Master JavaScript Interview
Questions & Concepts

JavaScript is a lightweight, interpreted, high-level programming language used to create dynamic and interactive behavior on websites and web applications.

JavaScript has eight data types: String, Number, BigInt, Boolean, Undefined, Null, Symbol, and Object (which includes Arrays and Functions).

var is function-scoped and hoisted; let is block-scoped and can be reassigned; const is block-scoped and cannot be reassigned after declaration.

Hoisting is JavaScript's behavior of moving variable and function declarations to the top of their scope before code execution.

== checks for value equality with type coercion, while === checks for both value and type equality without coercion (strict equality).

Undefined is the default value of a variable that has been declared but not yet assigned a value.

Null is an intentional assignment representing the absence of a value or object, explicitly set by the developer.

A function is a reusable block of code designed to perform a specific task, defined using the function keyword or arrow function syntax.

An arrow function is a concise ES6 syntax for writing functions using =>, which also lexically binds the this keyword from the surrounding scope.

Template literals are string literals enclosed in backticks (`) that support multi-line strings and embedded expressions using ${} syntax.

An array is an ordered collection of values stored in a single variable, accessible via zero-based index positions.

An object is a collection of key-value pairs where keys are strings (or Symbols) and values can be any data type.

The typeof operator returns a string indicating the data type of a given value or expression.

NaN (Not a Number) is a value returned when a mathematical operation fails to produce a valid number result.

Type coercion is the automatic conversion of a value from one data type to another during operations or comparisons.

Falsy values are false, 0, "", null, undefined, and NaN. All other values are truthy and evaluate to true in boolean contexts.

The DOM (Document Object Model) is a programming interface that represents an HTML document as a tree of objects that JavaScript can manipulate.

An event listener is a function attached to an element that waits for a specific event (like click or keypress) and executes when it occurs.

innerHTML sets or gets HTML markup including tags, while textContent sets or gets only the plain text content without parsing HTML.

A loop repeatedly executes a block of code as long as a condition is true, with common types being for, while, do...while, for...of, and for...in.

A closure is a function that retains access to its outer scope variables even after the outer function has finished executing.

Scope determines the visibility and accessibility of variables. JavaScript has global scope, function scope, and block scope (with let and const).

this refers to the object that is currently executing the function. Its value depends on how and where the function is called.

A Promise is an object representing the eventual completion or failure of an asynchronous operation, with states: pending, fulfilled, or rejected.

Async/Await is syntactic sugar over Promises that allows writing asynchronous code in a synchronous-looking style for better readability.

The Event Loop is a mechanism that continuously checks the call stack and callback queue, pushing queued callbacks to the stack when it is empty.

Destructuring is an ES6 syntax that extracts values from arrays or properties from objects into distinct variables in a single statement.

The spread operator (...) expands an iterable (like an array or object) into individual elements, useful for copying or merging data.

The rest parameter (...) collects all remaining function arguments into a single array, allowing functions to accept an indefinite number of arguments.

A higher-order function is a function that takes another function as an argument or returns a function as its result, such as map, filter, and reduce.

map transforms each array element; filter returns elements matching a condition; reduce accumulates array elements into a single output value.

Prototypal inheritance allows objects to inherit properties and methods from other objects through the prototype chain.

The prototype chain is a series of linked objects where JavaScript looks up properties and methods if they are not found on the current object.

Event bubbling is when an event triggered on a child element propagates upward through its parent elements in the DOM tree.

Event delegation attaches a single event listener to a parent element to handle events from its child elements using bubbling.

A callback is a function passed as an argument to another function, executed after that function completes its operation.

Callback hell is deeply nested callbacks that make code difficult to read and maintain, solved by using Promises or Async/Await.

ES6 modules allow splitting JavaScript code into reusable files using import and export statements for better organization and maintainability.

call invokes a function with a given this and arguments individually; apply uses an array of arguments; bind returns a new function with a bound this.

localStorage stores data with no expiration across sessions; sessionStorage stores data only for the duration of the browser tab session.