Question 1

What is a closure?

Accepted Answer

A closure is a function that retains access to variables from its outer (enclosing) scope, even after the outer function has finished executing. Closures are created every time a function is created in JavaScript and are commonly used for data privacy and creating factory functions.

Question 2

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

Accepted Answer

== (loose equality) performs type coercion before comparison, converting operands to the same type. === (strict equality) compares both value and type without conversion. Always prefer === to avoid unexpected type coercion bugs.

Question 3

What is the Event Loop?

Accepted Answer

The Event Loop is JavaScript's mechanism for handling asynchronous operations. It continuously monitors the call stack and callback queue, pushing queued callbacks to the stack when it's empty. This enables non-blocking behavior in single-threaded JavaScript.

Question 4

What is the Difference Between let and var?

Accepted Answer

var is function-scoped and hoisted to the top of its scope, while let is block-scoped and exists in a temporal dead zone until declared. let prevents accidental redeclaration and is preferred in modern JavaScript.

Question 5

What is hoisting?

Accepted Answer

Hoisting is JavaScript's behavior of moving variable and function declarations to the top of their scope during compilation. Function declarations are fully hoisted, var declarations are hoisted but not their values, and let/const are hoisted but remain in a temporal dead zone.

Question 6

What is a Promise?

Accepted Answer

A Promise is an object representing the eventual completion or failure of an asynchronous operation. It can be in one of three states: pending, fulfilled, or rejected. Promises provide cleaner async code than callbacks through .then(), .catch(), and .finally() methods.

Question 7

What is the difference between null and undefined?

Accepted Answer

undefined means a variable has been declared but not assigned a value - it's the default state. null is an intentional assignment indicating 'no value' or 'empty'. typeof undefined returns 'undefined', while typeof null returns 'object' (a known JavaScript quirk).

Question 8

What is prototypal inheritance?

Accepted Answer

Prototypal inheritance is JavaScript's mechanism where objects can inherit properties and methods from other objects through the prototype chain. Every object has an internal link to a prototype object, from which it can access inherited properties.

Question 9

What is async/await?

Accepted Answer

async/await is syntactic sugar over Promises that makes asynchronous code look synchronous. An async function always returns a Promise, and await pauses execution until the Promise resolves. It simplifies error handling with try/catch blocks.

Question 10

What is Event Bubbling?

Accepted Answer

Event bubbling is the propagation pattern where an event triggered on a nested element 'bubbles up' through its ancestors in the DOM tree. When you click a button inside a div, the click event fires on the button first, then the div, then each parent element up to the document.

Question 11

What is the difference between call() and apply()?

Accepted Answer

Both methods invoke a function with a specified 'this' context. The difference is in how arguments are passed: call() accepts arguments individually (fn.call(context, arg1, arg2)), while apply() accepts them as an array (fn.apply(context, [arg1, arg2])).

Question 12

What is scope?

Accepted Answer

Scope determines the accessibility of variables in JavaScript. There are three types: global scope (accessible everywhere), function scope (accessible within a function), and block scope (accessible within curly braces, for let/const). Scope controls variable lifetime and prevents naming conflicts.

Question 13

What are Higher-Order Functions?

Accepted Answer

Higher-order functions are functions that either take other functions as arguments or return functions as their result. Common examples include map(), filter(), reduce(), and forEach(). They enable functional programming patterns and code reusability.

Question 14

What is the Spread Operator?

Accepted Answer

The spread operator (...) expands an iterable (array, string, object) into individual elements. It's used for copying arrays ([...arr]), merging objects ({...obj1, ...obj2}), and passing array elements as function arguments (fn(...args)).

Question 15

What is CORS?

Accepted Answer

CORS (Cross-Origin Resource Sharing) is a security mechanism that allows or restricts web pages from making requests to a different domain than the one serving the page. Servers must include specific headers (Access-Control-Allow-Origin) to permit cross-origin requests.

JavaScript API pt. 1(15 questions)

What is the difference between setTimeout() and setInterval()?

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

How to clear an array of objects in JavaScript

What is the Difference Between let and var?

What are the Data Types in JavaScript?

How to Create an Object in JavaScript?

What are the Falsy Values in JavaScript?

What Distinguishes null from undefined?

What is the Spread Operator?

What is the Rest Operator?

What Does NaN Mean?

How to Create a Valid URL?

What Distinguishes call() from apply()?

What is the Purpose of the bind() Function?

What is the Difference Between slice() and splice()?

Definitions pt. 1(15 questions)

What is scope?

What types of scope do you know?

What is prototypal inheritance?

How does hoisting work?

What is a closure?

What is an IIFE?

How do you change the context of a function call?

What is the purpose of use strict;?

Object.freeze() vs Object.seal()

What is the Event Loop?

What are Persistent Data Structures?

How to Create a Namespace?

How to Debug JavaScript Code?

How to Prevent an Object from Being Extended?

What Is the Difference Between Synchronous and Asynchronous Functions?

Functions pt. 1(5 questions)

How Do the map(), filter(), and reduce() Functions Work?

What Is the Difference Between forEach() and map()?

What Are First-Class Functions?

What Are Higher-Order Functions?

What Is a Pure Function?

Browser API(10 questions)

What is Event Bubbling?

What is Event Capturing?

What is Event Delegation?

What is the purpose of the history object?

What is the difference between an attribute and a property?

What is the purpose of web storage?

How do you use web storage?

Is it possible to read localStorage from a different domain?

What distinguishes cookies, sessionStorage, and localStorage?

What methods are available in sessionStorage?

Asynchronous Code(5 questions)

What is a Promise?

What is a callback?

What is callback hell?

What are the benefits of using a Promise?

What is async/await?

JavaScript API pt. 2(10 questions)

How Can We Create an Array in JavaScript?

How to Clone an Object in JavaScript?

What Is JSON and How Do We Handle It?

How Do We Read Data Stored in JSON?

What Is the Effect of Calling the setTimeout Function with a 0 Delay?

Why Do We Use Object.seal()?

Why Do We Use Object.freeze()?

What is the difference between Object.values and Object.entries?

What is the difference between for...in and for...of?

How to check if an object has a property?

Definitions pt. 2(10 questions)

What Are the Benefits of Using Modules?

What Is a Call Stack?

What Is the Memory Heap?

What Is a Polyfill?

What Is Tree Shaking?

What Is the Prototype Chain?

What Is the Difference Between __proto__ and prototype?

What Are ES6 Classes?

What Is Obfuscation?

What is Minification?

Functions pt. 2(5 questions)

What is memoization?

What is currying?

What is the difference between `setTimeout()` and `setInterval()`?

What is the difference between `==` and `===`?

What is the Difference Between `let` and `var`?

What Distinguishes `null` from `undefined`?

What Does `NaN` Mean?

What Distinguishes `call()` from `apply()`?

What is the Purpose of the `bind()` Function?

What is the Difference Between `slice()` and `splice()`?

What is the purpose of `use strict;`?

How Do the `map()`, `filter()`, and `reduce()` Functions Work?

What Is the Difference Between `forEach()` and `map()`?

What Is the Effect of Calling the `setTimeout` Function with a 0 Delay?

Why Do We Use `Object.seal()`?

Why Do We Use `Object.freeze()`?

What is the difference between `Object.values` and `Object.entries`?

What is the difference between `for...in` and `for...of`?

What Is the Difference Between `proto` and `prototype`?

What Is the Purpose of `preventDefault()`?

What Is the Purpose of `stopPropagation()`?

What Is the Difference Between `load` and `DOMContentLoaded`?

What Is the `void` Operator?

What is the difference between `window` and `document`?

What is the purpose of `Promise.all()`?

What is the purpose of `Promise.race()`?

How can you cancel a `Promise()`?

What is the difference between `<script async>` and `<script defer>`?

Loading Fonts via `<link>` vs. `@import`

What is a `WeakSet`?

How does a `WeakSet` differ from a `Set`?

What is a `WeakMap`?