Top 20+ JavaScript Interview Questions

JavaScript is a client-side as well as server side scripting language; And it was created by Brendan Eich in the year of 1995.

Basically, the JavaScript is an object-based, lightweight, and cross-platform translated language and it is responsible for translating the JavaScript code for the web browser. JavaScript is completely different language from Java language.

javascript interview question
JavaScript Interview Questions

JavaScript interview questions have been designed specially to get you to make familiar with the nature of questions you may experience during your interview for the subject of JavaScript. In this section, we are listed basically three levels of JavaScript interview questions:

  • Beginner Interview Questions
  • Intermediate Interview Questions
  • Advanced Interview Questions

Q1- Which company developed JavaScript and When?

"Netscape" software company is developed JavaScript in "1995".

Q2- Is JavaScript case-sensitive language?

YES! JavaScript is a case-sensitive language. It means that language keywords, variables, function names, and any other identifiers must always be typed with a consistent capitalization of letters in JavaScript language.

Q3- What is the difference between Java & JavaScript?

Java Programming Language:

  • Java is a complete Object-Oriented Programming (OOPS) language.
  • Java creates applications that run in a virtual machine or browser.
  • Java code needs to be compiled before running it.

JavaScript Language:

  • JavaScript is an Oops scripting language that can be introduced to HTML pages.
  • JavaScript code is run on a browser only.
  • JavaScript codes are all in the form of text.

Q4- What is the name of JavaScript data types?

There are following data types supported in JavaScript:

  • Number
  • String
  • Boolean
  • Object
  • Undefined
  • Symbol
  • Null

Q5- How can read & write the properties of an object in JavaScript?

By using the dot(.) notation you can read and write the properties of an object in JavaScript.

Q6- What are the advantages of using JavaScript?

  • Less Server Interaction: The Less Server Interaction is used to save server traffic that means validate user input before sending the page off to the server.
  • Immediate Feedback tothe Visitors: They don't have to wait for a page reload to see if they have forgotten to enter something.
  • Increased Interactivity: Increase interactivity means that create interfaces that react when the user hovers over them with a mouse or activates them via the keyboard.
  • Richer Interfaces: Use JavaScript to include such items as drag-and-drop components and sliders to give a Rich Interface to your site visitors.

Q7- What are the disadvantages of using JavaScript?

  • JavaScript is not supporting for multithreading and multiprocessing.
  • In JavaScript, Reading and writing of files is not allowed.
  • No support for networking applications in JavaScript.

Q8- How to write simple example of JavaScript?

The most important thing write all JavaScript code between the open <script> and close </script> tags:

Q9- What is a prompt box?

A prompt box in JavaScript allows the user to enter input by providing a text box.

Q10- How to write a comment in JavaScript?

The comment allows to descript about code OR instruct about code BUT the comment is not visible OR display when the code run on the browser it means that if the user wants to hide some codes on the browser then use comment.

Types of JavaScript Comments

There are two types of comments in JavaScrip:

  • Single-Line Comment: It is started by double forward slash "//".
  • Multi-Line Comment: It is started with asterisk symbol like /* write comment here */.

Q11- What is the purpose of 'this' keyword?

In JavaScrip, 'this' keyword is used to always refers to the current context.

Q12- What is closure in JavaScript

The closure is created in JavaScript when a variable which is defined outside the scope in reference is accessed from some inner scope.

For Example:

Q13- What is variable typing in JavaScript?

The variable typing is used to assign any characteristics and same variable use for a number, or quantity to a variable, its called variable typing in JavaScript.

For Example:

Q14- What are the types of Functions in JavaScript?

In JavaScript, There are two types of Function:

  • Named
  • Anonymous

Q15- What is the difference between 'var' and 'let' keywords in JS?

var: The 'var' is used to declared a variable in javascript; the difference 'var' has function scope.

let: The 'let' is also used to declared a variable in javascript; It has block scope.

Q16- What is the difference between defined and undefined variable in JavaScript?

Defined Variable

Use a variable that exist and it has been declared with a value, then JavaScript will return the value of the variable. The declaration of variable like typeof undeclared_variable = value of the variable, this will return defined variable:

For Example:

Undefined Variable

Use a variable that doesn't exist and has not been declared, then JavaScript will throw an error var name is not defined and script will stop executing. Declaration like typeof undeclared_variable, then it will return undefined variable:

For Example:

Q17- What are the phases of Event Propagation?

The Event Propagation has divided into three phases:

  • Capturing Phase
  • Target Phase
  • Bubbling Phase

Q18- What's the difference between == and === ?

== OR Abstract Equality: The abstract equality is used to compare by value after coercion.

=== OR Strict Equality: The strict equality is used to compare by value and type without coercion.

Q19- How to empty an array in JavaScript?

1st needs to create an empty array (arrayList1 =[], arrayList2 = []); the name of an arrayList1 & arrayList2; after that declaration the value of arrayList1 for more understanding see the below example:

Q20- What is the Scope in JavaScript?

The Scope is the area where you are able to declare variables OR able to write functions, and valid to access all it; this is called Scope in JavaScrip. Basically, JavaScript has three types of Scopes:

  1. Global Scope
  2. Function Scope
  3. Block Scope

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