Top 18 JavaScript Interview Questions

Here you can see top 18 JavaScript Interview Questions and answers. read each JavaScript Interview Questions and make prepare for interview. most of the JavaScript Interview based on the this type of questions.

1) What is JavaScript ?

Answer : JavaScript is a high-level, interpreted programming language used to create interactive web pages and applications.

2) What are the different data types in JavaScript?

Primitive Data Types : Number, String Boolean, Undefined, Null and Symbol.

Non-Primitive Data Types : Object, Array, Function, Date, Map and Set.

3) What is hoisting in JavaScript?

Answer : Hoisting is a JavaScript mechanism where variables and function declarations are moved to the top of their scope before code execution.

4) What is the difference between null and undefined?

Answer : Null is a assignment value that represents no value, while undefined means a variable has been declared but not assigned a value.

5) What is a callback function?

Answer : A callback function is a function that passed into another function as an argument, which is invoked inside the outer function to complete some kind of action.

6) What is the purpose of the setTimeout() function?

Answer : setTimeout() is used to execute a function after a specified delay.

7) How can you check if a array includes a certain value?

Answer : Using the includes() method: array.includes(value).

8) What is the purpose of async and await in JavaScript?

Answer : async and await are used to handle asynchronous operations more easily, 

9) Explain the difference between let, var, and const.

Answer : var is a Function-scoped but let and const are block-scoped. const variable cannot be reassigned.

10) What is the DOM in JavaScript?

Answer : The DOM stands for Document Object Model. it is a represent the tree structure of HTML elements.

11) What is the difference between == and ===?

Answer : == compares values after type conversion, while === compares values without type conversion.

12) What is the difference between synchronous and asynchronous code?

Answer : Synchronous code is blocking further execution until the current operation completes. Asynchronous code allows other operations to run while Synchronous waiting for the current one to complete.

13) What is the fetch() API?

Answer : The fetch API provides a modern and powerful way to make network requests and returning promises that resolve to the response of the request.

14) What is the reduce() method in JavaScript?

Answer : The reduce() method executes a reducer function on each element of the array and give result in a single output value.

15) What is the typeof operator?

Answer : The typeof operator in JavaScript is used to determine the data type of a variable. which can be any variable, value, or expression It returns a string indicating the type of operand.

16) What is the new keyword in JavaScript?

Answer : JavaScript new keyword is used to create an instance of an object that has a constructor function.

17) What is the difference between slice() and splice() methods?

Answer : Slice() in JavaScript extracts a section of an array and returns a new array containing the selected elements, without modifying the original array.

Splice() method in JavaScript is used to change the contents of an array by removing or replacing existing elements and/or adding new elements in place, modifying the original array.

18) What is the Promise.all() method?

Answer : Promise.all() method takes array of promises and returns a single promise that resolve when all the promises in the array have resolved.

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