30-seconds-of-interviews

30 Seconds of Interviews

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

> _This README is built using [markdown-builder](https://github.com/30-seconds/markdown-builder)._ ## Foreword Interviews are daunting and can make even the most seasoned expert forget things under pressure. Review and learn what questions are commonly encountered in interviews curated by the community that's answered them and go prepared for anything they'll ask. By bringing together experience and real-world examples, you can go from being nervous to being prepared for that next big opportunity. ## [View online](https://30secondsofinterviews.org/)
## Contributing > 30 seconds of interviews is a community effort, so feel free to contribute in any way you can. Every contribution helps! Do you have an excellent idea or know some cool questions that aren't on the list? Read the [contribution guidelines](https://github.com/30-seconds/30-seconds-of-interviews/blob/master/CONTRIBUTING.md) and submit a pull request. Join our [Gitter channel](https://gitter.im/30-seconds-of-interviews/Lobby) to help with the development of the project. #### Related projects - [30 Seconds of Code](https://30secondsofcode.org) - [30 Seconds of CSS](https://github.com/30-seconds/30-seconds-of-css) - [30 Seconds of React](https://github.com/30-seconds/30-seconds-of-react) - [30 Seconds of Knowledge](https://chrome.google.com/webstore/detail/30-seconds-of-knowledge/mmgplondnjekobonklacmemikcnhklla) ## Table of Contents ### JavaScript View contents * [Create a function `batches` that returns the maximum number of whole batches that can be cooked from a recipe.](#create-a-function-batches-that-returns-the-maximum-number-of-whole-batches-that-can-be-cooked-from-a-recipe) * [What is Big O Notation?](#what-is-big-o-notation) * [Create a standalone function `bind` that is functionally equivalent to the method `Function.prototype.bind`.](#create-a-standalone-function-bind-that-is-functionally-equivalent-to-the-method-functionprototypebind) * [How can you avoid callback hells?](#how-can-you-avoid-callback-hells) * [What is the purpose of callback function as an argument of `setState`?](#what-is-the-purpose-of-callback-function-as-an-argument-of-setstate) * [Which is the preferred option between callback refs and findDOMNode()?](#which-is-the-preferred-option-between-callback-refs-and-finddomnode) * [What is a callback? Can you show an example using one?](#what-is-a-callback-can-you-show-an-example-using-one) * [What is the `children` prop?](#what-is-the-children-prop) * [How do you clone an object in JavaScript?](#how-do-you-clone-an-object-in-javascript) * [What is a closure? Can you give a useful example of one?](#what-is-a-closure-can-you-give-a-useful-example-of-one) * [How do you compare two objects in JavaScript?](#how-do-you-compare-two-objects-in-javascript) * [What is context?](#what-is-context) * [What is CORS?](#what-is-cors) * [What is the DOM?](#what-is-the-dom) * [What is the difference between the equality operators `==` and `===`?](#what-is-the-difference-between-the-equality-operators--and-) * [What is the difference between an element and a component in React?](#what-is-the-difference-between-an-element-and-a-component-in-react) * [What is event delegation and why is it useful? Can you show an example of how to use it?](#what-is-event-delegation-and-why-is-it-useful-can-you-show-an-example-of-how-to-use-it) * [What is event-driven programming?](#what-is-event-driven-programming) * [What is the difference between an expression and a statement in JavaScript?](#what-is-the-difference-between-an-expression-and-a-statement-in-javascript) * [What are truthy and falsy values in JavaScript?](#what-are-truthy-and-falsy-values-in-javascript) * [Generate an array, containing the Fibonacci sequence, up until the nth term.](#generate-an-array-containing-the-fibonacci-sequence-up-until-the-nth-term) * [What does `0.1 + 0.2 === 0.3` evaluate to?](#what-does-01--02--03-evaluate-to) * [What is the difference between the array methods `map()` and `forEach()`?](#what-is-the-difference-between-the-array-methods-map-and-foreach) * [What are fragments?](#what-are-fragments) * [What is functional programming?](#what-is-functional-programming) * [What will the console log in this example?](#what-will-the-console-log-in-this-example) * [How does hoisting work in JavaScript?](#how-does-hoisting-work-in-javascript) * [What is the difference between HTML and React event handling?](#what-is-the-difference-between-html-and-react-event-handling) * [What is the reason for wrapping the entire contents of a JavaScript source file in a function that is immediately invoked?](#what-is-the-reason-for-wrapping-the-entire-contents-of-a-javascript-source-file-in-a-function-that-is-immediately-invoked) * [Explain the differences between imperative and declarative programming.](#explain-the-differences-between-imperative-and-declarative-programming) * [What are inline conditional expressions?](#what-are-inline-conditional-expressions) * [What is a key? What are the benefits of using it in lists?](#what-is-a-key-what-are-the-benefits-of-using-it-in-lists) * [What is the difference between lexical scoping and dynamic scoping?](#what-is-the-difference-between-lexical-scoping-and-dynamic-scoping) * [Create a function that masks a string of characters with `#` except for the last four (4) characters.](#create-a-function-that-masks-a-string-of-characters-with--except-for-the-last-four-4-characters) * [What is memoization?](#what-is-memoization) * [How do you ensure methods have the correct `this` context in React component classes?](#how-do-you-ensure-methods-have-the-correct-this-context-in-react-component-classes) * [What is a MIME type and what is it used for?](#what-is-a-mime-type-and-what-is-it-used-for) * [Contrast mutable and immutable values, and mutating vs non-mutating methods.](#contrast-mutable-and-immutable-values-and-mutating-vs-non-mutating-methods) * [What is the only value not equal to itself in JavaScript?](#what-is-the-only-value-not-equal-to-itself-in-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?](#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) * [What is the event loop in Node.js?](#what-is-the-event-loop-in-nodejs) * [What is the difference between `null` and `undefined`?](#what-is-the-difference-between-null-and-undefined) * [Describe the different ways to create an object. When should certain ways be preferred over others?](#describe-the-different-ways-to-create-an-object-when-should-certain-ways-be-preferred-over-others) * [What is the difference between a parameter and an argument?](#what-is-the-difference-between-a-parameter-and-an-argument) * [Does JavaScript pass by value or by reference?](#does-javascript-pass-by-value-or-by-reference) * [How do you pass an argument to an event handler or callback?](#how-do-you-pass-an-argument-to-an-event-handler-or-callback) * [Create a function `pipe` that performs left-to-right function composition by returning a function that accepts one argument.](#create-a-function-pipe-that-performs-left-to-right-function-composition-by-returning-a-function-that-accepts-one-argument) * [What are portals in React?](#what-are-portals-in-react) * [What is the difference between the postfix `i++` and prefix `++i` increment operators?](#what-is-the-difference-between-the-postfix-i-and-prefix-i-increment-operators) * [In which states can a Promise be?](#in-which-states-can-a-promise-be) * [What are Promises?](#what-are-promises) * [How does prototypal inheritance differ from classical inheritance?](#how-does-prototypal-inheritance-differ-from-classical-inheritance) * [What is a pure function?](#what-is-a-pure-function) * [What is recursion and when is it useful?](#what-is-recursion-and-when-is-it-useful) * [What is the output of the following code?](#what-is-the-output-of-the-following-code) * [What are refs in React? When should they be used?](#what-are-refs-in-react-when-should-they-be-used) * [What does the following function return?](#what-does-the-following-function-return) * [Are semicolons required in JavaScript?](#are-semicolons-required-in-javascript) * [What is short-circuit evaluation in JavaScript?](#what-is-short-circuit-evaluation-in-javascript) * [What is a stateful component in React?](#what-is-a-stateful-component-in-react) * [What is a stateless component?](#what-is-a-stateless-component) * [Explain the difference between a static method and an instance method.](#explain-the-difference-between-a-static-method-and-an-instance-method) * [What is the difference between synchronous and asynchronous code in JavaScript?](#what-is-the-difference-between-synchronous-and-asynchronous-code-in-javascript) * [What is the `this` keyword and how does it work?](#what-is-the-this-keyword-and-how-does-it-work) * [What does the following code evaluate to?](#what-does-the-following-code-evaluate-to) * [What are JavaScript data types?](#what-are-javascript-data-types) * [What is the purpose of JavaScript UI libraries/frameworks like React, Vue, Angular, Hyperapp, etc?](#what-is-the-purpose-of-javascript-ui-librariesframeworks-like-react-vue-angular-hyperapp-etc) * [What does `'use strict'` do and what are some of the key benefits to using it?](#what-does-use-strict-do-and-what-are-some-of-the-key-benefits-to-using-it) * [What are the differences between `var`, `let`, `const` and no keyword statements?](#what-are-the-differences-between-var-let-const-and-no-keyword-statements) * [What is a virtual DOM and why is it used in libraries/frameworks?](#what-is-a-virtual-dom-and-why-is-it-used-in-librariesframeworks) * [What is a cross-site scripting attack (XSS) and how do you prevent it?](#what-is-a-cross-site-scripting-attack-xss-and-how-do-you-prevent-it) ### React View contents * [What is the purpose of callback function as an argument of `setState`?](#what-is-the-purpose-of-callback-function-as-an-argument-of-setstate) * [Which is the preferred option between callback refs and findDOMNode()?](#which-is-the-preferred-option-between-callback-refs-and-finddomnode) * [What is the `children` prop?](#what-is-the-children-prop) * [Why does React use `className` instead of `class` like in HTML?](#why-does-react-use-classname-instead-of-class-like-in-html) * [What is context?](#what-is-context) * [What is the difference between an element and a component in React?](#what-is-the-difference-between-an-element-and-a-component-in-react) * [What are error boundaries in React?](#what-are-error-boundaries-in-react) * [What are fragments?](#what-are-fragments) * [What are higher-order components?](#what-are-higher-order-components) * [What is the difference between HTML and React event handling?](#what-is-the-difference-between-html-and-react-event-handling) * [What are inline conditional expressions?](#what-are-inline-conditional-expressions) * [What is a key? What are the benefits of using it in lists?](#what-is-a-key-what-are-the-benefits-of-using-it-in-lists) * [What are the lifecy