Most Frequently Asked JavaScript Interview Questions

Javascript has been the backbone of many web development aspects. Getting a job in web development as a javascript(er) is not that hard. Regular practice and some tips and tricks can get you to shine in the job interview. Here are a few frequently asked questions in Javascript.

Pat yourself if you know them.

Question 1: Difference between “==” and “===”?

== only compares values. === compare values and type both.

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In the above code snippet, the value of true is 1, whereas the type of true is boolean.

So true === 1 will return false as its type is not 1.

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Question 2: What is the output of A and B?

A. 
document.write({});

B. 
console.log({});

console.log is used to log useful information on the console. It is more popularly used as a debugging strategy in JavaScript programs.

document.write modifies the DOM with arguments passed to it.

The curly braces {} are another way for creating objects in javascript. This is equivalent to the new Object() syntax which creates new Object literal.

So, document.write({}); will clear the webpage content and replace it with a new object. Like below:

Awesome JavaScript Tricks

Question 3: What is the strict mode in JavaScript and how can it be enabled?

Strict mode is a way to introduce better error-checking into your code.

When you use strict mode, you cannot use implicitly declared variables, or assign a value to a read-only property, or add a property to an object that is not extensible.

You can enable strict mode by adding “use strict” at the beginning of a file, a program, or a function.

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Question 4: Compute the output.

var Y = 1;
if (function F(){})
{
    Y += typeof(F);
}
console.log(Y);

The output would be 1undefined.

The if condition statement evaluates using eval, so eval(function F(){}) returns function F(){} - which is true.

Therefore, inside the if statement, executing typeof F returns undefined because the if statement code executes at run time, and the statement inside the if condition is evaluated during run time.

Question 5: Compute the output.

var ob1= null;
var ob2= function a(){};
console.log(typeof ob1 === "object");
console.log(typeof ob2 === "object");

null is an object whereas a function isn't. The function can be stored in a variable. The type of that stored variable will be of type function only

Question 6: What is the difference between unescape() and escape()

Unescape

It decodes the coded string.

Also computes a new string in which hexadecimal escape sequences are replaced with the character that it represents

Escape

It encodes the coded string.

Also computes a new string in which certain characters have been replaced by a hexadecimal escape sequence

Question 7: What do you understand by Closures in JavaScript?

A closure gives you access to an outer function's scope from an inner function. Even you can pass arguments to the inner function

The function sumMsg() uses the variables a and b defined in the outer function sum(a,b)

JavaScript closures made super easy

Question 8: Event Bubbling in JavaScript

Event propagation in HTML DOM API

If you click on "HI", the "I am the first" will be invoked, then followed by "I am in between then I am last" as shown below

It's like the circular waves. It starts from the inside.

body  {
    margin: 10px;
    border: 1px solid blue;
  }
</style>

<form onclick="alert('I am Last')">BYE
  <div onclick="alert('I am inbetween')">INBETWEEN
    <p onclick="alert('I am the first')">HI</p>
  </div>
</form>

onclick is invoked when HI is clicked

The next onclick is invoked after OK is clicked in the dialog box

The last onclick is invoked after OK is clicked in the dialog box

Conclusion

I hope these questions and answers got you more excited about JavaScript. If you are looking for a career in web development these JavaScript questions are very popular in the interviews.

Please comment below other such interesting questions that you might have come across.

Author: Jairam Bala
I am Jairam, a Software Developer at Accenture. Software and Tech Enthusiast. Freelancer too.
Email: b.jairam0512@gmail.com

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