JavaScript Interview Questions and Answers

Prepare for your next interview with these Top 15 JavaScript Interview Questions and Answers covering key concepts, best practices, and coding examples.

 

Q.1) What is JavaScript definition?

Ans. JavaScript is a dynamic computer programming language. It is lightweight and most commonly used as a part of web pages, whose implementations allow client-side scripts to interact with the user and make dynamic pages. It is an interpreted programming language with object-oriented capabilities.

 

Q.2) What do you mean by scripting language?

Ans. They are usually short, fast, and interpreted from source code or bytecode. A scripting language is a programming language that executes tasks within a special run-time environment by an interpreter instead of a compiler. 

 

Q.3) What are the different data types present in javascript?

Ans.1) Boolean=>true/false

        2) String

       3) Null

     4)Number

     5) Undefined.

 

Q.4) Explain the concept of hoisting in JavaScript.

Ans. Hoisting is a default behavior of javascript where all the variable and function declarations are moved on top. This means that irrespective of where the variables and functions are declared, they are moved on top of the scope. The scope can be both local and global.

 

Q.5) Is JavaScript a statically typed or a dynamically typed language?

Ans.JavaScript is a dynamically typed language. In a dynamically typed language, the type of a variable is checked during run-time in contrast to statically typed language.

 

Q.6) What is an Immediately Invoked Function in JavaScript and what is its syntax?

Ans. An Immediately Invoked Function ( known as IIFE and pronounced as IIFY) is a function that runs as soon as it is defined.

Syntax of IIFE :

(function(){ 

  // Do something;

})();

 

Q.7) Explain the “this” keyword in javascript.

Ans. The “this” keyword refers to the object that the function is a property of. The value of “this” keyword will always depend on the object that is invoking the function.

 

Q.8) Explain call(), apply() and, bind() methods.

Ans.

call()

It’s a predefined method in JavaScript.

This method invokes a method (function) by specifying the owner object.

call() method allows an object to use the method (function) of another object.

call() accepts arguments.

apply():

The apply method is similar to the call() method. The only difference is that

call() method takes arguments separately whereas, the apply() method takes arguments as an array.

bind():

This method returns a new function, where the value of the “this” keyword will be bound to the owner object, which is provided as a parameter.

 

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Q.9) . Explain the Scope and Scope Chain in JavaScript.

Ans. Scope in JS determines the accessibility of variables and functions at various parts of one’s code. In general terms, the scope will let us know at a given part of code, what are the variables and functions that we can or cannot access.

There are three types of scopes in JS:

• Global Scope

• Local or Function Scope

• Block Scope

Scope Chain:

 if the javascript engine does not find the variable in the local scope, it tries to check for the variable in the outer scope. If the variable does not exist in the outer scope, it tries to find the variable in the global scope.

 

Q/10) What is DOM?

Ans.DOM stands for Document Object Model.

DOM is a programming interface for HTML and XML documents. When the browser tries to render an HTML document, it creates an object based on the HTML document called DOM. Using this DOM, we can manipulate or change various elements inside the HTML document.

 

Q.11) What are arrow functions?

Ans. Arrow functions were introduced in the ES6 version of JavaScript.

They provide us with a new and shorter syntax for declaring functions.

Arrow functions can only be used as a function expression.

 

Q.12) What is the use of promises in javascript?

Ans. Promises are used to handle asynchronous operations in javascript.

Promise object has four states –

Pending – Initial state of promise. This state represents that the promise has neither been fulfilled nor been rejected, it is in the pending state.

Fulfilled – This state represents that the promise has been fulfilled, meaning the async operation is completed.

Rejected – This state represents that the promise has been rejected for some reason, meaning the async operation has failed.

Settled – This state represents that the promise has been either rejected or fulfilled.

A promise is created using the Promise constructor which takes in a callback function with two parameters, resolve and reject respectively.

 

Q.13) Explain WeakMap in JavaScript.

Ans. In JavaScript, Map is used to store key-value pairs. The key-value pairs can be of both primitive and non-primitive types.WeakMap is similar to Map with key differences:

The keys and values in weakmap should always be an object.

If there are no references to the object, the object will be garbage collected.

 

Q.14) What is Object restructuring?

Ans. Object destructuring is a new way to extract elements from an object or an array.

 

Q.15) What is the rest parameter and spread operator?

Ans. Rest parameter ( … )

It provides an improved way of handling the parameters of a function.

Using the rest parameter syntax, we can create functions that can take a variable number of arguments. Any number of arguments will be converted into an array using the rest parameter.

It also helps in extracting all or some parts of the arguments.

Rest parameters can be used by applying three dots (…) before the parameters.

 

Spread operator (…)

Although the syntax of the spread operator is exactly the same as the rest parameter, the spread operator is used to spread an array and object literals. We also use spread operators where one or more arguments are expected in a function call.

 

Key differences between the rest parameter and spread operator:

The rest parameter is used to take a variable number of arguments and turn them into an array while the spread operator takes an array or an object and spreads it

The rest parameter is used in function declaration whereas the spread operator is used in function calls.

 

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