Javascript Interview Questions and Answers: Freshers

JavaScript Interview Questions for Freshers

In this Article, you will learn JavaScript interview questions and answers that are most frequently asked in interviews. Before proceeding to learn JavaScript interview questions and answers, first we learn the complete JavaScript Tutorial.

What is JavaScript and Who developed JavaScript?

JavaScript (JS) is the most widely used lightweight, scripting and compiled programming language with first-class functions, and it was developed by Brenden Eich in 1995. It is well-known as a scripting language for web pages, mobile apps, web servers, and many more.

JavaScript is the most used language in top companies such as Uber, Airbnb, Google, Netflix, Instagram, Spotify, Amazon, and many more because of its features and performance. To get into these companies and other software companies as a web developer, you need to master some important JavaScript interview questions to crack their JavaScript Online Assessment round and JavaScript Technical interview.

Let’s discuss some common questions that you should prepare for the interviews. These questions will be helpful in clearing the interviews specially for the frontend development role.

What are JavaScript Data Types?

The Data types supported by JavaScript are:

Which symbol is used for comments in Javascript?

Comments are used to prevent the execution of statements. Comments are ignored while the compiler executes the code. There are two types of symbols used to represent comments in JavaScript:

Double slash: It is known as a single-line comment.

// Single line comment

Slash with Asterisk: It is known as a multi-line comment.

/* 
Multi-line comments
...
*/

What is the use of isNaN function?

isNan function returns true if the argument is not a number; otherwise, it is false.

<p>isNaN() returns true if a value is NaN:</p>

<p id="demo"></p>

<script>
let result =
"Is '123' NaN? " + isNaN('123') + "<br>" +
"Is 'Hello' NaN? " + isNaN('Hello') + "<br>" +
"Is '2005/12/12' NaN? " + isNaN('2005/12/12');
document.getElementById("demo").innerHTML = result;
</script>

Output

isNaN() returns true if a value is NaN:

Is '123' NaN? false
Is 'Hello' NaN? true
Is '2005/12/12' NaN? true

What is the ‘this’ Keyword in Javascript?

 Functions in JavaScript are essential objects. Like objects, they can be assigned to variables, passed to other functions, and returned from functions. And much like objects, they have their own properties. ‘this’ stores the current execution context of the JavaScript program. Thus, when it is used inside a function, the value of ‘this’ will change depending on how the function is defined, how it is invoked, and the default execution context.

<p id="demo"></p>

<script>
// Create an object:
const person = {
  firstName: "webs",
  lastName: "codex",
  id: 5566,
  fullName : function() {
    return this.firstName + " " + this.lastName;
  }
};

// Display data from the object:
document.getElementById("demo").innerHTML = person.fullName();
</script>

Output: Here this keyword is referring to the person object so it can access firstnName and lastName values.

webs codex

Why do we use the word “debugger” in javascript?

The debugger for the browser must be activated in order to debug the code. Built-in debuggers may be switched on and off, requiring the user to report faults. The remaining section of the code should stop execution before moving on to the next line while debugging.

Difference between “ == “ and “ === “ operators.

Both are comparison operators. The difference between both the operators is that “==” is used to compare values whereas, “ === “ is used to compare both values and types.

var x = 6;
var y = "6";
(x == y)  // Returns true since the value of both x and y is the same
(x === y) // Returns false since the typeof x is "number" and typeof y is "string"

Difference between var and let, const keyword in Javascript.

There are some difference between var and let.

1. From the very beginning, the ‘var’ keyword was used in JavaScript programming whereas the keyword ‘let’ was just added in 2015.
2. The keyword ‘Var’ has a function scope. Anywhere in the function, the variable specified using var is accessible but in ‘let’ the scope of a variable declared with the ‘let’ keyword is limited to the block in which it is declared. Let’s start with a Block Scope.
3. In ECMAScript 2015, let and const are hoisted but not initialized. Referencing the variable in the block before the variable declaration results in a ReferenceError because the variable is in a “temporal dead zone” from the start of the block until the declaration is processed.
4. const keyword in JavaScript: The const keyword has all the properties that are the same as the let keyword, except the user cannot update it.

Write the code for adding new elements dynamically?

<html> 
<head> 
<title>Write the code for adding new elements dynamically?</title> 
<script type="text/javascript"> 

function addNode () { 
    var newP = document. createElement("p"); 
    var textNode = document.createTextNode(" This is a new text node"); 
    newP.appendChild(textNode); document.getElementById("demo").appendChild(newP); 
}

</script> 
</head> 
<body> 
   <p id="demo">This is the paragraph <p> 
</body> 
</html>

How you can submit a form using JavaScript?

To submit a form using JavaScript use

document.form[0].submit();
document.form[0].submit();

What a pop() method in JavaScript is?

The pop() method is similar to the shift() method, but the difference is that the Shift method works at the array’s start. The pop() method takes the last element off of the given array and returns it. The array on which it is called is then altered.

var fruits = ["Mango", "Banana", "Papaya", "Orange"];
fruits.pop();
//Now fruit becomes Mango,Banana,Papaya

How to handle exceptions?

Try… Catch—finally is used to handle exceptions in the JavaScript.

try{
   //  Code here
}
Catch(exp){
   // Code to throw an exception.
}
Finally{
   // Code runs either it finishes successfully or after catch
}