30-seconds-of-interviews

A curated collection of common interview questions to help you prepare for your next interview.

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30secondsofinterviews.org

30 Seconds of Interviews / Contributing
30 Seconds of Code
30 Seconds of CSS 16,111 over 1 year ago
30 Seconds of React 5,080 over 1 year ago
30 Seconds of Knowledge

30 Seconds of Interviews / JavaScript / What is a stateless component?
React docs on State and Lifecycle

30 Seconds of Interviews / JavaScript / What is the difference between the equality operators == and ===?
MDN docs for comparison operators

30 Seconds of Interviews / JavaScript / What is the difference between an element and a component in React?
React docs on Rendering Elements
React docs on Components and Props

30 Seconds of Interviews / JavaScript / What is a stateful component in React?
React docs on State and Lifecycle

30 Seconds of Interviews / JavaScript / In which states can a Promise be?
Official Web Docs Promise

30 Seconds of Interviews / JavaScript / What does the following code evaluate to?
MDN docs for typeof

30 Seconds of Interviews / JavaScript / How do you clone an object in JavaScript?
MDN docs for Object.assign()
Clone an object in vanilla JS

30 Seconds of Interviews / JavaScript / How do you compare two objects in JavaScript?
Object Equality in JavaScript
Deep comparison between two values

30 Seconds of Interviews / JavaScript / What is a cross-site scripting attack (XSS) and how do you prevent it?
Cross-Site Scripting Attack (XSS)

30 Seconds of Interviews / JavaScript / What is CORS?
MDN docs for CORS

30 Seconds of Interviews / JavaScript / What is the DOM?
MDN docs for DOM

30 Seconds of Interviews / JavaScript / What are the differences between var, let, const and no keyword statements?
let vs const

30 Seconds of Interviews / JavaScript / What is event delegation and why is it useful? Can you show an example of how to use it?
Event Delegation

30 Seconds of Interviews / JavaScript / What is the purpose of callback function as an argument of setState?
React docs on setState

30 Seconds of Interviews / JavaScript / What is the difference between an expression and a statement in JavaScript?
What is the difference between a statement and an expression?

30 Seconds of Interviews / JavaScript / What are truthy and falsy values in JavaScript?
Truthy on MDN
Falsy on MDN

30 Seconds of Interviews / JavaScript / Generate an array, containing the Fibonacci sequence, up until the nth term.
Similar problem 121,836 about 18 hours ago

30 Seconds of Interviews / JavaScript / What does 0.1 + 0.2 === 0.3 evaluate to?
A simple helper function to check equality 121,836 about 18 hours ago
Fix "0.1 + 0.2 = 0.300000004" in JavaScript

30 Seconds of Interviews / JavaScript / What is the difference between the array methods map() and forEach()?
MDN docs for forEach
MDN docs for map
JavaScript — Map vs. ForEach

30 Seconds of Interviews / JavaScript / What is short-circuit evaluation in JavaScript?
JavaScript: What is short-circuit evaluation?

30 Seconds of Interviews / JavaScript / What will the console log in this example?
MDN docs for hoisting

30 Seconds of Interviews / JavaScript / How does hoisting work in JavaScript?
MDN docs for hoisting
Understanding Hoisting in JavaScript

30 Seconds of Interviews / JavaScript / What is the difference between HTML and React event handling?
React docs on Handling Events

30 Seconds of Interviews / JavaScript / What is the reason for wrapping the entire contents of a JavaScript source file in a function that is immediately invoked?
MDN docs for closures

30 Seconds of Interviews / JavaScript / What does the following function return?
Automatic semicolon insertion in JavaScript

30 Seconds of Interviews / JavaScript / What are inline conditional expressions?
React docs on Conditional Rendering

30 Seconds of Interviews / JavaScript / What is a key? What are the benefits of using it in lists?
React docs on Lists and Keys

30 Seconds of Interviews / JavaScript / What is the difference between lexical scoping and dynamic scoping?
Mozilla Docs Closures & Lexical Scoping

30 Seconds of Interviews / JavaScript / What is the output of the following code?
JavaScript Value vs Reference

30 Seconds of Interviews / JavaScript / How does prototypal inheritance differ from classical inheritance?
MDN docs for inheritance and the prototype chain

30 Seconds of Interviews / JavaScript / What is a MIME type and what is it used for?
MIME Type MDN

30 Seconds of Interviews / JavaScript / What are Promises?
Master the JavaScript Interview: What is a Promise?

30 Seconds of Interviews / JavaScript / What are JavaScript data types?
MDN docs for data types and data structures
Understanding Data Types in JavaScript

30 Seconds of Interviews / JavaScript / NodeJS often uses a callback pattern where if an error is encountered during execution, this error is passed as the first argument to the callback. What are the advantages of this pattern?
The Node.js Way Understanding Error-First Callbacks
What are the error conventions?

30 Seconds of Interviews / JavaScript / What is a callback? Can you show an example using one?
MDN docs for callbacks

30 Seconds of Interviews / JavaScript / What is the difference between null and undefined?
MDN docs for null
MDN docs for undefined

30 Seconds of Interviews / JavaScript / Does JavaScript pass by value or by reference?
JavaScript Value vs Reference

30 Seconds of Interviews / JavaScript / How do you pass an argument to an event handler or callback?
React docs on Handling Events

30 Seconds of Interviews / JavaScript / What are fragments?
React docs on Fragments

30 Seconds of Interviews / JavaScript / Create a function pipe that performs left-to-right function composition by returning a function that accepts one argument.
What is function composition?

30 Seconds of Interviews / JavaScript / What is the event loop in Node.js?
Node.js docs on event loop, timers and process.nextTick()

30 Seconds of Interviews / JavaScript / What is the only value not equal to itself in JavaScript?
MDN docs for NaN

30 Seconds of Interviews / JavaScript / Contrast mutable and immutable values, and mutating vs non-mutating methods.
Mutating vs non-mutating array methods

30 Seconds of Interviews / JavaScript / What is Big O Notation?
Big O Notation in JavaScript

30 Seconds of Interviews / JavaScript / What is a pure function?
Pure functions in JavaScript

30 Seconds of Interviews / JavaScript / What is recursion and when is it useful?
In plain English, what is recursion?

30 Seconds of Interviews / JavaScript / What is memoization?
Implementing memoization in JavaScript

30 Seconds of Interviews / JavaScript / What are refs in React? When should they be used?
React docs on Refs and the DOM

30 Seconds of Interviews / JavaScript / Explain the differences between imperative and declarative programming.
Declarative vs Imperative Programming

30 Seconds of Interviews / JavaScript / What is functional programming?
Javascript and Functional Programming: An Introduction
Master the JavaScript Interview: What is Functional Programming?

30 Seconds of Interviews / JavaScript / What are portals in React?
React docs on Portals

30 Seconds of Interviews / JavaScript / What is event-driven programming?
MDN docs on Events and Handlers
Understanding Node.js event-driven architecture

30 Seconds of Interviews / JavaScript / What is context?
React docs on Context

30 Seconds of Interviews / JavaScript / Explain the difference between a static method and an instance method.
Classes on MDN

30 Seconds of Interviews / JavaScript / What is a closure? Can you give a useful example of one?
MDN docs for closures
What is a closure
I never understood JavaScript closures

30 Seconds of Interviews / JavaScript / What is the this keyword and how does it work?
this on MDN

30 Seconds of Interviews / JavaScript / What is the children prop?
React docs on Children

30 Seconds of Interviews / JavaScript / Which is the preferred option between callback refs and findDOMNode()?
React docs on Refs and the DOM

30 Seconds of Interviews / JavaScript / What is the purpose of JavaScript UI libraries/frameworks like React, Vue, Angular, Hyperapp, etc?
Virtual DOM in Hyperapp 19,082 about 1 month ago

30 Seconds of Interviews / JavaScript / What does 'use strict' do and what are some of the key benefits to using it?
MDN docs for strict mode

30 Seconds of Interviews / JavaScript / How can you avoid callback hells?
Avoiding Callback Hell in Node.js
Asynchronous JavaScript: From Callback Hell to Async and Await

30 Seconds of Interviews / JavaScript / What is a virtual DOM and why is it used in libraries/frameworks?
The difference between Virtual DOM and DOM

30 Seconds of Interviews / JavaScript / How do you ensure methods have the correct this context in React component classes?
React docs on Handling Events
React docs on Passing Functions to Components

30 Seconds of Interviews / React / What is a stateless component?
React docs on State and Lifecycle

30 Seconds of Interviews / React / What is a stateful component in React?
React docs on State and Lifecycle

30 Seconds of Interviews / React / What is the difference between an element and a component in React?
React docs on Rendering Elements
React docs on Components and Props

30 Seconds of Interviews / React / How do you pass an argument to an event handler or callback?
React docs on Handling Events

30 Seconds of Interviews / React / What is the purpose of callback function as an argument of setState?
React docs on setState

30 Seconds of Interviews / React / What is the difference between HTML and React event handling?
React docs on Handling Events

30 Seconds of Interviews / React / What are inline conditional expressions?
React docs on Conditional Rendering

30 Seconds of Interviews / React / What is a key? What are the benefits of using it in lists?
React docs on Lists and Keys

30 Seconds of Interviews / React / Which is the preferred option between callback refs and findDOMNode()?
React docs on Refs and the DOM

30 Seconds of Interviews / React / What are fragments?
React docs on Fragments

30 Seconds of Interviews / React / How do you ensure methods have the correct this context in React component classes?
React docs on Handling Events
React docs on Passing Functions to Components

30 Seconds of Interviews / React / What are error boundaries in React?
https://reactjs.org/docs/error-boundaries.html

30 Seconds of Interviews / React / What is context?
React docs on Context

30 Seconds of Interviews / React / What are refs in React? When should they be used?
React docs on Refs and the DOM

30 Seconds of Interviews / React / What is the children prop?
React docs on Children

30 Seconds of Interviews / React / What are portals in React?
React docs on Portals

30 Seconds of Interviews / HTML / What is the purpose of the alt attribute on images?
A good basis for accessibility

30 Seconds of Interviews / HTML / What is the purpose of cache busting and how can you achieve it?
Strategies for cache-busting CSS

30 Seconds of Interviews / HTML / Can a web page contain multiple <header> elements? What about <footer> elements?
StackOverflow Using header or footer tag twice

30 Seconds of Interviews / HTML / Briefly describe the correct usage of the following HTML5 semantic elements: <header>, <article>,<section>, <footer>
HTML 5 Semantic Elements

30 Seconds of Interviews / HTML / Where and why is the rel="noopener" attribute used?
Open external anchors using rel="noopener"
About rel="noopener"

30 Seconds of Interviews / HTML / What are defer and async attributes on a <script> tag?
async vs defer attributes

30 Seconds of Interviews / HTML / What is the difference between HTML and React event handling?
React docs on Handling Events

30 Seconds of Interviews / HTML / What are some differences that XHTML has compared to HTML?
W3Schools docs for HTML and XHTML

30 Seconds of Interviews / HTML / What is the DOM?
MDN docs for DOM

30 Seconds of Interviews / HTML / Discuss the differences between an HTML specification and a browser’s implementation thereof.
HTML 5.2 WWW Specifications

30 Seconds of Interviews / HTML / What is HTML5 Web Storage? Explain localStorage and sessionStorage.
W3Schools HTML5 Webstorage

30 Seconds of Interviews / CSS / What is CSS BEM?
Writing clean and maintainable CSS

30 Seconds of Interviews / CSS / What are the advantages of using CSS preprocessors?
CSS Preprocessors

30 Seconds of Interviews / CSS / Using flexbox, create a 3-column layout where each column takes up a col-{n} / 12 ratio of the container.
MDN docs for basic concepts of flexbox
A complete guide to Flexbox

30 Seconds of Interviews / CSS / Can you name the four types of @media properties?
MDN docs for @media rule
MDN docs for using media queries

30 Seconds of Interviews / CSS / Describe the layout of the CSS Box Model and briefly describe each component.
W3School's CSS Box Model Page
Mozilla's Intro to the CSS Box Model

30 Seconds of Interviews / CSS / What is the difference between em and rem units?
CSS units for font-size: px | em | rem

30 Seconds of Interviews / CSS / What are the advantages of using CSS sprites and how are they utilized?
CSS Sprites explained by CSS Tricks

30 Seconds of Interviews / CSS / What is the difference between '+' and '~' sibling selectors?.
W3School's CSS Combinators Page
Mozilla's Combinators and groups of selectors page

30 Seconds of Interviews / CSS / Can you describe how CSS specificity works?
CSS Specificity

30 Seconds of Interviews / CSS / What is a focus ring? What is the correct solution to handle them?
:focus-visible

30 Seconds of Interviews / Accessibility / What is WCAG? What are the differences between A, AA, and AAA compliance?
Web Content Accessibility Guidelines (WCAG) Overview
How to Meet WCAG

30 Seconds of Interviews / Accessibility / What is ARIA and when should you use it?
WAI-ARIA Overview
WAI-ARIA Spec
ARIA Serious? Eric Eggert presentation

30 Seconds of Interviews / Accessibility / What is the Accessibility Tree?
Accessibility APIs

30 Seconds of Interviews / Accessibility / What are landmark roles and how can they be useful?
ARIA Landmark Roles
Using ARIA landmarks to identify regions of a page

30 Seconds of Interviews / Node / NodeJS often uses a callback pattern where if an error is encountered during execution, this error is passed as the first argument to the callback. What are the advantages of this pattern?
The Node.js Way Understanding Error-First Callbacks
What are the error conventions?

30 Seconds of Interviews / Node / What is REST?
https://medium.com/extend/what-is-rest-a-simple-explanation-for-beginners-part-1-introduction-b4a072f8740f ]( )

30 Seconds of Interviews / Node / How can you avoid callback hells?
Avoiding Callback Hell in Node.js
Asynchronous JavaScript: From Callback Hell to Async and Await

30 Seconds of Interviews / Node / What is the event loop in Node.js?
Node.js docs on event loop, timers and process.nextTick()

30 Seconds of Interviews / Security / What is a cross-site scripting attack (XSS) and how do you prevent it?
Cross-Site Scripting Attack (XSS)

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