JavaScript Interview Questions

JavaScript is a high-level, interpreted scripting language used to create dynamic and interactive web applications.

JavaScript is interpreted, but modern engines use Just-In-Time (JIT) compilation for performance.

Primitive: string, number, boolean, null, undefined, symbol, bigint. Non-primitive: object, array, function.

var is function-scoped, let and const are block-scoped; const cannot be reassigned.

Hoisting moves variable and function declarations to the top of their scope during compilation.

A closure allows a function to access variables from its outer scope even after the outer function has finished.

Scope defines the accessibility of variables (global, function, block).

this refers to the object that is executing the current function.

A shorter syntax for functions that does not bind its own this.

Events propagate from the target element up to parent elements.

Events propagate from parent elements down to the target.

Handling events using a parent element instead of individual child elements.

Document Object Model represents the HTML structure as objects.

Browser Object Model provides access to browser features like window and navigator.

A function passed as an argument and executed later.

An object representing the eventual completion or failure of an asynchronous operation.

A cleaner syntax for handling promises and asynchronous code.

Synchronous blocks execution; asynchronous allows non-blocking execution.

Executes code after a specified delay.

Executes code repeatedly at fixed time intervals.

A lightweight data-exchange format using key-value pairs.

Stores data in the browser with no expiration.

Stores data for a single browser session.

Small data stored in the browser and sent with HTTP requests.

== compares values; === compares value and type.

Represents “Not a Number”.

Mechanism through which JavaScript objects inherit properties.

Series of linked prototypes used for property lookup.

Extracting values from arrays or objects into variables.

Expands iterable elements into individual values.

Collects remaining elements into an array.

Creates a new array by applying a function to each element.

Creates a new array with elements that pass a condition.

Reduces an array to a single value.

Immediately Invoked Function Expression executes immediately after definition.

Enforces stricter parsing and error handling in JavaScript.

Limits function execution until after a delay.

Ensures function runs at most once in a given time period.

JavaScript executes one task at a time but handles async via event loop.

Handles asynchronous callbacks and task execution.

Tracks function execution order.

Unused memory not released by the program.

Copies references, not nested objects.

Creates completely independent copy of objects.

Reusable piece of code exported and imported across files.

Modern JavaScript version introducing classes, arrow functions, let/const.

JavaScript runtime built on Chrome’s V8 engine for server-side development.

Node Package Manager used to manage JavaScript packages.

Framework controls flow; library is used when needed.

TypeScript adds static typing and better tooling to JavaScript.