JavaScript topics for interview preparation

Murad Hossain
3 min readNov 5, 2020


React Hook: ‘Hooks are functions that let you “hook into” React state and lifecycle features from function component. They do not work within a class. They let you use React without a class.’ — from React official blog post.

Hooks avoid the complexity of classes and higher-order components altogether.

Closure: is the combination of a function and the lexical scope within which the particular function is declared.

function start() {

var topic = ‘closure’; // topic is a local variable created by init

function display () { // display() is the inner function, a closure

alert(topic); // use variable declared in the parent function





Accessibility: design and creation of websites that can be used by everyone. Which allows assistive technology to interpret web pages.

A number of tools exist that can run accessibility audits on web pages:

  • Web Accessibility Evaluation Tool
  • NVDA in firefox
  • JAWX in Internet Explorer
  • ChromeVox in Google Chrome

Truthy and Falsy: each value has an inherent boolean value, generally know as either truthy or falsy.

Values that are always Falsy:

  • False
  • 0 (zero)
  • ‘’ or “”(empty string)
  • Null
  • undefined
  • Nan

Everything else is Truthy:

  • ‘0’
  • ‘false’
  • [ ]
  • { }
  • function ( ) { }

undefined vs null:

undefined: anything that is not defined.

let name;

console.log(name); // output: undefined

null: is an assignment value, we can assign it to a variable.

let name = null;

console.log(name); // output: null

Scope: is the accessibility of variables, functions, and objects in some particular part of our code during runtime. It provides the same level of security to our code.

Types of Scope:

  • Local Scope: variable defines inside a function.
  • Global Scope: variable defines outside a function.

JavaScript coding proble:

Find the largest element of an Array:

const array1 = [1, 2, 3, 4];


// output: 4

Sume of all numbers in an Array:

arrSum = function(arr){

return arr.reduce(function(a,b){

return a + b

}, 0);


Remove duplicates:

let arr = [1, 2, 3, 2, 4, 5];

function removeDuplicates(items) {

return items.filter((value, index) => items.indexof(value) === index);



// [1, 2, 3, 4, 5]

Asynchronous in JavaScript: In asynchronous programs, we can have two lines of code, where the first-line schedules some task to be run in the future, but second-line runs before that task completes.

setTimeout(function() {

console.log(‘asynchronous message’);

}, 1000);

console.log(‘synchronous message’);

JavaScript Methods: are actions that can be performed on objects. A method is a property that contains a function definition.

Object property: is a variable that is attached to the object.

Difference between object method and object property: we can think of object properties like object variable of the object and method like function. For example, a string has .length property it’s like a variable and here .toLowerCase() method looks like a function.

Check a number is a Prime number or not:

// return true if num is prime

function isPrime(num)


// if num is less than 2 => is not prime

if ( num < 2 ) return false;

for (let i = 2; i <= Math.sqrt(num); i++) {

if (n % i == 0) return false;


return true;




Murad Hossain

MERN Stack Developer. Make web and mobile application.