JavaScript Interview Questions and Answers


What is JavaScript?
JavaScript is a lightweight, interpreted programming language primarily used to create interactive and dynamic web pages. It's part of the core technologies of the web, along with HTML and CSS.


What are the data types in JavaScript? JavaScript has the following data types:
Primitive: String, Number, Boolean, Null, Undefined, Symbol, BigInt
Non-primitive: Object, Array, Function


What is the difference between null and undefined?
null is an assigned value representing no value.
undefined means a variable has been declared but not assigned a value.


Explain the concept of hoisting in JavaScript.
Hoisting is JavaScript's default behavior of moving declarations to the top of the scope before code execution. var declarations are hoisted and initialized as undefined; let and const are hoisted but not initialized.


What is a closure in JavaScript?
A closure is a function that retains access to its lexical scope, even when the function is executed outside of that scope.


What is the difference between “==” and “===” operators in JavaScript?

== checks for value equality (performs type coercion)
=== checks for value and type equality (strict equality.


Explain the concept of prototypal inheritance in JavaScript.
Objects in JavaScript can inherit properties from other objects using the prototype chain. Every object has an internal link to another object called its prototype.


What are the different ways to define a function in JavaScript?
Function declaration: function greet() {}
Function expression: const greet = function() {}
Arrow function: const greet = () => {}


How does event delegation work in JavaScript?
Event delegation uses event bubbling by attaching a single event listener to a parent element that handles events triggered by its children.


What is the purpose of the “this” keyword in JavaScript?
this refers to the object that is executing the current function. Its value depends on how the function is called.


What are the different ways to create objects in JavaScript?
Object literals: const obj = {}
Constructor functions
Object.create()
Classes


Explain the concept of callback functions in JavaScript.
A callback is a function passed as an argument to another function and executed after some operation is completed.


What is event bubbling and event capturing in JavaScript?
Bubbling: event goes from target to root.
Capturing: event goes from root to target. JavaScript uses bubbling by default.


What is the purpose of the “bind” method in JavaScript?
The bind() method creates a new function with a specified this context and optional arguments.


Explain the concept of AJAX in JavaScript.
AJAX (Asynchronous JavaScript and XML) allows web pages to be updated asynchronously by exchanging data with a server behind the scenes.


What is the “typeof” operator used for? The typeof operator returns a string indicating the type of a given operand.


How does JavaScript handle errors and exceptions?
Using try...catch...finally blocks. Errors can also be thrown manually using throw.


Explain the concept of event-driven programming in JavaScript. Event-driven programming is a paradigm where the flow is determined by events such as user actions, sensor outputs, or messages.


What is the purpose of the “async” and “await” keywords in JavaScript?
They simplify working with promises, allowing asynchronous code to be written like synchronous code.


What is the difference between a deep copy and a shallow copy in JavaScript?
Shallow copy copies top-level properties.
Deep copy duplicates all nested levels.


How does JavaScript handle memory management?
JavaScript uses garbage collection to manage memory. It frees memory that is no longer referenced.


Explain the concept of event loop in JavaScript.

The event loop handles asynchronous operations. It takes tasks from the queue and pushes them to the call stack when it is empty.