JavaScript Interview Questions

Advanced and Basic JavaScript Interview Questions

Javascript is a scripting language(supports scripts) for Web pages but it is also used in non-browser environments as well. It is a powerful, lightweight, interpreted, scripting language with first-class functions (i.e. the language supports passing functions as arguments to other functions).

In order to add the dynamic interactivity to web pages Javascript is embedded within Hypertext Markup Language (HTML). Since it runs on the client side of the web so it can be used to design/program how the web pages behave on the occurrence of a particular event. That's the reason why it is widely used for the behavior of the web pages.

Read Advanced and Basic Javascript Interview Questions and Answers. We have listed some Javascript Developer interview questions with their answers that help you to crack javascript interviews

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Advanced and Basic JavaScript Interview Questions

In JavaScript, we use the function setInterval() to call any function in every x seconds.

Syntax: setInterval(function, milliseconds, param1, param2, ...)

Function: it is a required parameter which includes the function to be execute.

Milliseconds: required parameter which tells how often the function will execute.

Others are an additional parameter.

For example: setInterval(function (){ alert("Hello"); }, 3000);

In the above example, this function calls hello function in very 3 seconds.

Higher order function is the best feature of functional programming available in JavaScript. It is the function which takes a function as an argument and returns a function as a result. Some of the inbuilt higher-order functions are mapping, filtering, reduction, zipping, etc.

InnerHTML property of HTML DOM is used to get inner Html of an element in JavaScript.

Example Usage:

This is inner Element
<script type="text/javascript">
	var  inner= document.getElementById("inner").innerHTML ;
	console.log(inner); // This is inner Element
	document.getElementById("inner").innerHTML = "Html changed!";
	var  inner= document.getElementById("inner").innerHTML ;
	console.log(inner); // Html changed!

JavaScript’s default behavior that allows moving declarations to the top is called Hoisting. The 2 ways of creating functions in JavaScript are Function Declaration and Function Expression. Let’s find out more about these:

Function Declaration

A function with the specific parameters is known as function declarations. To create a variable in JavaScript is called declarations.


hoisted(); // logs "foo"

function hoisted() {



Function Expression

When a function is created by using an expression it is called function expression.


notHoisted(); // TypeError: notHoisted is not a function

var notHoisted = function() {



The design pattern is a general reusable solution to a commonly occurring problem in software design. Some of the design patterns are:

  1. Creational design pattern: These patterns dealt with the mechanism of object creation which optimize object creation with the basic approach.
  2. Structural design pattern: these patterns deal with different classes and objects to provide new functionality.
  3. Behavioral Patterns: These patterns are to improve communication between objects and to recognize patterns.
  4. Concurrency design patterns: These patterns handle with multi-thread programming paradigms.
  5. Architectural design patterns: These patterns used to deal with architectural designs.
  • The primitive data types are numbers, strings, Boolean, undefined, null and anything other than these data types are known as non-primitive such as objects and functions.
  • Primitive data types are immutable while non-primitives are mutable.
  • Primitives are known immutable as they can't be changed once they created but non-primitive are changeable, means once an object is created, it can be changed.
  • Primitives data types are compared with their values, it means two values are strictly equal if they have the same data type and holds the same value.
  • Non-primitives are not compared with values. For example, if two objects have the same properties and values, they are strictly not equal.

Closures are the combination of lexical environment and function within which the function was declared. This allows JavaScript programmers to write better, more creative, concise and expressive codes. The closure will consist of all the local variables that were in-scope when the closure was created.

Sure, closures appear to be complex and beyond the scope, but after you read this article, closures will be much more easy to understand and more simple for your everyday JavaScript programming tasks. JavaScript is  a very function-oriented language it gives the user freedom to use functions as the wish of the programmer.

#C program to remove duplicate programme:

#include <stdio.h>
int main(){
  int n, a[100], b[100], calc = 0, i, j;
  printf("Enter no. of elements in array\n");
  scanf("%d", &n);
   printf("Enter %d integers\n", n);
   for (i = 0; i < n; i++)
    scanf("%d", &a[i]);
   for (i = 0; i<n; i++) {
    for (j = 0; j < calc; j++) {
      if(a[i] == b[j])
        break;   }
    if (j== calc){
      b[count] = a[i];
      calc++;  }  }
   printf("Array obtained after removing duplicate elements:\n");
   for (i = 0; i < calc; i++)
    printf("%d\n", b[i]);
  return 0;}
In modern javascript let & const are different ways of creating variables.Earlier in javascript, we use the var keyword for creating variables. let & const keyword is introduced in version ES6 with the vision of creating two different types of variables in javascript one is immutable and other is mutable.
const: It is used to create an immutable variable. Immutable variables are variables whose value is never changed in the complete life cycle of the program.
let: let is used to create a mutable variable. Mutable variables are normal variables like var that can be changed any number of time.

Both var and let are used for variable/ method declaration in javascript but the main difference between let and var is that var is function scoped whereas let is block scoped.

  • The array.slice() removes items from the array and then return those removed items as an array whereas array.slice() method is selected items from an array and then those elements as a new array object.
  • The splice() method affects the original array whereas slice() method doesn’t affect the original array.
  • Splice() method takes n number of arguments whereas slice() can take only two arguments.

Syntax of splice(): array.splice(index, howmany, item1, ....., itemX)

Syntax of slice(): array.slice(start, end)

Host objects: These are those objects which environment gives. It means they are different for different environments. For example, browsers include objects such as windows but Node.js environments give objects such as Node List.

Native Objects: these are built-in objects in JavaScript. They are also known as Global Objects because they will be available to you independent of ay environment if you working in JavaScript.

toISOString() method is used to convert javascript date to ISO standard. It converts JavaScript Date object into a string, using the ISO standard.


var date = new Date();
var n = date.toISOString();
// YYYY-MM-DDTHH:mm:ss.sssZ