JavaScript for Interviews

This is my first post/blog/article of any sort, so any feedback will be appreciated.

These are my notes which I prepared to revise for JavaScript interviews. Some of the topics are advanced but are a must-know for a intermediate JavaScript developer.

💪 Basics

JavaScript is a dynamically typed language

Data type of variables can be changed during runtime

var justAVariable = 10;
justAVariable = "I am a string now";

/* Does not throw any error */

Variables

	Scope	Can be updated?	Can be redeclared?
var	FUNCTIONAL	YES	YES
let	BLOCK	YES	NO
const	BLOCK	NO	NO

Data Types

Primitive Values

Boolean, String, Number, BigInt, Symbol, Null, Undefined
Objects

Everything except primitives are of type Object in JS.

== v/s ===

== checks only for value (type coersion may occur)
=== checks for both value and type (no type coersion)

Spread Operator

Spreads an array into elements.

Spreads an object into key value pairs.

const arr = [1, 2, 3];
const copyArr = [...arr]; // copyArr = [1, 2, 3]

const sum = (a, b, c) => a + b + c;
sum(...arr); // Equivalent to sum(1, 2, 3)

const obj = { x: 10 };
const copyObj = { ...obj }; // copyObj = { x: 10 }

Rest Parameter

Condense elements into an array.

Function can be called using variable no. of arguments.

const condense = (...params) => {
    return params;
}

console.log(condense(1, 2, 3, 4)); // prints [1, 2, 3, 4]
console.log(condense(1, 2)); // prints [1, 2]

🫡 Array Methods

map() method

arr.map((element, index, array) ⇒ { /* Modify element */ })

/* returns a new array with modified elements */

const numbers = [1, 2, 3, 4];
const double = numbers.map((ele) => ele * 2); // [2, 4, 6, 8]

filter() method

arr.filter((element, index, array) ⇒ { /* Condition */ })

/* returns a new array whose elements satisfy the condition */

const numbers = [1, 2, 3, 4];
const evenNumbers = numbers.filter((ele) => ele % 2 == 0); // [2, 4]

find() method

arr.find((element, index, array) ⇒ { /* Condition */ })

/* returns the first element satisfying the condition */

const numbers = [1, 2, 3, 4];
const greaterThanTwo = numbers.find((ele) => ele > 2); // 3

reduce() method

arr.reduce((prevEle, currEle, currIndex, array) ⇒ { /* Code */ }, initialVal)

/* returns a single value - the accumulated result of calling the callback function on each element */

const numbers = [1, 2, 3, 4];
const sum = numbers.reduce((prev, curr) => prev + curr, 0); // 10

☝️ Hoisting

Function, variable and class declarations are moved to the top of the scope.

Variable Hoisting

var is hoisted and initialized with undefined.

console.log(a); // prints "undefined"
var a = 10;

let and const are hoisted, but not initialized.

console.log(b); // ReferenceError: b is not defined
let b = 20;

No hoisting for variables that are not declared.

console.log(c); // ReferenceError: c is not defined
c = 30;

Function Hoisting

Function declarations are hoisted. (! Arrow functions are not hoisted)

helloWorld(); // Works

function helloWorld() {
  console.log("Hello World");
}

Class Hoisting

Class declarations are hoisted, but not initialised.

⚠️ Function and Class expressions are not hoisted.

🤝 Closure

A function and it’s lexical scope together form a closure.
Due to closures, we can access the outer scope of our function.

If we return a function, it stores the reference to variables and functions in the outer scope even after the outer function is executed.

function outerFunc() {
  let str = "Hello World";
    return () => console.log(str);
}

const testClosure = outerFunc();
testClosure(); // prints "Hello World"

/* The returned function still remembers the value of str" */

NOTE: Only a reference to the variables is stored and not the actual value. So, if you mutate the variable before calling the returned function, it will reflect the updated value.

let arr = [];
function outerFunc() {
  arr.push(1);
    return () => console.log(arr);
}

const testClosure = outerFunc();
arr.push(2);
testClosure(); // prints [2, 1]

😮‍💨 IIFE (Immediately Invoked Function Expression)

Do not have a name ⇒ Anonymous function

Executed just after it is defined.

/* Syntax */

( **function(){}** )();
( **function(){}**() );
( **() => {}** )();
!**function(){}**();
+**function(){}**();
-**function(){}**();

Used for keeping data private and exposing selected functions.

const iife = (() => {
  let val = 0;
  return {
    getVal: () => val,
    setVal: (n) => {
      val = n;
    },
  }
})();

iife.setVal(10)
console.log(iife.getVal());

👩‍👦 Prototypal Inheritance

All objects inherit properties with the help of the prototype object.

eg. Arrays inherit from Array prototype
The prototype object has a prototype of it’s own. This forms a prototype chain with the last object being null.
The prototype object: __proto__
When we try to access something in an object, it first checks the object, then in it’s prototype, then in the prototype’s prototype, etc.
```
const arr = [];

arr.push(1); // We can acces this due to the arr.__proto__.push() method
```