting
Same ... syntax, opposite job: gathers remaining items into one array/object. Context tells them apart — spread is used where a value is expected (expanding), rest is used in a destructuring/parameter position (collecting).
// Rest in function parameters
function sum(...nums) {
return nums.reduce((acc, n) => acc + n, 0);
}
console.log(sum(1, 2, 3, 4)); // 10
// Rest in array destructuring
const [first2, ...remaining] = [1, 2, 3, 4];
console.log(first2, remaining); // 1 [2, 3, 4]
// Rest in object destructuring
const { name: n2, ...others } = user;
console.log(n2, others); // Manpreet { age: 25, city: "Delhi" }
Optional Chaining (?.)
Safely access deeply nested properties without a chain of manual && checks — if anything in the chain is null/undefined, it short-circuits to undefined instead of throwing.
const user2 = { name: "Manpreet" };
console.log(user2.address.city);
// ❌ Uncaught TypeError: Cannot read properties of undefined
console.log(user2.address?.city); // ✅ undefined, no crash
// Also works for optional function calls and array access
user2.greet?.(); // does nothing, since greet doesn't exist — no crash
console.log(user2.hobbies?.[0]); // undefined, no crash
Nullish Coalescing (??)
Provides a fallback value, but only when the left side is null or undefined — unlike ||, which also treats 0, "", and false as "falsy" and wrongly falls back for them.
const count = 0;
console.log(count || 10); // 10 ❌ wrong! 0 is a valid value here
console.log(count ?? 10); // 0 ✅ correct — only null/undefined trigger the fallback
let userInput;
console.log(userInput ?? "default"); // "default"
ES Modules — import / export
Modules let you split code across files and share pieces between them, without polluting the global scope.
// mathUtils.js
export function add(a, b) {
return a + b;
}
export const PI = 3.14159;
export default function multiply(a, b) {
return a * b;
}
// app.js
import multiply, { add, PI } from "./mathUtils.js"; // default + named imports together
import * as MathUtils from "./mathUtils.js"; // import everything as a namespace object
console.log(add(2, 3)); // 5
console.log(multiply(2, 3)); // 6
console.log(MathUtils.PI); // 3.14159
-
export default→ one per file, imported without curly braces, and you can name it anything on import. -
export { name }(named export) → can have many per file, imported with curly braces, and the name must match (unless aliased withas).
CommonJS vs ES Modules (the interview-favorite comparison)
| CommonJS (Node's original system) | ES Modules (modern standard) | |
|---|---|---|
| Syntax |
require() / module.exports
|
import / export
|
| Loading | Synchronous | Asynchronous (supports top-level await) |
| When resolved | At runtime | At parse time (statically analyzable — enables tree-shaking) |
| Default in | Node.js (.js files) |
Browsers, and Node with .mjs or "type": "module" in package.json |
// CommonJS
const { add } = require("./mathUtils");
module.exports = { add };
Episode 29 : Polyfills & Advanced Async — map/filter/reduce/bind, Generators, Iterators, Proxy
==================================================================================================
A polyfill is code that implements a feature manually, so it works even in environments that don't natively support it — and interviewers love asking you to write one, because it proves you actually understand what the built-in method does internally, not just how to call it.
Polyfill for Array.prototype.map
Array.prototype.myMap = function (callback) {
const result = [];
for (let i = 0; i < this.length; i++) {
result.push(callback(this[i], i, this));
}
return result;
};
console.log([1, 2, 3].myMap((x) => x * 2)); // [2, 4, 6]
Polyfill for Array.prototype.filter
Array.prototype.myFilter = function (callback) {
const result = [];
for (let i = 0; i < this.length; i++) {
if (callback(this[i], i, this)) {
result.push(this[i]);
}
}
return result;
};
console.log([1, 2, 3, 4].myFilter((x) => x % 2 === 0)); // [2, 4]
Polyfill for Array.prototype.reduce
Array.prototype.myReduce = function (callback, initialValue) {
let accumulator = initialValue;
for (let i = 0; i < this.length; i++) {
accumulator = accumulator !== undefined
? callback(accumulator, this[i], i, this)
: this[i]; // if no initialValue given, first element becomes the seed
}
return accumulator;
};
console.log([1, 2, 3].myReduce((acc, cur) => acc + cur, 0)); // 6
Polyfill for Function.prototype.bind
Function.prototype.myBind = function (context, ...args) {
const fn = this; // `this` here is the function myBind was called on
return function (...newArgs) {
return fn.apply(context, [...args, ...newArgs]);
};
};
const obj = { name: "Manpreet" };
function greet(greeting) {
console.log(`${greeting}, ${this.name}`);
}
const boundGreet = greet.myBind(obj, "Hello");
boundGreet(); // "Hello, Manpreet"
call, apply, bind — the quick difference (since notes mentioned but didn't spell this out)
function introduce(city, country) {
console.log(`${this.name} from ${city}, ${country}`);
}
const person = { name: "Manpreet" };
introduce.call(person, "Delhi", "India"); // invokes immediately, args passed individually
introduce.apply(person, ["Delhi", "India"]); // invokes immediately, args passed as an array
const boundFn = introduce.bind(person, "Delhi", "India"); // returns a NEW function, doesn't invoke
boundFn(); // invoke it later, whenever you want
Generators (function* / yield)
A generator function can pause its own execution at a yield and resume later, right where it left off — unlike a normal function which always runs start-to-finish in one go.
function* numberGenerator() {
console.log("start");
yield 1;
console.log("resumed after first yield");
yield 2;
console.log("resumed after second yield");
yield 3;
}
const gen = numberGenerator(); // nothing runs yet — just creates a generator object
console.log(gen.next()); // logs "start", returns { value: 1, done: false }
console.log(gen.next()); // logs "resumed after first yield", returns { value: 2, done: false }
console.log(gen.next()); // logs "resumed after second yield", returns { value: 3, done: false }
console.log(gen.next()); // returns { value: undefined, done: true }
Generators are also directly for...of iterable, and are the mechanism async/await is built on top of under the hood (an async function is conceptually a generator that auto-resumes on promise resolution).
for (const num of numberGenerator()) {
console.log(num); // 1, 2, 3 (logs happen interleaved too)
}
Iterators & Symbol.iterator
An iterable is any object that defines how to be looped over — this is the protocol that powers for...of, spread (...), and destructuring on arrays, strings, Maps, and Sets.
const myIterable = {
from: 1,
to: 3,
[Symbol.iterator]() {
let current = this.from;
const last = this.to;
return {
next() {
if (current <= last) {
return { value: current++, done: false };
}
return { value: undefined, done: true };
},
};
},
};
console.log([...myIterable]); // [1, 2, 3]
for (const num of myIterable) {
console.log(num); // 1, 2, 3
}
Plain objects ({}) are NOT iterable by default (no Symbol.iterator) — that's why for...of throws on a plain object but works fine on arrays, which is a common gotcha question.
Proxy & Reflect
A Proxy lets you wrap an object and intercept fundamental operations on it (get, set, delete, etc.) — useful for validation, logging, or reactive frameworks (Vue 3's reactivity system is built on Proxy).
const target = { name: "Manpreet", age: 25 };
const handler = {
get(obj, prop) {
console.log(`Reading property: ${prop}`);
return Reflect.get(obj, prop); // default behavior, forwarded via Reflect
},
set(obj, prop, value) {
if (prop === "age" && typeof value !== "number") {
throw new TypeError("Age must be a number");
}
console.log(`Setting property: ${prop} = ${value}`);
return Reflect.set(obj, prop, value);
},
};
const proxyUser = new Proxy(target, handler);
console.log(proxyUser.name); // logs "Reading property: name", then "Manpreet"
proxyUser.age = 26; // logs "Setting property: age = 26"
proxyUser.age = "twenty-six"; // throws TypeError
Reflect is a built-in object with methods that mirror the internal operations Proxy traps intercept (Reflect.get, Reflect.set, Reflect.has, etc.) — it exists so you have a clean, functional way to forward the "default" behavior from inside a trap, instead of manually reimplementing it.
Episode 30 : Type Coercion, Equality (== vs ===), Truthy/Falsy & NaN
===========================================================================
This entire topic wasn't covered anywhere in the notes above, despite being one of the most-asked JS fundamentals questions — almost every interview has at least one "what does this print?" question built on type coercion.
Type Coercion
JS is loosely typed (mentioned back in Episode 6), which means it will automatically convert values from one type to another when an operation expects a specific type. This automatic conversion is called type coercion.
console.log("5" + 3); // "53" — number 3 gets coerced to a string, then concatenated
console.log("5" - 3); // 2 — string "5" gets coerced to a number, then subtracted
console.log("5" * "2"); // 10 — both strings coerced to numbers
console.log(1 + true); // 2 — true coerced to 1
console.log(1 + false); // 1 — false coerced to 0
console.log("5" + null); // "5null" — null coerced to the string "null"
console.log(1 + null); // 1 — null coerced to the number 0
console.log(1 + undefined); // NaN — undefined coerces to NaN, and NaN poisons any arithmetic
The rule of thumb: the + operator prefers string concatenation (if either operand is a string, the other gets coerced to a string). Every other arithmetic operator (-, *, /) prefers numeric coercion.
== (loose equality) vs === (strict equality)
=== checks both value AND type — no coercion happens. == first coerces both operands to the same type, then compares.
console.log(5 == "5"); // true — "5" is coerced to number 5, then 5 == 5
console.log(5 === "5"); // false — different types, no coercion, so not equal
console.log(null == undefined); // true — special case, JS treats them as loosely equal to each other
console.log(null === undefined); // false — different types
console.log(0 == false); // true — false coerced to 0
console.log(0 === false); // false — different types
console.log("" == 0); // true — "" coerced to 0
console.log([] == false); // true — [] coerced to "" coerced to 0, false coerced to 0
That last line is exactly why == has a bad reputation — the coercion rules chain through multiple steps and become genuinely hard to predict. This is why the standard advice (mentioned nowhere in these notes until now) is:
Always use
===and!==unless you have a specific, well-understood reason to use==(the most common legitimate one beingx == null, which conveniently catches bothnullandundefinedin one check).
Truthy and Falsy values
Every value in JS is inherently "truthy" or "falsy" when used in a boolean context (like an if condition). There are only 8 falsy values in JS — everything else is truthy:
// The complete list of falsy values:
false
0
-0
0n // BigInt zero
"" // empty string
null
undefined
NaN
// Everything else is truthy, including these commonly-mistaken ones:
if ("0") console.log("truthy!"); // runs — non-empty string, even "0", is truthy
if ([]) console.log("truthy!"); // runs — empty array is truthy
if ({}) console.log("truthy!"); // runs — empty object is truthy
if ("false") console.log("truthy!"); // runs — non-empty string, even the string "false", is truthy
[] and {} being truthy despite "looking empty" is one of the most common gotchas asked directly in interviews.
NaN — Not a Number, but weirder than it sounds
console.log(typeof NaN); // "number" — NaN is, ironically, of type "number"
console.log(NaN === NaN); // false — NaN is the ONLY value in JS that is not equal to itself
console.log(NaN == NaN); // false — still false, coercion doesn't help here
// So how do you actually check if a value is NaN?
console.log(isNaN(NaN)); // true
console.log(isNaN("hello")); // true — ⚠️ misleading! isNaN coerces its argument to a number first ("hello" -> NaN), THEN checks
console.log(Number.isNaN("hello")); // false — ✅ Number.isNaN does NOT coerce, so it correctly says "hello" is not NaN
console.log(Number.isNaN(NaN)); // true
// The reliable modern way to check for NaN is always Number.isNaN(), not the global isNaN().
Episode 31 : Shallow vs Deep Copy, JSON.stringify/parse, Map, Set, WeakMap & WeakSet
=======================================================================================
Also missing from the notes above — object copying pitfalls and the newer collection types (Map/Set) are extremely common in day-to-day code and come up constantly in interviews around state management (React, Redux, etc.).
Shallow Copy
A shallow copy duplicates only the "top level" of an object/array — if a property is itself an object, both the original and the copy end up pointing to the same nested object in memory.
const original = { name: "Manpreet", address: { city: "Delhi" } };
// Common ways to shallow-copy:
const copy1 = { ...original }; // spread operator
const copy2 = Object.assign({}, original); // Object.assign
copy1.name = "Ashish"; // fine — doesn't affect original, since `name` is a primitive
console.log(original.name); // "Manpreet" — untouched
copy1.address.city = "Mumbai"; // ⚠️ mutates the SHARED nested object
console.log(original.address.city); // "Mumbai" — original got mutated too! Same reference.
Deep Copy
A deep copy duplicates every level, recursively, so nested objects are also fully independent copies.
// Option 1: structuredClone() — modern, built into browsers and Node 17+
const deepCopy = structuredClone(original);
deepCopy.address.city = "Mumbai";
console.log(original.address.city); // "Delhi" — untouched, fully independent copy
// Option 2: JSON.parse(JSON.stringify(...)) — the classic pre-structuredClone trick
const deepCopy2 = JSON.parse(JSON.stringify(original));
Why the JSON.stringify/parse trick is risky (a very common interview gotcha)
const obj = {
date: new Date(),
fn: function () {},
undef: undefined,
sym: Symbol("id"),
num: NaN,
bigNum: 10n,
};
console.log(JSON.parse(JSON.stringify(obj)));
// { date: "2024-01-01T00:00:00.000Z", num: null }
// - Date objects get silently converted to strings, not real Date objects
// - functions are silently DROPPED entirely
// - undefined properties are silently DROPPED entirely
// - Symbol properties are silently DROPPED entirely
// - NaN and Infinity are silently converted to null
// - BigInt values THROW an error — "Do not know how to serialize a BigInt"
This is exactly why structuredClone() is now the recommended approach — it correctly handles Date, Map, Set, and circular references, none of which the JSON trick can handle safely. (Functions still can't be cloned by either method — functions are never structurally cloneable, since they close over live scope.)
Map — like an object, but better for key-value data
const map = new Map();
map.set("name", "Manpreet");
map.set(1, "one"); // keys can be ANY type, not just strings (unlike plain objects)
map.set(true, "yes");
console.log(map.get("name")); // "Manpreet"
console.log(map.size); // 3
for (const [key, value] of map) {
console.log(key, value); // iterates in insertion order, guaranteed
}
Why prefer Map over a plain object sometimes? Keys can be any type (objects, functions, numbers — not just strings), iteration order is guaranteed, .size gives you the count directly (no Object.keys(obj).length needed), and a Map doesn't have any inherited prototype properties that could accidentally collide with your data keys.
Set — a collection of unique values
const set = new Set([1, 2, 2, 3, 3, 3]);
console.log(set); // Set(3) {1, 2, 3} — duplicates automatically removed
set.add(4);
set.has(2); // true
set.delete(1);
console.log([...set]); // [2, 3, 4] — easily converted back to an array
// Classic use: dedupe an array in one line
const nums = [1, 2, 2, 3, 4, 4, 5];
const unique = [...new Set(nums)]; // [1, 2, 3, 4, 5]
WeakMap and WeakSet — same idea, but "weakly" held
The keys in a WeakMap (and the values in a WeakSet) must be objects, and they are held weakly — meaning if there's no other reference to that object anywhere else in the program, the garbage collector is free to remove it, and its entry disappears from the WeakMap/WeakSet automatically.
let user = { name: "Manpreet" };
const weakMap = new WeakMap();
weakMap.set(user, "some metadata about this user");
user = null; // no more references to the original object anywhere
// The entry in weakMap can now be garbage collected automatically — no memory leak risk.
This is precisely why WeakMap/WeakSet are the right tool for attaching metadata to objects you don't own the lifecycle of (e.g. DOM nodes) — a regular Map would keep that DOM node alive in memory forever just because it's a key, even after it's removed from the page. Note: because entries can vanish at any time, WeakMap/WeakSet are NOT iterable — there's no .size, no for...of, and no way to list their contents.
This article was originally published by DEV Community and written by Manpreet Singh.
Read original article on DEV Community