JavaScript Interview Questions and Answers

Getting ready for a JS Interview? No matter whether you are a fresh graduate or an experienced developer looking for the Top 15 JavaScript interview questions, this guide will come in handy to brush up on the essentials of JavaScript interviews.

From JavaScript basic interview questions and Answers like data types, to the bread and butter of JavaScript coding interviews, such as hoisting and promises, we’ve got you covered.

Q.1) Define JavaScript?

Ans. JavaScript is a high-level interpreted and dynamic programming language. It is best known as the script language for Web pages, but it is also used in many non-browser environments. It is a multi-paradigm language, i.e., object-oriented, imperative, and declarative (e.g, functional programming) are all supported.

Q.2) What do you know about scripting language?

Ans. A scripting language is a kind of programming language whose programs are usually not compiled but interpreted at runtime. These are languages that “script,” or automate a set of operations on a given runtime (for example, your web browser).

Q.3) What are the various types of data in JavaScript?

Ans. JavaScript has two general categories of data types: 1- Primitives. 2- Objects.

String: Textual data.

Number: Integers and floats.

BigInt: Large integers.

Boolean: true or false.

Undefined: A variable that has been declared but not initialised.

Null: Intentional absence of value.

Symbol: Unique and immutable identifiers.

Object: Compound values (such as Arrays and Functions).

Q4. What is Hoisting in JavaScript?

Ans. Hoisting is JavaScript's way of still allowing you to reference an identifier even if you have not declared it yet by moving the declaration to the top of its current scope.

Variables: The var is hoisted , not the value. var gets hoisted as undefined, let and const get hoisted , but they remain in what is called the Temporal Dead Zone.

Functions: When you declare a function, it is hoisted entirely, which means that even if you call the function before your code, you can do so without issues.

Q.5) What is the type of JavaScript? Is it a statically typed or a dynamically typed language?

Ans. JavaScript is a weakly typed language. What this means is that you can leave off the specific type of a variable (like int or string). Its type is inferred at runtime from the value given for it.

Q6) What is IIFE?

Ans. An IIFE is a function that is executed immediately after it is created. One of the common uses is to avoid cluttering up your global namespace. Syntax:

(function() {

console. log("I run immediately!" );

})();

Q7) What is the "this” keyword in JavaScript?

Ans. The this keyword refers to the method in which a function is executed.

In one way, this is the owner object.

In the standalone form or when used in a function, this is the Global object (or undefined for strict mode).

This will be lexically bound from the outer scope (in an arrow function).

Q.8) What are call(), apply() and bind() functions?

Ans. These procedures are applied to the context of this.

call(): Calls a function with a given this value, and arguments provided individually.

apply(): Where call()’s arguments are listed explicitly, those of apply() should be in an array.

bind(): Doesn't call the function right away. Instead, it creates a new function with this bound to the passed-in object.

Q.9) What is an explanation of the scope and the scope chain?

Ans. Scope is what determines who can see a piece of data.

Global Scope: Visible everywhere.

Function Scope: Available only within the function.

Only in the block (let/const) Block scope: inside {}. Scope Chain: If a variable cannot be found in the immediate scope, JavaScript will continue to look "out" into the encapsulating environment until it hits the global scope.

Q.10) What is the DOM?

Ans. The DOM is a standard for interacting with HTML or XML documents. It depicts the page as a tree (nodes and objects) that can be programmed to modify the document structure, style, and content.

Q.11) What are Arrow Functions?

Ans. Arrow Functions are included in ES6 and provide a more concise syntax. They do not have their own this, arguments, or super bindings, unlike regular functions.

const add = (a, b) => a + b;

Q.12) What is Promises in JavaScript used for?

Ans. Promises handle asynchronous operations. They stand for variable values that can be had now, later, or not at all. A Promise has four states:

Pending: Initial state.

Fulfilled: Operation completed successfully.

Rejected: Operation failed.

Settled: Either fulfilled or rejected.

Q.13) Explain WeakMap in JavaScript.

Ans. A WeakMap provides a map from key to value, which requires keys be objects.

Garbage Collection: If you don’t have any other reference to the key object, it can be garbage collected even if we still hold a reference in WeakMap.

Not enumerable: You can't iterate over a WeakMap.

Q.14) What is Object Destructuring?

Ans. Destructuring is when you assign values of an object to variables with their keys in the body of a JSON object based on ES6.

const user = { name: 'Alex', age: 25 };

const { name, age } = user;

Q.15) Explain the Rest Parameter and Spread Operator?

Ans. Both will use the ... syntax but for opposite reasons:

Rest Parameter: Allows us to gather the remaining parts of arguments and place them into an array.

Spread Operator: Spreads the array (or object) into a series of elements in function calls or literals.

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