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14/Aug/2020 | 10 minutes to read

Best Answers to JavaScript Interview Questions

QFLES is listing some Essential JavaScript Interview Questions and Answers for Freshers and mid level of Experienced Professionals prepared by Industry Experts. All answers for these JavaScript interview questions are given based on standard documents and tried to explain in simple and easiest way.

1. What is JavaScript? JavaScript is interpreted what does this imply?

JavaScript is a programming language for Web. It's light weight language to build functionality into HTML static pages. JavaScript also provide some Object oriented capabilities.
It's interpreted language means it's translated line by line not compiled like any other languages C#, Java etc.

2. What is Object in Java Script? In how many ways can you create the object?

In JavaScript everything is in the form of object. These types can be object if defined with new keyword - Booleans, Numbers, Strings.
And these types are always objects - Dates, Maths, Regular Expressions, Arrays, Functions, Objects.

In Java Script all values except primitives are objects. A value with no properties or methods is called primitive value and has primitive data type. Java script has 5 types of primitive data type. - string, number, boolean, null, undefined

Java script objects are collection of named values. Java script object example: Object creation using an Object Literal

var employee = {firstName:"Bhanu", lastName:"pratap", age:40};
Object creation using New Keyword
var employee = new Object();
employee.firstName = "Bhanu";
employee.lastName = "Pratap";
employee.age = 50;

JavaScript Objects are Mutable

It means they are addressed by reference (memory address) not by their value.
 var  x = employee;
If employee is an object then this code will not create copy of employee rather it's employee only. Any changes to x will apply to employee also because both objects are same.

3. What is dataType and contentType in jQuery Ajax calls

contentType

it's header information sent to server, that will specify a particular format. For example you are going to send JSON or XML data.

dataType

it's telling the jQuery , what kind of response you will receive from server. For example JSON, XML or HTML etc.

4. What is Closures in JavaScript? Why closure is required?

Closure

A closure is a inner function having access to the outer function's scope. A closure has access to:
  • it's own scope (variables defined in it's curly braces)
  • outer functions's variables
  • global variables

Why closure is required?

Let's understand with one example. Suppose you want a counter to count something.
// Initiate count
var count = 0;
// Function to increment count
function add() {
  count += 1;
}

// Call add() 4 times
add();
add();
add();
add();

// The count should now be 4
There is a problem with above code, Any code on page can change the counter value without calling 'add' function. So let's' modify the code as below.
// Initiate count
var count = 0;
// Function to increment count
function add() {
var count = 0;
  count += 1;
}

// Call add() 4 times
add();
add();
add();
add();

// The count should now be 4 but it is 0.
Now we display here count variable value but that is global so it's 0. Let's modify code again by removing global variable.
// Function to increment count
function add() {
var count = 0;
  count += 1;
}

// Call add() 4 times
add();
add();
add();
add();

// The count should now be 4 but it is 1.
Again problem, as every time count variable will initialize so it's 1. This problem can be solved with inner functions by executing outer function only once that is called closure.
var add = (function () {
  var count = 0;
  return function () {count += 1; return count}
})();

add();
add();
add();

// the count is now 3
So closure is a function that has access to it's outer scope and can perform operation on variables defined in outer scope.

5. What is the difference between null and undefined>

When you declare a variable but not initialize to any value is treated as undefined. Whereas null is an assignment value to any variable with no value. Type of undefined is undefined but type of null is an object. So undefined is an type whereas null is an object.

6. What is the difference between '==' and '===' operators?

Both are the operators in JavaScript. '==' checks only value whereas '===' checks for both value and type of that value. For example, if you compare number '2' and string "2" then '==' will return true but '===' will return false as it's checking for both value and type.

7. What is event bubbling?

8. What are the disadvantages of using closures?

9. What are memory leaks in JavaScript?

10. How to implement inheritance in JS?

11. What is the difference between classical and prototypical inheritance?

12. How to implement polyfill for .bind function in JS?

13. What is hoisting in JavaScript?

14. What is the event loop in JS?

15. What is the use of strict in JavaScript?

16. What is JavaScript typecasting?

17. Explain this, (call, apply, bind) in JS.

18. Explain Spread syntax in JavaScript.

19. How to modify the URL without reloading the page?

20. How to execute JavaScript code after page load?

Some General Interview Questions for JavaScript:

1. How much will you rate your self in JavaScript?

When you attend an interview, Interviewer may ask you to rate your self in specific Technology like JavaScript, So It's depend on your knowledge and work experience in JavaScript.

2. What challenges did you face while working on JavaScript?

This question may be specific to your technology and completely depends on your past work experience. So you need to just explain the challenges you faced related to JavaScript in your Project.

3. What was your role in last Project related to JavaScript?

It's based on your role and responsibilities assigned to you and what functionality you implemented using JavaScript in your project. This question is generally asked in every interview.

4. How much experience do you have in JavaScript?

Here you can tell about your overall work experience on JavaScript.

5. Have you done any JavaScript Certification or Training?

It's depend on candidate like you have done any JavaScript training or certification. Certifications or trainings are not essential but good to have.

Conclusion:

We have covered some frequently asked JavaScript Interview Questions and Answers to help you for your Interview. All these Essential JavaScript Interview Questions are targeted for mid level of experienced Professionals and freshers.
While attending any JavaScript Interview if you face any difficulty to answer any question please write to us at [email protected]. Our IT Expert team will find the best answer and will update on portal. In case if we find any new JavaScript questions, we will update the same here.