Top 27 JavaScript Interview Questions for 2020

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1) Define Undeclared and Undefined Variables?

Undeclared variable are defined as these variable is neither defined nor exist in the program.

If program calls this variable then it causes Runtime error.

Undefined variable are those which are declared but not define i.e. No values is assigned to it.

If program calls undefined variable then it returns an undefined number.

2) What does Prompt Box do?

Prompt Box is similar to the alert box but it prompts i.e. label and box is provided to user for input.

3) Diffrentiate between Session State and View State?

In session state for an user specific data can be accessed for all the pages of web application whereas View state is only for a page in a session.

4) What is the difference between ‘==’ and ‘===’ operator?

EqualsTo operator(==) only checks the values of a variable is same or not.

Strictly EqualsTo operator(===) it not only checks value of variable but also datatype same or not.

eg. 5==5 o/p True

‘a’===”a” o/p False

5) Describe parseInt function?

ParseInt() Function is designed for type conversion i.e. convert number between different base.

eg. parseInt (“F”,16) //here F of base 16 converted to int.

6) How is void(0) differ from void()?

Void(0) does not allow the page to refresh and ‘0’ parameter is passed also if another method is called then page is not refreshed.

7) How can you group data type in JavaScript?

Datatype is grouped into 2 groups

  1. Primitive Data type
  2. Refrence Data type

8) Describe Blur Function.

Blur function is used to remove the focus from particular object.

9) Enlist the type of error in JavaScript?

There are 3 types of error

  1. Load time errors: when web page is loaded then due to wrong syntax etc error arises known as load time error.
  2. Run time errors: if the command in the html is not used properly then run time error.
  3. Logical errors: these are user genrated error by applieng wrong logic is an logical error.

10) Define push method in JavaScript.

Push method is implemented in the end of array. It allows to add one or more elements. We can also add multiple element by passing multiple argument.

11) How Callback function used in JavaScript?

Callback function is a JavaScript function which is passed through some method with some argument. It is called only when certain state is triggered.

12) How Exception Handling works in JavaScript?

Exception Handling is implemented by using Try,Catch,Finally also Throws allow exception to handle by JavaScript.

13) What is the use of isNaN function?

isNaN function is used to check if the argument is not a number then returns true else vice versa.

14) What is the drawback of creating true private methods in JavaScript?

The drawbacks of creating true private methods in JavaScript is that they are very memory-inefficient. As a new copy of the method would be created for each instance.

15) What is a “closure” in JavaScript?

A closure is a function defined inside the parent function. It has access to variables that are declared and defined in the parent function scope.

16) What is the scope of closure function in JavaScript?

The closure function has access to variables in three scopes:

  1. Variables declared in their own scope.
  2. Variables declared in a parent function scope.
  3. Variables declared in the global namespace.

17) Enlist the types of JavaScript statement.

18) Explain JavaScript Object?

Objects are the data types of JavaScript. Each variable which is created is an object. Object has its specific methods and properties which can be accessed when working with that type of data.

19) Enlist some standard JavaScript Objects.

JavaScript objects are as follows:

20) What is screen object in JavaScript?

Screen object helps in getting the information of the user’s screen. We can get the width, height, colorDepth etc. of the user’s screen.

21) Is JavaScript case sensitive?

Yes, JavaScript is a case sensitive scripting language. Variable names, keywords, methods, event handlers are all case sensitive.

22) What is Anonymous Functions?

A function without a name is an anonymous function. It is stored inside a variable name. Thus, the invocation happens , using the variable name.

23) Explain Generic Function in JavaScript.

A generic function in JavaScript is used to check whether the specified string is null or empty. It is similar to “String.IsNullOrEmpty” of C# language.

24) What is InnerHTML Property in JavaScript?

InnerHTML property can be used to modify an HTML document. It is used to change the page’s content without refreshing the page. It makes website feel quicker and more responsive to user input.

25) What are Cookies in JavaScript?

Cookies are the variables stored on the client’s computer. When the client’s Browser requests a page, the cookies are obtained too. JavaScript can be used to create and retrieve the cookie values.

26) Enlist main component of Cookies.

  1. Expire: The date on which the cookie will expire. If this is blank, the cookie will expire, when the visitor quits the Browser.
  2. Domain: The domain is the name of our Website.
  3. Path: The path to the directory or the Web page, that sets the cookie.
  4. Secure: If this field contains the word “secure”, the cookie may only be retrieved with a secure Server. If this field is blank, no such restriction exists.
  5. Storing Cookies: The simplest way to create a cookie is to assign a string value to the “document.cookie” object.

26) Enlist the Popup boxes in JavaScript.

There are the following types of Popup Boxes in JavScript, which are:

27) Explain Concept of chaining in JavaScript.

In this technique we write the series of statements, one after the other like a chain. The Chaining Method is also known as Cascading, because it repeatedly calls one method on an object, forming a chain/continuous line of the code.

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