# 30-seconds-of-code **Repository Path**: seconds-of-code/30-seconds-of-code ## Basic Information - **Project Name**: 30-seconds-of-code - **Description**: 精选一些有用的 JavaScript 代码片段集合，你可以在30秒或更少的时间内学习并理解 - **Primary Language**: JavaScript - **License**: CC0-1.0 - **Default Branch**: master - **Homepage**: https://30secondsofcode.org/ - **GVP Project**: No ## Statistics - **Stars**: 219 - **Forks**: 44 - **Created**: 2018-10-06 - **Last Updated**: 2025-05-10 ## Categories & Tags **Categories**: sample-code **Tags**: None ## README ![Logo](/logo.png) # 30 seconds of code [![License](https://img.shields.io/badge/license-CC0--1.0-blue.svg)](https://github.com/30-seconds/30-seconds-of-code/blob/master/LICENSE) [![npm Downloads](https://img.shields.io/npm/dt/30-seconds-of-code.svg)](https://www.npmjs.com/package/30-seconds-of-code) [![npm Version](https://img.shields.io/npm/v/30-seconds-of-code.svg)](https://www.npmjs.com/package/30-seconds-of-code) [![Gitter chat](https://img.shields.io/badge/chat-on%20gitter-4FB999.svg)](https://gitter.im/30-seconds-of-code/Lobby) [![PRs Welcome](https://img.shields.io/badge/PRs-welcome-brightgreen.svg)](http://makeapullrequest.com) [![Travis Build](https://travis-ci.com/30-seconds/30-seconds-of-code.svg?branch=master)](https://travis-ci.com/30-seconds/30-seconds-of-code) [![Codacy Badge](https://api.codacy.com/project/badge/Grade/6ab7791fb1ea40b4a576d658fb96807f)](https://www.codacy.com/app/Chalarangelo/30-seconds-of-code?utm_source=github.com&utm_medium=referral&utm_content=30-seconds/30-seconds-of-code&utm_campaign=Badge_Grade) [![Maintainability](https://api.codeclimate.com/v1/badges/4b8c1e099135f2d53413/maintainability)](https://codeclimate.com/github/30-seconds/30-seconds-of-code/maintainability) [![js-semistandard-style](https://img.shields.io/badge/code%20style-semistandard-brightgreen.svg)](https://github.com/Flet/semistandard) [![Known Vulnerabilities](https://snyk.io/test/github/30-seconds/30-seconds-of-code/badge.svg?targetFile=package.json)](https://snyk.io/test/github/30-seconds/30-seconds-of-code?targetFile=package.json) [![ProductHunt](https://img.shields.io/badge/producthunt-vote-orange.svg)](https://www.producthunt.com/posts/30-seconds-of-code) > Curated collection of useful JavaScript snippets that you can understand in 30 seconds or less. [![Sponsored by DigitalOcean](/sponsored_by_DigitalOcean.png)](https://www.digitalocean.com) - Use Ctrl + F or command + F to search for a snippet. - Contributions welcome, please read the [contribution guide](CONTRIBUTING.md). - Snippets are written in ES6, use the [Babel transpiler](https://babeljs.io/) to ensure backwards-compatibility. - You can import these snippets into your text editor of choice (VSCode, Atom, Sublime) using the files found in [this repo](https://github.com/Rob-Rychs/30-seconds-of-code-texteditorsnippets). - You can import these snippets into Alfred 3, using [this file](https://github.com/lslvxy/30-seconds-of-code-alfredsnippets). - If you want to follow 30-seconds-of-code on social media, you can find us on [Facebook](https://www.facebook.com/30secondsofcode), [Instagram](https://www.instagram.com/30secondsofcode) and [Twitter](https://twitter.com/30secondsofcode). #### Related projects - [30 Seconds of CSS](https://30-seconds.github.io/30-seconds-of-css/) - [30 Seconds of Interviews](https://30secondsofinterviews.org/) - [30 Seconds of Python](https://github.com/kriadmin/30-seconds-of-python-code) *(unofficial)* - [30 Seconds of PHP](https://github.com/appzcoder/30-seconds-of-php-code) *(unofficial)* #### Package ⚠️ **NOTICE:** A few of our snippets are not yet optimized for production (see disclaimers for individual snippet issues). You can find a package with all the snippets on [npm](https://www.npmjs.com/package/30-seconds-of-code). ```bash # With npm npm install 30-seconds-of-code # With yarn yarn add 30-seconds-of-code ``` CDN links - [ES2017 Full (UMD)](https://unpkg.com/30-seconds-of-code) - [ES5 Minified (UMD)](https://unpkg.com/30-seconds-of-code/dist/_30s.es5.min.js)
Details **Browser** > IMPORTANT: replace the `src` with the full version link and desired target spec (such as ES5 minified)): ```html ``` **Node** ```js // CommonJS const _30s = require('30-seconds-of-code'); _30s.average(1, 2, 3); // ES Modules import _30s from '30-seconds-of-code'; _30s.average(1, 2, 3); ``` To import snippets directly: ```js // CommonJS const { average } = require('30-seconds-of-code'); average(1, 2, 3); // ES Modules import { average } from '30-seconds-of-code'; average(1, 2, 3); ```
## Table of Contents ### 🔌 Adapter
View contents * [`ary`](#ary) * [`call`](#call) * [`collectInto`](#collectinto) * [`flip`](#flip) * [`over`](#over) * [`overArgs`](#overargs) * [`pipeAsyncFunctions`](#pipeasyncfunctions) * [`pipeFunctions`](#pipefunctions) * [`promisify`](#promisify) * [`rearg`](#rearg) * [`spreadOver`](#spreadover) * [`unary`](#unary)
### 📚 Array
View contents * [`all`](#all) * [`allEqual`](#allequal) * [`any`](#any) * [`arrayToCSV`](#arraytocsv) * [`bifurcate`](#bifurcate) * [`bifurcateBy`](#bifurcateby) * [`chunk`](#chunk) * [`compact`](#compact) * [`countBy`](#countby) * [`countOccurrences`](#countoccurrences) * [`deepFlatten`](#deepflatten) * [`difference`](#difference) * [`differenceBy`](#differenceby) * [`differenceWith`](#differencewith) * [`drop`](#drop) * [`dropRight`](#dropright) * [`dropRightWhile`](#droprightwhile) * [`dropWhile`](#dropwhile) * [`everyNth`](#everynth) * [`filterNonUnique`](#filternonunique) * [`filterNonUniqueBy`](#filternonuniqueby) * [`findLast`](#findlast) * [`findLastIndex`](#findlastindex) * [`flatten`](#flatten) * [`forEachRight`](#foreachright) * [`groupBy`](#groupby) * [`head`](#head) * [`indexOfAll`](#indexofall) * [`initial`](#initial) * [`initialize2DArray`](#initialize2darray) * [`initializeArrayWithRange`](#initializearraywithrange) * [`initializeArrayWithRangeRight`](#initializearraywithrangeright) * [`initializeArrayWithValues`](#initializearraywithvalues) * [`initializeNDArray`](#initializendarray) * [`intersection`](#intersection) * [`intersectionBy`](#intersectionby) * [`intersectionWith`](#intersectionwith) * [`isSorted`](#issorted) * [`join`](#join) * [`JSONtoCSV`](#jsontocsv-) * [`last`](#last) * [`longestItem`](#longestitem) * [`mapObject`](#mapobject-) * [`maxN`](#maxn) * [`minN`](#minn) * [`none`](#none) * [`nthElement`](#nthelement) * [`offset`](#offset) * [`partition`](#partition) * [`permutations`](#permutations-) * [`pull`](#pull) * [`pullAtIndex`](#pullatindex-) * [`pullAtValue`](#pullatvalue-) * [`pullBy`](#pullby-) * [`reducedFilter`](#reducedfilter) * [`reduceSuccessive`](#reducesuccessive) * [`reduceWhich`](#reducewhich) * [`reject`](#reject) * [`remove`](#remove) * [`sample`](#sample) * [`sampleSize`](#samplesize) * [`shank`](#shank) * [`shuffle`](#shuffle) * [`similarity`](#similarity) * [`sortedIndex`](#sortedindex) * [`sortedIndexBy`](#sortedindexby) * [`sortedLastIndex`](#sortedlastindex) * [`sortedLastIndexBy`](#sortedlastindexby) * [`stableSort`](#stablesort-) * [`symmetricDifference`](#symmetricdifference) * [`symmetricDifferenceBy`](#symmetricdifferenceby) * [`symmetricDifferenceWith`](#symmetricdifferencewith) * [`tail`](#tail) * [`take`](#take) * [`takeRight`](#takeright) * [`takeRightWhile`](#takerightwhile) * [`takeWhile`](#takewhile) * [`toHash`](#tohash) * [`union`](#union) * [`unionBy`](#unionby) * [`unionWith`](#unionwith) * [`uniqueElements`](#uniqueelements) * [`uniqueElementsBy`](#uniqueelementsby) * [`uniqueElementsByRight`](#uniqueelementsbyright) * [`uniqueSymmetricDifference`](#uniquesymmetricdifference) * [`unzip`](#unzip) * [`unzipWith`](#unzipwith-) * [`without`](#without) * [`xProd`](#xprod) * [`zip`](#zip) * [`zipObject`](#zipobject) * [`zipWith`](#zipwith-)
### 🌐 Browser
View contents * [`arrayToHtmlList`](#arraytohtmllist) * [`bottomVisible`](#bottomvisible) * [`copyToClipboard`](#copytoclipboard-) * [`counter`](#counter-) * [`createElement`](#createelement) * [`createEventHub`](#createeventhub-) * [`currentURL`](#currenturl) * [`detectDeviceType`](#detectdevicetype) * [`elementContains`](#elementcontains) * [`elementIsVisibleInViewport`](#elementisvisibleinviewport-) * [`getScrollPosition`](#getscrollposition) * [`getStyle`](#getstyle) * [`hasClass`](#hasclass) * [`hashBrowser`](#hashbrowser-) * [`hide`](#hide) * [`httpsRedirect`](#httpsredirect) * [`insertAfter`](#insertafter) * [`insertBefore`](#insertbefore) * [`isBrowserTabFocused`](#isbrowsertabfocused) * [`nodeListToArray`](#nodelisttoarray) * [`observeMutations`](#observemutations-) * [`off`](#off) * [`on`](#on) * [`onUserInputChange`](#onuserinputchange-) * [`prefix`](#prefix) * [`recordAnimationFrames`](#recordanimationframes) * [`redirect`](#redirect) * [`runAsync`](#runasync-) * [`scrollToTop`](#scrolltotop) * [`setStyle`](#setstyle) * [`show`](#show) * [`smoothScroll`](#smoothscroll) * [`toggleClass`](#toggleclass) * [`triggerEvent`](#triggerevent) * [`UUIDGeneratorBrowser`](#uuidgeneratorbrowser)
### ⏱️ Date
View contents * [`dayOfYear`](#dayofyear) * [`formatDuration`](#formatduration) * [`getColonTimeFromDate`](#getcolontimefromdate) * [`getDaysDiffBetweenDates`](#getdaysdiffbetweendates) * [`getMeridiemSuffixOfInteger`](#getmeridiemsuffixofinteger) * [`isAfterDate`](#isafterdate) * [`isBeforeDate`](#isbeforedate) * [`isSameDate`](#issamedate) * [`maxDate`](#maxdate) * [`minDate`](#mindate) * [`tomorrow`](#tomorrow)
### 🎛️ Function
View contents * [`attempt`](#attempt) * [`bind`](#bind) * [`bindKey`](#bindkey) * [`chainAsync`](#chainasync) * [`compose`](#compose) * [`composeRight`](#composeright) * [`converge`](#converge) * [`curry`](#curry) * [`debounce`](#debounce) * [`defer`](#defer) * [`delay`](#delay) * [`functionName`](#functionname) * [`hz`](#hz) * [`memoize`](#memoize-) * [`negate`](#negate) * [`once`](#once) * [`partial`](#partial) * [`partialRight`](#partialright) * [`runPromisesInSeries`](#runpromisesinseries) * [`sleep`](#sleep) * [`throttle`](#throttle-) * [`times`](#times) * [`uncurry`](#uncurry) * [`unfold`](#unfold) * [`when`](#when)
### ➗ Math
View contents * [`approximatelyEqual`](#approximatelyequal) * [`average`](#average) * [`averageBy`](#averageby) * [`binomialCoefficient`](#binomialcoefficient) * [`clampNumber`](#clampnumber) * [`degreesToRads`](#degreestorads) * [`digitize`](#digitize) * [`distance`](#distance) * [`elo`](#elo-) * [`factorial`](#factorial) * [`fibonacci`](#fibonacci) * [`gcd`](#gcd) * [`geometricProgression`](#geometricprogression) * [`hammingDistance`](#hammingdistance) * [`inRange`](#inrange) * [`isDivisible`](#isdivisible) * [`isEven`](#iseven) * [`isPrime`](#isprime) * [`lcm`](#lcm) * [`luhnCheck`](#luhncheck-) * [`maxBy`](#maxby) * [`median`](#median) * [`minBy`](#minby) * [`percentile`](#percentile) * [`powerset`](#powerset) * [`primes`](#primes) * [`radsToDegrees`](#radstodegrees) * [`randomIntArrayInRange`](#randomintarrayinrange) * [`randomIntegerInRange`](#randomintegerinrange) * [`randomNumberInRange`](#randomnumberinrange) * [`round`](#round) * [`sdbm`](#sdbm) * [`standardDeviation`](#standarddeviation) * [`sum`](#sum) * [`sumBy`](#sumby) * [`sumPower`](#sumpower) * [`toSafeInteger`](#tosafeinteger)
### 📦 Node
View contents * [`atob`](#atob) * [`btoa`](#btoa) * [`colorize`](#colorize) * [`hasFlags`](#hasflags) * [`hashNode`](#hashnode) * [`isDuplexStream`](#isduplexstream) * [`isReadableStream`](#isreadablestream) * [`isStream`](#isstream) * [`isTravisCI`](#istravisci) * [`isWritableStream`](#iswritablestream) * [`JSONToFile`](#jsontofile) * [`readFileLines`](#readfilelines) * [`untildify`](#untildify) * [`UUIDGeneratorNode`](#uuidgeneratornode)
### 🗃️ Object
View contents * [`bindAll`](#bindall) * [`deepClone`](#deepclone) * [`deepFreeze`](#deepfreeze) * [`defaults`](#defaults) * [`dig`](#dig) * [`equals`](#equals-) * [`findKey`](#findkey) * [`findLastKey`](#findlastkey) * [`flattenObject`](#flattenobject) * [`forOwn`](#forown) * [`forOwnRight`](#forownright) * [`functions`](#functions) * [`get`](#get) * [`invertKeyValues`](#invertkeyvalues) * [`lowercaseKeys`](#lowercasekeys) * [`mapKeys`](#mapkeys) * [`mapValues`](#mapvalues) * [`matches`](#matches) * [`matchesWith`](#matcheswith) * [`merge`](#merge) * [`nest`](#nest) * [`objectFromPairs`](#objectfrompairs) * [`objectToPairs`](#objecttopairs) * [`omit`](#omit) * [`omitBy`](#omitby) * [`orderBy`](#orderby) * [`pick`](#pick) * [`pickBy`](#pickby) * [`renameKeys`](#renamekeys) * [`shallowClone`](#shallowclone) * [`size`](#size) * [`transform`](#transform) * [`truthCheckCollection`](#truthcheckcollection) * [`unflattenObject`](#unflattenobject-)
### 📜 String
View contents * [`byteSize`](#bytesize) * [`capitalize`](#capitalize) * [`capitalizeEveryWord`](#capitalizeeveryword) * [`CSVToArray`](#csvtoarray) * [`CSVToJSON`](#csvtojson-) * [`decapitalize`](#decapitalize) * [`escapeHTML`](#escapehtml) * [`escapeRegExp`](#escaperegexp) * [`fromCamelCase`](#fromcamelcase) * [`indentString`](#indentstring) * [`isAbsoluteURL`](#isabsoluteurl) * [`isAnagram`](#isanagram) * [`isLowerCase`](#islowercase) * [`isUpperCase`](#isuppercase) * [`mapString`](#mapstring) * [`mask`](#mask) * [`pad`](#pad) * [`palindrome`](#palindrome) * [`pluralize`](#pluralize) * [`removeNonASCII`](#removenonascii) * [`reverseString`](#reversestring) * [`sortCharactersInString`](#sortcharactersinstring) * [`splitLines`](#splitlines) * [`stringPermutations`](#stringpermutations-) * [`stripHTMLTags`](#striphtmltags) * [`toCamelCase`](#tocamelcase) * [`toKebabCase`](#tokebabcase) * [`toSnakeCase`](#tosnakecase) * [`truncateString`](#truncatestring) * [`unescapeHTML`](#unescapehtml) * [`URLJoin`](#urljoin-) * [`words`](#words)
### 📃 Type
View contents * [`getType`](#gettype) * [`is`](#is) * [`isArrayLike`](#isarraylike) * [`isBoolean`](#isboolean) * [`isEmpty`](#isempty) * [`isFunction`](#isfunction) * [`isNil`](#isnil) * [`isNull`](#isnull) * [`isNumber`](#isnumber) * [`isObject`](#isobject) * [`isObjectLike`](#isobjectlike) * [`isPlainObject`](#isplainobject) * [`isPrimitive`](#isprimitive) * [`isPromiseLike`](#ispromiselike) * [`isString`](#isstring) * [`isSymbol`](#issymbol) * [`isUndefined`](#isundefined) * [`isValidJSON`](#isvalidjson)
### 🔧 Utility
View contents * [`castArray`](#castarray) * [`cloneRegExp`](#cloneregexp) * [`coalesce`](#coalesce) * [`coalesceFactory`](#coalescefactory) * [`extendHex`](#extendhex) * [`getURLParameters`](#geturlparameters) * [`hexToRGB`](#hextorgb-) * [`httpGet`](#httpget) * [`httpPost`](#httppost) * [`isBrowser`](#isbrowser) * [`mostPerformant`](#mostperformant) * [`nthArg`](#ntharg) * [`parseCookie`](#parsecookie) * [`prettyBytes`](#prettybytes-) * [`randomHexColorCode`](#randomhexcolorcode) * [`RGBToHex`](#rgbtohex) * [`serializeCookie`](#serializecookie) * [`timeTaken`](#timetaken) * [`toCurrency`](#tocurrency) * [`toDecimalMark`](#todecimalmark) * [`toOrdinalSuffix`](#toordinalsuffix) * [`validateNumber`](#validatenumber) * [`yesNo`](#yesno)
--- ## 🔌 Adapter ### ary Creates a function that accepts up to `n` arguments, ignoring any additional arguments. Call the provided function, `fn`, with up to `n` arguments, using `Array.prototype.slice(0,n)` and the spread operator (`...`). ```js const ary = (fn, n) => (...args) => fn(...args.slice(0, n)); ```
Examples ```js const firstTwoMax = ary(Math.max, 2); [[2, 6, 'a'], [8, 4, 6], [10]].map(x => firstTwoMax(...x)); // [6, 8, 10] ```

[⬆ Back to top](#table-of-contents) ### call Given a key and a set of arguments, call them when given a context. Primarily useful in composition. Use a closure to call a stored key with stored arguments. ```js const call = (key, ...args) => context => context[key](...args); ```
Examples ```js Promise.resolve([1, 2, 3]) .then(call('map', x => 2 * x)) .then(console.log); //[ 2, 4, 6 ] const map = call.bind(null, 'map'); Promise.resolve([1, 2, 3]) .then(map(x => 2 * x)) .then(console.log); //[ 2, 4, 6 ] ```

[⬆ Back to top](#table-of-contents) ### collectInto Changes a function that accepts an array into a variadic function. Given a function, return a closure that collects all inputs into an array-accepting function. ```js const collectInto = fn => (...args) => fn(args); ```
Examples ```js const Pall = collectInto(Promise.all.bind(Promise)); let p1 = Promise.resolve(1); let p2 = Promise.resolve(2); let p3 = new Promise(resolve => setTimeout(resolve, 2000, 3)); Pall(p1, p2, p3).then(console.log); // [1, 2, 3] (after about 2 seconds) ```

[⬆ Back to top](#table-of-contents) ### flip Flip takes a function as an argument, then makes the first argument the last. Return a closure that takes variadic inputs, and splices the last argument to make it the first argument before applying the rest. ```js const flip = fn => (first, ...rest) => fn(...rest, first); ```
Examples ```js let a = { name: 'John Smith' }; let b = {}; const mergeFrom = flip(Object.assign); let mergePerson = mergeFrom.bind(null, a); mergePerson(b); // == b b = {}; Object.assign(b, a); // == b ```

[⬆ Back to top](#table-of-contents) ### over Creates a function that invokes each provided function with the arguments it receives and returns the results. Use `Array.prototype.map()` and `Function.prototype.apply()` to apply each function to the given arguments. ```js const over = (...fns) => (...args) => fns.map(fn => fn.apply(null, args)); ```
Examples ```js const minMax = over(Math.min, Math.max); minMax(1, 2, 3, 4, 5); // [1,5] ```

[⬆ Back to top](#table-of-contents) ### overArgs Creates a function that invokes the provided function with its arguments transformed. Use `Array.prototype.map()` to apply `transforms` to `args` in combination with the spread operator (`...`) to pass the transformed arguments to `fn`. ```js const overArgs = (fn, transforms) => (...args) => fn(...args.map((val, i) => transforms[i](val))); ```
Examples ```js const square = n => n * n; const double = n => n * 2; const fn = overArgs((x, y) => [x, y], [square, double]); fn(9, 3); // [81, 6] ```

[⬆ Back to top](#table-of-contents) ### pipeAsyncFunctions Performs left-to-right function composition for asynchronous functions. Use `Array.prototype.reduce()` with the spread operator (`...`) to perform left-to-right function composition using `Promise.then()`. The functions can return a combination of: simple values, `Promise`'s, or they can be defined as `async` ones returning through `await`. All functions must be unary. ```js const pipeAsyncFunctions = (...fns) => arg => fns.reduce((p, f) => p.then(f), Promise.resolve(arg)); ```
Examples ```js const sum = pipeAsyncFunctions( x => x + 1, x => new Promise(resolve => setTimeout(() => resolve(x + 2), 1000)), x => x + 3, async x => (await x) + 4 ); (async () => { console.log(await sum(5)); // 15 (after one second) })(); ```

[⬆ Back to top](#table-of-contents) ### pipeFunctions Performs left-to-right function composition. Use `Array.prototype.reduce()` with the spread operator (`...`) to perform left-to-right function composition. The first (leftmost) function can accept one or more arguments; the remaining functions must be unary. ```js const pipeFunctions = (...fns) => fns.reduce((f, g) => (...args) => g(f(...args))); ```
Examples ```js const add5 = x => x + 5; const multiply = (x, y) => x * y; const multiplyAndAdd5 = pipeFunctions(multiply, add5); multiplyAndAdd5(5, 2); // 15 ```

[⬆ Back to top](#table-of-contents) ### promisify Converts an asynchronous function to return a promise. Use currying to return a function returning a `Promise` that calls the original function. Use the `...rest` operator to pass in all the parameters. *In Node 8+, you can use [`util.promisify`](https://nodejs.org/api/util.html#util_util_promisify_original)* ```js const promisify = func => (...args) => new Promise((resolve, reject) => func(...args, (err, result) => (err ? reject(err) : resolve(result))) ); ```
Examples ```js const delay = promisify((d, cb) => setTimeout(cb, d)); delay(2000).then(() => console.log('Hi!')); // // Promise resolves after 2s ```

[⬆ Back to top](#table-of-contents) ### rearg Creates a function that invokes the provided function with its arguments arranged according to the specified indexes. Use `Array.prototype.map()` to reorder arguments based on `indexes` in combination with the spread operator (`...`) to pass the transformed arguments to `fn`. ```js const rearg = (fn, indexes) => (...args) => fn(...indexes.map(i => args[i])); ```
Examples ```js var rearged = rearg( function(a, b, c) { return [a, b, c]; }, [2, 0, 1] ); rearged('b', 'c', 'a'); // ['a', 'b', 'c'] ```

[⬆ Back to top](#table-of-contents) ### spreadOver Takes a variadic function and returns a closure that accepts an array of arguments to map to the inputs of the function. Use closures and the spread operator (`...`) to map the array of arguments to the inputs of the function. ```js const spreadOver = fn => argsArr => fn(...argsArr); ```
Examples ```js const arrayMax = spreadOver(Math.max); arrayMax([1, 2, 3]); // 3 ```

[⬆ Back to top](#table-of-contents) ### unary Creates a function that accepts up to one argument, ignoring any additional arguments. Call the provided function, `fn`, with just the first argument given. ```js const unary = fn => val => fn(val); ```
Examples ```js ['6', '8', '10'].map(unary(parseInt)); // [6, 8, 10] ```

[⬆ Back to top](#table-of-contents) --- ## 📚 Array ### all Returns `true` if the provided predicate function returns `true` for all elements in a collection, `false` otherwise. Use `Array.prototype.every()` to test if all elements in the collection return `true` based on `fn`. Omit the second argument, `fn`, to use `Boolean` as a default. ```js const all = (arr, fn = Boolean) => arr.every(fn); ```
Examples ```js all([4, 2, 3], x => x > 1); // true all([1, 2, 3]); // true ```

[⬆ Back to top](#table-of-contents) ### allEqual Check if all elements in an array are equal. Use `Array.prototype.every()` to check if all the elements of the array are the same as the first one. ```js const allEqual = arr => arr.every(val => val === arr[0]); ```
Examples ```js allEqual([1, 2, 3, 4, 5, 6]); // false allEqual([1, 1, 1, 1]); // true ```

[⬆ Back to top](#table-of-contents) ### any Returns `true` if the provided predicate function returns `true` for at least one element in a collection, `false` otherwise. Use `Array.prototype.some()` to test if any elements in the collection return `true` based on `fn`. Omit the second argument, `fn`, to use `Boolean` as a default. ```js const any = (arr, fn = Boolean) => arr.some(fn); ```
Examples ```js any([0, 1, 2, 0], x => x >= 2); // true any([0, 0, 1, 0]); // true ```

[⬆ Back to top](#table-of-contents) ### arrayToCSV Converts a 2D array to a comma-separated values (CSV) string. Use `Array.prototype.map()` and `Array.prototype.join(delimiter)` to combine individual 1D arrays (rows) into strings. Use `Array.prototype.join('\n')` to combine all rows into a CSV string, separating each row with a newline. Omit the second argument, `delimiter`, to use a default delimiter of `,`. ```js const arrayToCSV = (arr, delimiter = ',') => arr.map(v => v.map(x => `"${x}"`).join(delimiter)).join('\n'); ```
Examples ```js arrayToCSV([['a', 'b'], ['c', 'd']]); // '"a","b"\n"c","d"' arrayToCSV([['a', 'b'], ['c', 'd']], ';'); // '"a";"b"\n"c";"d"' ```

[⬆ Back to top](#table-of-contents) ### bifurcate Splits values into two groups. If an element in `filter` is truthy, the corresponding element in the collection belongs to the first group; otherwise, it belongs to the second group. Use `Array.prototype.reduce()` and `Array.prototype.push()` to add elements to groups, based on `filter`. ```js const bifurcate = (arr, filter) => arr.reduce((acc, val, i) => (acc[filter[i] ? 0 : 1].push(val), acc), [[], []]); ```
Examples ```js bifurcate(['beep', 'boop', 'foo', 'bar'], [true, true, false, true]); // [ ['beep', 'boop', 'bar'], ['foo'] ] ```

[⬆ Back to top](#table-of-contents) ### bifurcateBy Splits values into two groups according to a predicate function, which specifies which group an element in the input collection belongs to. If the predicate function returns a truthy value, the collection element belongs to the first group; otherwise, it belongs to the second group. Use `Array.prototype.reduce()` and `Array.prototype.push()` to add elements to groups, based on the value returned by `fn` for each element. ```js const bifurcateBy = (arr, fn) => arr.reduce((acc, val, i) => (acc[fn(val, i) ? 0 : 1].push(val), acc), [[], []]); ```
Examples ```js bifurcateBy(['beep', 'boop', 'foo', 'bar'], x => x[0] === 'b'); // [ ['beep', 'boop', 'bar'], ['foo'] ] ```

[⬆ Back to top](#table-of-contents) ### chunk Chunks an array into smaller arrays of a specified size. Use `Array.from()` to create a new array, that fits the number of chunks that will be produced. Use `Array.prototype.slice()` to map each element of the new array to a chunk the length of `size`. If the original array can't be split evenly, the final chunk will contain the remaining elements. ```js const chunk = (arr, size) => Array.from({ length: Math.ceil(arr.length / size) }, (v, i) => arr.slice(i * size, i * size + size) ); ```
Examples ```js chunk([1, 2, 3, 4, 5], 2); // [[1,2],[3,4],[5]] ```

[⬆ Back to top](#table-of-contents) ### compact Removes falsey values from an array. Use `Array.prototype.filter()` to filter out falsey values (`false`, `null`, `0`, `""`, `undefined`, and `NaN`). ```js const compact = arr => arr.filter(Boolean); ```
Examples ```js compact([0, 1, false, 2, '', 3, 'a', 'e' * 23, NaN, 's', 34]); // [ 1, 2, 3, 'a', 's', 34 ] ```

[⬆ Back to top](#table-of-contents) ### countBy Groups the elements of an array based on the given function and returns the count of elements in each group. Use `Array.prototype.map()` to map the values of an array to a function or property name. Use `Array.prototype.reduce()` to create an object, where the keys are produced from the mapped results. ```js const countBy = (arr, fn) => arr.map(typeof fn === 'function' ? fn : val => val[fn]).reduce((acc, val, i) => { acc[val] = (acc[val] || 0) + 1; return acc; }, {}); ```
Examples ```js countBy([6.1, 4.2, 6.3], Math.floor); // {4: 1, 6: 2} countBy(['one', 'two', 'three'], 'length'); // {3: 2, 5: 1} ```

[⬆ Back to top](#table-of-contents) ### countOccurrences Counts the occurrences of a value in an array. Use `Array.prototype.reduce()` to increment a counter each time you encounter the specific value inside the array. ```js const countOccurrences = (arr, val) => arr.reduce((a, v) => (v === val ? a + 1 : a), 0); ```
Examples ```js countOccurrences([1, 1, 2, 1, 2, 3], 1); // 3 ```

[⬆ Back to top](#table-of-contents) ### deepFlatten Deep flattens an array. Use recursion. Use `Array.prototype.concat()` with an empty array (`[]`) and the spread operator (`...`) to flatten an array. Recursively flatten each element that is an array. ```js const deepFlatten = arr => [].concat(...arr.map(v => (Array.isArray(v) ? deepFlatten(v) : v))); ```
Examples ```js deepFlatten([1, [2], [[3], 4], 5]); // [1,2,3,4,5] ```

[⬆ Back to top](#table-of-contents) ### difference Returns the difference between two arrays. Create a `Set` from `b`, then use `Array.prototype.filter()` on `a` to only keep values not contained in `b`. ```js const difference = (a, b) => { const s = new Set(b); return a.filter(x => !s.has(x)); }; ```
Examples ```js difference([1, 2, 3], [1, 2, 4]); // [3] ```

[⬆ Back to top](#table-of-contents) ### differenceBy Returns the difference between two arrays, after applying the provided function to each array element of both. Create a `Set` by applying `fn` to each element in `b`, then use `Array.prototype.filter()` in combination with `fn` on `a` to only keep values not contained in the previously created set. ```js const differenceBy = (a, b, fn) => { const s = new Set(b.map(v => fn(v))); return a.filter(x => !s.has(fn(x))); }; ```
Examples ```js differenceBy([2.1, 1.2], [2.3, 3.4], Math.floor); // [1.2] differenceBy([{ x: 2 }, { x: 1 }], [{ x: 1 }], v => v.x); // [ { x: 2 } ] ```

[⬆ Back to top](#table-of-contents) ### differenceWith Filters out all values from an array for which the comparator function does not return `true`. Use `Array.prototype.filter()` and `Array.prototype.findIndex()` to find the appropriate values. ```js const differenceWith = (arr, val, comp) => arr.filter(a => val.findIndex(b => comp(a, b)) === -1); ```
Examples ```js differenceWith([1, 1.2, 1.5, 3, 0], [1.9, 3, 0], (a, b) => Math.round(a) === Math.round(b)); // [1, 1.2] ```

[⬆ Back to top](#table-of-contents) ### drop Returns a new array with `n` elements removed from the left. Use `Array.prototype.slice()` to slice the remove the specified number of elements from the left. ```js const drop = (arr, n = 1) => arr.slice(n); ```
Examples ```js drop([1, 2, 3]); // [2,3] drop([1, 2, 3], 2); // [3] drop([1, 2, 3], 42); // [] ```

[⬆ Back to top](#table-of-contents) ### dropRight Returns a new array with `n` elements removed from the right. Use `Array.prototype.slice()` to slice the remove the specified number of elements from the right. ```js const dropRight = (arr, n = 1) => arr.slice(0, -n); ```
Examples ```js dropRight([1, 2, 3]); // [1,2] dropRight([1, 2, 3], 2); // [1] dropRight([1, 2, 3], 42); // [] ```

[⬆ Back to top](#table-of-contents) ### dropRightWhile Removes elements from the end of an array until the passed function returns `true`. Returns the remaining elements in the array. Loop through the array, using `Array.prototype.slice()` to drop the last element of the array until the returned value from the function is `true`. Returns the remaining elements. ```js const dropRightWhile = (arr, func) => { while (arr.length > 0 && !func(arr[arr.length - 1])) arr = arr.slice(0, -1); return arr; }; ```
Examples ```js dropRightWhile([1, 2, 3, 4], n => n < 3); // [1, 2] ```

[⬆ Back to top](#table-of-contents) ### dropWhile Removes elements in an array until the passed function returns `true`. Returns the remaining elements in the array. Loop through the array, using `Array.prototype.slice()` to drop the first element of the array until the returned value from the function is `true`. Returns the remaining elements. ```js const dropWhile = (arr, func) => { while (arr.length > 0 && !func(arr[0])) arr = arr.slice(1); return arr; }; ```
Examples ```js dropWhile([1, 2, 3, 4], n => n >= 3); // [3,4] ```

[⬆ Back to top](#table-of-contents) ### everyNth Returns every nth element in an array. Use `Array.prototype.filter()` to create a new array that contains every nth element of a given array. ```js const everyNth = (arr, nth) => arr.filter((e, i) => i % nth === nth - 1); ```
Examples ```js everyNth([1, 2, 3, 4, 5, 6], 2); // [ 2, 4, 6 ] ```

[⬆ Back to top](#table-of-contents) ### filterNonUnique Filters out the non-unique values in an array. Use `Array.prototype.filter()` for an array containing only the unique values. ```js const filterNonUnique = arr => arr.filter(i => arr.indexOf(i) === arr.lastIndexOf(i)); ```
Examples ```js filterNonUnique([1, 2, 2, 3, 4, 4, 5]); // [1, 3, 5] ```

[⬆ Back to top](#table-of-contents) ### filterNonUniqueBy Filters out the non-unique values in an array, based on a provided comparator function. Use `Array.prototype.filter()` and `Array.prototype.every()` for an array containing only the unique values, based on the comparator function, `fn`. The comparator function takes four arguments: the values of the two elements being compared and their indexes. ```js const filterNonUniqueBy = (arr, fn) => arr.filter((v, i) => arr.every((x, j) => (i === j) === fn(v, x, i, j))); ```
Examples ```js filterNonUniqueBy( [ { id: 0, value: 'a' }, { id: 1, value: 'b' }, { id: 2, value: 'c' }, { id: 1, value: 'd' }, { id: 0, value: 'e' } ], (a, b) => a.id == b.id ); // [ { id: 2, value: 'c' } ] ```

[⬆ Back to top](#table-of-contents) ### findLast Returns the last element for which the provided function returns a truthy value. Use `Array.prototype.filter()` to remove elements for which `fn` returns falsey values, `Array.prototype.pop()` to get the last one. ```js const findLast = (arr, fn) => arr.filter(fn).pop(); ```
Examples ```js findLast([1, 2, 3, 4], n => n % 2 === 1); // 3 ```

[⬆ Back to top](#table-of-contents) ### findLastIndex Returns the index of the last element for which the provided function returns a truthy value. Use `Array.prototype.map()` to map each element to an array with its index and value. Use `Array.prototype.filter()` to remove elements for which `fn` returns falsey values, `Array.prototype.pop()` to get the last one. ```js const findLastIndex = (arr, fn) => arr .map((val, i) => [i, val]) .filter(([i, val]) => fn(val, i, arr)) .pop()[0]; ```
Examples ```js findLastIndex([1, 2, 3, 4], n => n % 2 === 1); // 2 (index of the value 3) ```

[⬆ Back to top](#table-of-contents) ### flatten Flattens an array up to the specified depth. Use recursion, decrementing `depth` by 1 for each level of depth. Use `Array.prototype.reduce()` and `Array.prototype.concat()` to merge elements or arrays. Base case, for `depth` equal to `1` stops recursion. Omit the second argument, `depth` to flatten only to a depth of `1` (single flatten). ```js const flatten = (arr, depth = 1) => arr.reduce((a, v) => a.concat(depth > 1 && Array.isArray(v) ? flatten(v, depth - 1) : v), []); ```
Examples ```js flatten([1, [2], 3, 4]); // [1, 2, 3, 4] flatten([1, [2, [3, [4, 5], 6], 7], 8], 2); // [1, 2, 3, [4, 5], 6, 7, 8] ```

[⬆ Back to top](#table-of-contents) ### forEachRight Executes a provided function once for each array element, starting from the array's last element. Use `Array.prototype.slice(0)` to clone the given array, `Array.prototype.reverse()` to reverse it and `Array.prototype.forEach()` to iterate over the reversed array. ```js const forEachRight = (arr, callback) => arr .slice(0) .reverse() .forEach(callback); ```
Examples ```js forEachRight([1, 2, 3, 4], val => console.log(val)); // '4', '3', '2', '1' ```

[⬆ Back to top](#table-of-contents) ### groupBy Groups the elements of an array based on the given function. Use `Array.prototype.map()` to map the values of an array to a function or property name. Use `Array.prototype.reduce()` to create an object, where the keys are produced from the mapped results. ```js const groupBy = (arr, fn) => arr.map(typeof fn === 'function' ? fn : val => val[fn]).reduce((acc, val, i) => { acc[val] = (acc[val] || []).concat(arr[i]); return acc; }, {}); ```
Examples ```js groupBy([6.1, 4.2, 6.3], Math.floor); // {4: [4.2], 6: [6.1, 6.3]} groupBy(['one', 'two', 'three'], 'length'); // {3: ['one', 'two'], 5: ['three']} ```

[⬆ Back to top](#table-of-contents) ### head Returns the head of a list. Use `arr[0]` to return the first element of the passed array. ```js const head = arr => arr[0]; ```
Examples ```js head([1, 2, 3]); // 1 ```

[⬆ Back to top](#table-of-contents) ### indexOfAll Returns all indices of `val` in an array. If `val` never occurs, returns `[]`. Use `Array.prototype.reduce()` to loop over elements and store indices for matching elements. Return the array of indices. ```js const indexOfAll = (arr, val) => arr.reduce((acc, el, i) => (el === val ? [...acc, i] : acc), []); ```
Examples ```js indexOfAll([1, 2, 3, 1, 2, 3], 1); // [0,3] indexOfAll([1, 2, 3], 4); // [] ```

[⬆ Back to top](#table-of-contents) ### initial Returns all the elements of an array except the last one. Use `arr.slice(0,-1)` to return all but the last element of the array. ```js const initial = arr => arr.slice(0, -1); ```
Examples ```js initial([1, 2, 3]); // [1,2] ```

[⬆ Back to top](#table-of-contents) ### initialize2DArray Initializes a 2D array of given width and height and value. Use `Array.prototype.map()` to generate h rows where each is a new array of size w initialize with value. If the value is not provided, default to `null`. ```js const initialize2DArray = (w, h, val = null) => Array.from({ length: h }).map(() => Array.from({ length: w }).fill(val)); ```
Examples ```js initialize2DArray(2, 2, 0); // [[0,0], [0,0]] ```

[⬆ Back to top](#table-of-contents) ### initializeArrayWithRange Initializes an array containing the numbers in the specified range where `start` and `end` are inclusive with their common difference `step`. Use `Array.from()` to create an array of the desired length, `(end - start + 1)/step`, and a map function to fill it with the desired values in the given range. You can omit `start` to use a default value of `0`. You can omit `step` to use a default value of `1`. ```js const initializeArrayWithRange = (end, start = 0, step = 1) => Array.from({ length: Math.ceil((end - start + 1) / step) }, (v, i) => i * step + start); ```
Examples ```js initializeArrayWithRange(5); // [0,1,2,3,4,5] initializeArrayWithRange(7, 3); // [3,4,5,6,7] initializeArrayWithRange(9, 0, 2); // [0,2,4,6,8] ```

[⬆ Back to top](#table-of-contents) ### initializeArrayWithRangeRight Initializes an array containing the numbers in the specified range (in reverse) where `start` and `end` are inclusive with their common difference `step`. Use `Array.from(Math.ceil((end+1-start)/step))` to create an array of the desired length(the amounts of elements is equal to `(end-start)/step` or `(end+1-start)/step` for inclusive end), `Array.prototype.map()` to fill with the desired values in a range. You can omit `start` to use a default value of `0`. You can omit `step` to use a default value of `1`. ```js const initializeArrayWithRangeRight = (end, start = 0, step = 1) => Array.from({ length: Math.ceil((end + 1 - start) / step) }).map( (v, i, arr) => (arr.length - i - 1) * step + start ); ```
Examples ```js initializeArrayWithRangeRight(5); // [5,4,3,2,1,0] initializeArrayWithRangeRight(7, 3); // [7,6,5,4,3] initializeArrayWithRangeRight(9, 0, 2); // [8,6,4,2,0] ```

[⬆ Back to top](#table-of-contents) ### initializeArrayWithValues Initializes and fills an array with the specified values. Use `Array(n)` to create an array of the desired length, `fill(v)` to fill it with the desired values. You can omit `val` to use a default value of `0`. ```js const initializeArrayWithValues = (n, val = 0) => Array(n).fill(val); ```
Examples ```js initializeArrayWithValues(5, 2); // [2, 2, 2, 2, 2] ```

[⬆ Back to top](#table-of-contents) ### initializeNDArray Create a n-dimensional array with given value. Use recursion. Use `Array.prototype.map()` to generate rows where each is a new array initialized using `initializeNDArray`. ```js const initializeNDArray = (val, ...args) => args.length === 0 ? val : Array.from({ length: args[0] }).map(() => initializeNDArray(val, ...args.slice(1))); ```
Examples ```js initializeNDArray(1, 3); // [1,1,1] initializeNDArray(5, 2, 2, 2); // [[[5,5],[5,5]],[[5,5],[5,5]]] ```

[⬆ Back to top](#table-of-contents) ### intersection Returns a list of elements that exist in both arrays. Create a `Set` from `b`, then use `Array.prototype.filter()` on `a` to only keep values contained in `b`. ```js const intersection = (a, b) => { const s = new Set(b); return a.filter(x => s.has(x)); }; ```
Examples ```js intersection([1, 2, 3], [4, 3, 2]); // [2, 3] ```

[⬆ Back to top](#table-of-contents) ### intersectionBy Returns a list of elements that exist in both arrays, after applying the provided function to each array element of both. Create a `Set` by applying `fn` to all elements in `b`, then use `Array.prototype.filter()` on `a` to only keep elements, which produce values contained in `b` when `fn` is applied to them. ```js const intersectionBy = (a, b, fn) => { const s = new Set(b.map(fn)); return a.filter(x => s.has(fn(x))); }; ```
Examples ```js intersectionBy([2.1, 1.2], [2.3, 3.4], Math.floor); // [2.1] ```

[⬆ Back to top](#table-of-contents) ### intersectionWith Returns a list of elements that exist in both arrays, using a provided comparator function. Use `Array.prototype.filter()` and `Array.prototype.findIndex()` in combination with the provided comparator to determine intersecting values. ```js const intersectionWith = (a, b, comp) => a.filter(x => b.findIndex(y => comp(x, y)) !== -1); ```
Examples ```js intersectionWith([1, 1.2, 1.5, 3, 0], [1.9, 3, 0, 3.9], (a, b) => Math.round(a) === Math.round(b)); // [1.5, 3, 0] ```

[⬆ Back to top](#table-of-contents) ### isSorted Returns `1` if the array is sorted in ascending order, `-1` if it is sorted in descending order or `0` if it is not sorted. Calculate the ordering `direction` for the first two elements. Use `Object.entries()` to loop over array objects and compare them in pairs. Return `0` if the `direction` changes or the `direction` if the last element is reached. ```js const isSorted = arr => { let direction = -(arr[0] - arr[1]); for (let [i, val] of arr.entries()) { direction = !direction ? -(arr[i - 1] - arr[i]) : direction; if (i === arr.length - 1) return !direction ? 0 : direction; else if ((val - arr[i + 1]) * direction > 0) return 0; } }; ```
Examples ```js isSorted([0, 1, 2, 2]); // 1 isSorted([4, 3, 2]); // -1 isSorted([4, 3, 5]); // 0 ```

[⬆ Back to top](#table-of-contents) ### join Joins all elements of an array into a string and returns this string. Uses a separator and an end separator. Use `Array.prototype.reduce()` to combine elements into a string. Omit the second argument, `separator`, to use a default separator of `','`. Omit the third argument, `end`, to use the same value as `separator` by default. ```js const join = (arr, separator = ',', end = separator) => arr.reduce( (acc, val, i) => i === arr.length - 2 ? acc + val + end : i === arr.length - 1 ? acc + val : acc + val + separator, '' ); ```
Examples ```js join(['pen', 'pineapple', 'apple', 'pen'], ',', '&'); // "pen,pineapple,apple&pen" join(['pen', 'pineapple', 'apple', 'pen'], ','); // "pen,pineapple,apple,pen" join(['pen', 'pineapple', 'apple', 'pen']); // "pen,pineapple,apple,pen" ```

[⬆ Back to top](#table-of-contents) ### JSONtoCSV ![advanced](/advanced.svg) Converts an array of objects to a comma-separated values (CSV) string that contains only the `columns` specified. Use `Array.prototype.join(demiliter)` to combine all the names in `columns` to create the first row. Use `Array.prototype.map()` and `Array.prototype.reduce()` to create a row for each object, substituting non-existent values with empty strings and only mapping values in `columns`. Use `Array.prototype.join('\n')` to combine all rows into a string. Omit the third argument, `delimiter`, to use a default delimiter of `,`. ```js const JSONtoCSV = (arr, columns, delimiter = ',') => [ columns.join(delimiter), ...arr.map(obj => columns.reduce( (acc, key) => `${acc}${!acc.length ? '' : delimiter}"${!obj[key] ? '' : obj[key]}"`, '' ) ) ].join('\n'); ```
Examples ```js JSONtoCSV([{ a: 1, b: 2 }, { a: 3, b: 4, c: 5 }, { a: 6 }, { b: 7 }], ['a', 'b']); // 'a,b\n"1","2"\n"3","4"\n"6",""\n"","7"' JSONtoCSV([{ a: 1, b: 2 }, { a: 3, b: 4, c: 5 }, { a: 6 }, { b: 7 }], ['a', 'b'], ';'); // 'a;b\n"1";"2"\n"3";"4"\n"6";""\n"";"7"' ```

[⬆ Back to top](#table-of-contents) ### last Returns the last element in an array. Use `arr.length - 1` to compute the index of the last element of the given array and returning it. ```js const last = arr => arr[arr.length - 1]; ```
Examples ```js last([1, 2, 3]); // 3 ```

[⬆ Back to top](#table-of-contents) ### longestItem Takes any number of iterable objects or objects with a `length` property and returns the longest one. If multiple objects have the same length, the first one will be returned. Returns `undefined` if no arguments are provided. Use `Array.prototype.reduce()`, comparing the `length` of objects to find the longest one. ```js const longestItem = (val, ...vals) => [val, ...vals].reduce((a, x) => (x.length > a.length ? x : a)); ```
Examples ```js longestItem('this', 'is', 'a', 'testcase'); // 'testcase' longestItem(...['a', 'ab', 'abc']); // 'abc' longestItem(...['a', 'ab', 'abc'], 'abcd'); // 'abcd' longestItem([1, 2, 3], [1, 2], [1, 2, 3, 4, 5]); // [1, 2, 3, 4, 5] longestItem([1, 2, 3], 'foobar'); // 'foobar' ```

[⬆ Back to top](#table-of-contents) ### mapObject ![advanced](/advanced.svg) Maps the values of an array to an object using a function, where the key-value pairs consist of the original value as the key and the mapped value. Use an anonymous inner function scope to declare an undefined memory space, using closures to store a return value. Use a new `Array` to store the array with a map of the function over its data set and a comma operator to return a second step, without needing to move from one context to another (due to closures and order of operations). ```js const mapObject = (arr, fn) => (a => ( (a = [arr, arr.map(fn)]), a[0].reduce((acc, val, ind) => ((acc[val] = a[1][ind]), acc), {}) ))(); ```
Examples ```js const squareIt = arr => mapObject(arr, a => a * a); squareIt([1, 2, 3]); // { 1: 1, 2: 4, 3: 9 } ```

[⬆ Back to top](#table-of-contents) ### maxN Returns the `n` maximum elements from the provided array. If `n` is greater than or equal to the provided array's length, then return the original array (sorted in descending order). Use `Array.prototype.sort()` combined with the spread operator (`...`) to create a shallow clone of the array and sort it in descending order. Use `Array.prototype.slice()` to get the specified number of elements. Omit the second argument, `n`, to get a one-element array. ```js const maxN = (arr, n = 1) => [...arr].sort((a, b) => b - a).slice(0, n); ```
Examples ```js maxN([1, 2, 3]); // [3] maxN([1, 2, 3], 2); // [3,2] ```

[⬆ Back to top](#table-of-contents) ### minN Returns the `n` minimum elements from the provided array. If `n` is greater than or equal to the provided array's length, then return the original array (sorted in ascending order). Use `Array.prototype.sort()` combined with the spread operator (`...`) to create a shallow clone of the array and sort it in ascending order. Use `Array.prototype.slice()` to get the specified number of elements. Omit the second argument, `n`, to get a one-element array. ```js const minN = (arr, n = 1) => [...arr].sort((a, b) => a - b).slice(0, n); ```
Examples ```js minN([1, 2, 3]); // [1] minN([1, 2, 3], 2); // [1,2] ```

[⬆ Back to top](#table-of-contents) ### none Returns `true` if the provided predicate function returns `false` for all elements in a collection, `false` otherwise. Use `Array.prototype.some()` to test if any elements in the collection return `true` based on `fn`. Omit the second argument, `fn`, to use `Boolean` as a default. ```js const none = (arr, fn = Boolean) => !arr.some(fn); ```
Examples ```js none([0, 1, 3, 0], x => x == 2); // true none([0, 0, 0]); // true ```

[⬆ Back to top](#table-of-contents) ### nthElement Returns the nth element of an array. Use `Array.prototype.slice()` to get an array containing the nth element at the first place. If the index is out of bounds, return `undefined`. Omit the second argument, `n`, to get the first element of the array. ```js const nthElement = (arr, n = 0) => (n === -1 ? arr.slice(n) : arr.slice(n, n + 1))[0]; ```
Examples ```js nthElement(['a', 'b', 'c'], 1); // 'b' nthElement(['a', 'b', 'b'], -3); // 'a' ```

[⬆ Back to top](#table-of-contents) ### offset Moves the specified amount of elements to the end of the array. Use `Array.prototype.slice()` twice to get the elements after the specified index and the elements before that. Use the spread operator(`...`) to combine the two into one array. If `offset` is negative, the elements will be moved from end to start. ```js const offset = (arr, offset) => [...arr.slice(offset), ...arr.slice(0, offset)]; ```
Examples ```js offset([1, 2, 3, 4, 5], 2); // [3, 4, 5, 1, 2] offset([1, 2, 3, 4, 5], -2); // [4, 5, 1, 2, 3] ```

[⬆ Back to top](#table-of-contents) ### partition Groups the elements into two arrays, depending on the provided function's truthiness for each element. Use `Array.prototype.reduce()` to create an array of two arrays. Use `Array.prototype.push()` to add elements for which `fn` returns `true` to the first array and elements for which `fn` returns `false` to the second one. ```js const partition = (arr, fn) => arr.reduce( (acc, val, i, arr) => { acc[fn(val, i, arr) ? 0 : 1].push(val); return acc; }, [[], []] ); ```
Examples ```js const users = [{ user: 'barney', age: 36, active: false }, { user: 'fred', age: 40, active: true }]; partition(users, o => o.active); // [[{ 'user': 'fred', 'age': 40, 'active': true }],[{ 'user': 'barney', 'age': 36, 'active': false }]] ```

[⬆ Back to top](#table-of-contents) ### permutations ![advanced](/advanced.svg) ⚠️ **WARNING**: This function's execution time increases exponentially with each array element. Anything more than 8 to 10 entries will cause your browser to hang as it tries to solve all the different combinations. Generates all permutations of an array's elements (contains duplicates). Use recursion. For each element in the given array, create all the partial permutations for the rest of its elements. Use `Array.prototype.map()` to combine the element with each partial permutation, then `Array.prototype.reduce()` to combine all permutations in one array. Base cases are for array `length` equal to `2` or `1`. ```js const permutations = arr => { if (arr.length <= 2) return arr.length === 2 ? [arr, [arr[1], arr[0]]] : arr; return arr.reduce( (acc, item, i) => acc.concat( permutations([...arr.slice(0, i), ...arr.slice(i + 1)]).map(val => [item, ...val]) ), [] ); }; ```
Examples ```js permutations([1, 33, 5]); // [ [ 1, 33, 5 ], [ 1, 5, 33 ], [ 33, 1, 5 ], [ 33, 5, 1 ], [ 5, 1, 33 ], [ 5, 33, 1 ] ] ```

[⬆ Back to top](#table-of-contents) ### pull Mutates the original array to filter out the values specified. Use `Array.prototype.filter()` and `Array.prototype.includes()` to pull out the values that are not needed. Use `Array.prototype.length = 0` to mutate the passed in an array by resetting it's length to zero and `Array.prototype.push()` to re-populate it with only the pulled values. _(For a snippet that does not mutate the original array see [`without`](#without))_ ```js const pull = (arr, ...args) => { let argState = Array.isArray(args[0]) ? args[0] : args; let pulled = arr.filter((v, i) => !argState.includes(v)); arr.length = 0; pulled.forEach(v => arr.push(v)); }; ```
Examples ```js let myArray = ['a', 'b', 'c', 'a', 'b', 'c']; pull(myArray, 'a', 'c'); // myArray = [ 'b', 'b' ] ```

[⬆ Back to top](#table-of-contents) ### pullAtIndex ![advanced](/advanced.svg) Mutates the original array to filter out the values at the specified indexes. Use `Array.prototype.filter()` and `Array.prototype.includes()` to pull out the values that are not needed. Use `Array.prototype.length = 0` to mutate the passed in an array by resetting it's length to zero and `Array.prototype.push()` to re-populate it with only the pulled values. Use `Array.prototype.push()` to keep track of pulled values ```js const pullAtIndex = (arr, pullArr) => { let removed = []; let pulled = arr .map((v, i) => (pullArr.includes(i) ? removed.push(v) : v)) .filter((v, i) => !pullArr.includes(i)); arr.length = 0; pulled.forEach(v => arr.push(v)); return removed; }; ```
Examples ```js let myArray = ['a', 'b', 'c', 'd']; let pulled = pullAtIndex(myArray, [1, 3]); // myArray = [ 'a', 'c' ] , pulled = [ 'b', 'd' ] ```

[⬆ Back to top](#table-of-contents) ### pullAtValue ![advanced](/advanced.svg) Mutates the original array to filter out the values specified. Returns the removed elements. Use `Array.prototype.filter()` and `Array.prototype.includes()` to pull out the values that are not needed. Use `Array.prototype.length = 0` to mutate the passed in an array by resetting it's length to zero and `Array.prototype.push()` to re-populate it with only the pulled values. Use `Array.prototype.push()` to keep track of pulled values ```js const pullAtValue = (arr, pullArr) => { let removed = [], pushToRemove = arr.forEach((v, i) => (pullArr.includes(v) ? removed.push(v) : v)), mutateTo = arr.filter((v, i) => !pullArr.includes(v)); arr.length = 0; mutateTo.forEach(v => arr.push(v)); return removed; }; ```
Examples ```js let myArray = ['a', 'b', 'c', 'd']; let pulled = pullAtValue(myArray, ['b', 'd']); // myArray = [ 'a', 'c' ] , pulled = [ 'b', 'd' ] ```

[⬆ Back to top](#table-of-contents) ### pullBy ![advanced](/advanced.svg) Mutates the original array to filter out the values specified, based on a given iterator function. Check if the last argument provided in a function. Use `Array.prototype.map()` to apply the iterator function `fn` to all array elements. Use `Array.prototype.filter()` and `Array.prototype.includes()` to pull out the values that are not needed. Use `Array.prototype.length = 0` to mutate the passed in an array by resetting it's length to zero and `Array.prototype.push()` to re-populate it with only the pulled values. ```js const pullBy = (arr, ...args) => { const length = args.length; let fn = length > 1 ? args[length - 1] : undefined; fn = typeof fn == 'function' ? (args.pop(), fn) : undefined; let argState = (Array.isArray(args[0]) ? args[0] : args).map(val => fn(val)); let pulled = arr.filter((v, i) => !argState.includes(fn(v))); arr.length = 0; pulled.forEach(v => arr.push(v)); }; ```
Examples ```js var myArray = [{ x: 1 }, { x: 2 }, { x: 3 }, { x: 1 }]; pullBy(myArray, [{ x: 1 }, { x: 3 }], o => o.x); // myArray = [{ x: 2 }] ```

[⬆ Back to top](#table-of-contents) ### reducedFilter Filter an array of objects based on a condition while also filtering out unspecified keys. Use `Array.prototype.filter()` to filter the array based on the predicate `fn` so that it returns the objects for which the condition returned a truthy value. On the filtered array, use `Array.prototype.map()` to return the new object using `Array.prototype.reduce()` to filter out the keys which were not supplied as the `keys` argument. ```js const reducedFilter = (data, keys, fn) => data.filter(fn).map(el => keys.reduce((acc, key) => { acc[key] = el[key]; return acc; }, {}) ); ```
Examples ```js const data = [ { id: 1, name: 'john', age: 24 }, { id: 2, name: 'mike', age: 50 } ]; reducedFilter(data, ['id', 'name'], item => item.age > 24); // [{ id: 2, name: 'mike'}] ```

[⬆ Back to top](#table-of-contents) ### reduceSuccessive Applies a function against an accumulator and each element in the array (from left to right), returning an array of successively reduced values. Use `Array.prototype.reduce()` to apply the given function to the given array, storing each new result. ```js const reduceSuccessive = (arr, fn, acc) => arr.reduce((res, val, i, arr) => (res.push(fn(res.slice(-1)[0], val, i, arr)), res), [acc]); ```
Examples ```js reduceSuccessive([1, 2, 3, 4, 5, 6], (acc, val) => acc + val, 0); // [0, 1, 3, 6, 10, 15, 21] ```

[⬆ Back to top](#table-of-contents) ### reduceWhich Returns the minimum/maximum value of an array, after applying the provided function to set comparing rule. Use `Array.prototype.reduce()` in combination with the `comparator` function to get the appropriate element in the array. You can omit the second parameter, `comparator`, to use the default one that returns the minimum element in the array. ```js const reduceWhich = (arr, comparator = (a, b) => a - b) => arr.reduce((a, b) => (comparator(a, b) >= 0 ? b : a)); ```
Examples ```js reduceWhich([1, 3, 2]); // 1 reduceWhich([1, 3, 2], (a, b) => b - a); // 3 reduceWhich( [{ name: 'Tom', age: 12 }, { name: 'Jack', age: 18 }, { name: 'Lucy', age: 9 }], (a, b) => a.age - b.age ); // {name: "Lucy", age: 9} ```

[⬆ Back to top](#table-of-contents) ### reject Takes a predicate and array, like `Array.prototype.filter()`, but only keeps `x` if `pred(x) === false`. ```js const reject = (pred, array) => array.filter((...args) => !pred(...args)); ```
Examples ```js reject(x => x % 2 === 0, [1, 2, 3, 4, 5]); // [1, 3, 5] reject(word => word.length > 4, ['Apple', 'Pear', 'Kiwi', 'Banana']); // ['Pear', 'Kiwi'] ```

[⬆ Back to top](#table-of-contents) ### remove Removes elements from an array for which the given function returns `false`. Use `Array.prototype.filter()` to find array elements that return truthy values and `Array.prototype.reduce()` to remove elements using `Array.prototype.splice()`. The `func` is invoked with three arguments (`value, index, array`). ```js const remove = (arr, func) => Array.isArray(arr) ? arr.filter(func).reduce((acc, val) => { arr.splice(arr.indexOf(val), 1); return acc.concat(val); }, []) : []; ```
Examples ```js remove([1, 2, 3, 4], n => n % 2 === 0); // [2, 4] ```

[⬆ Back to top](#table-of-contents) ### sample Returns a random element from an array. Use `Math.random()` to generate a random number, multiply it by `length` and round it of to the nearest whole number using `Math.floor()`. This method also works with strings. ```js const sample = arr => arr[Math.floor(Math.random() * arr.length)]; ```
Examples ```js sample([3, 7, 9, 11]); // 9 ```

[⬆ Back to top](#table-of-contents) ### sampleSize Gets `n` random elements at unique keys from `array` up to the size of `array`. Shuffle the array using the [Fisher-Yates algorithm](https://github.com/30-seconds/30-seconds-of-code#shuffle). Use `Array.prototype.slice()` to get the first `n` elements. Omit the second argument, `n` to get only one element at random from the array. ```js const sampleSize = ([...arr], n = 1) => { let m = arr.length; while (m) { const i = Math.floor(Math.random() * m--); [arr[m], arr[i]] = [arr[i], arr[m]]; } return arr.slice(0, n); }; ```
Examples ```js sampleSize([1, 2, 3], 2); // [3,1] sampleSize([1, 2, 3], 4); // [2,3,1] ```

[⬆ Back to top](#table-of-contents) ### shank Has the same functionality as [`Array.prototype.prototype.splice()`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/splice), but returning a new array instead of mutating the original array. Use `Array.prototype.slice()` and `Array.prototype.concat()` to get a new array with the new contents after removing existing elements and/or adding new elements. Omit the second argument, `index`, to start at `0`. Omit the third argument, `delCount`, to remove `0` elements. Omit the fourth argument, `elements`, in order to not add any new elements. ```js const shank = (arr, index = 0, delCount = 0, ...elements) => arr .slice(0, index) .concat(elements) .concat(arr.slice(index + delCount)); ```
Examples ```js const names = ['alpha', 'bravo', 'charlie']; const namesAndDelta = shank(names, 1, 0, 'delta'); // [ 'alpha', 'delta', 'bravo', 'charlie' ] const namesNoBravo = shank(names, 1, 1); // [ 'alpha', 'charlie' ] console.log(names); // ['alpha', 'bravo', 'charlie'] ```

[⬆ Back to top](#table-of-contents) ### shuffle Randomizes the order of the values of an array, returning a new array. Uses the [Fisher-Yates algorithm](https://github.com/30-seconds/30-seconds-of-code#shuffle) to reorder the elements of the array. ```js const shuffle = ([...arr]) => { let m = arr.length; while (m) { const i = Math.floor(Math.random() * m--); [arr[m], arr[i]] = [arr[i], arr[m]]; } return arr; }; ```
Examples ```js const foo = [1, 2, 3]; shuffle(foo); // [2, 3, 1], foo = [1, 2, 3] ```

[⬆ Back to top](#table-of-contents) ### similarity Returns an array of elements that appear in both arrays. Use `Array.prototype.filter()` to remove values that are not part of `values`, determined using `Array.prototype.includes()`. ```js const similarity = (arr, values) => arr.filter(v => values.includes(v)); ```
Examples ```js similarity([1, 2, 3], [1, 2, 4]); // [1, 2] ```

[⬆ Back to top](#table-of-contents) ### sortedIndex Returns the lowest index at which value should be inserted into array in order to maintain its sort order. Check if the array is sorted in descending order (loosely). Use `Array.prototype.findIndex()` to find the appropriate index where the element should be inserted. ```js const sortedIndex = (arr, n) => { const isDescending = arr[0] > arr[arr.length - 1]; const index = arr.findIndex(el => (isDescending ? n >= el : n <= el)); return index === -1 ? arr.length : index; }; ```
Examples ```js sortedIndex([5, 3, 2, 1], 4); // 1 sortedIndex([30, 50], 40); // 1 ```

[⬆ Back to top](#table-of-contents) ### sortedIndexBy Returns the lowest index at which value should be inserted into array in order to maintain its sort order, based on a provided iterator function. Check if the array is sorted in descending order (loosely). Use `Array.prototype.findIndex()` to find the appropriate index where the element should be inserted, based on the iterator function `fn`. ```js const sortedIndexBy = (arr, n, fn) => { const isDescending = fn(arr[0]) > fn(arr[arr.length - 1]); const val = fn(n); const index = arr.findIndex(el => (isDescending ? val >= fn(el) : val <= fn(el))); return index === -1 ? arr.length : index; }; ```
Examples ```js sortedIndexBy([{ x: 4 }, { x: 5 }], { x: 4 }, o => o.x); // 0 ```

[⬆ Back to top](#table-of-contents) ### sortedLastIndex Returns the highest index at which value should be inserted into array in order to maintain its sort order. Check if the array is sorted in descending order (loosely). Use `Array.prototype.reverse()` and `Array.prototype.findIndex()` to find the appropriate last index where the element should be inserted. ```js const sortedLastIndex = (arr, n) => { const isDescending = arr[0] > arr[arr.length - 1]; const index = arr.reverse().findIndex(el => (isDescending ? n <= el : n >= el)); return index === -1 ? 0 : arr.length - index; }; ```
Examples ```js sortedLastIndex([10, 20, 30, 30, 40], 30); // 4 ```

[⬆ Back to top](#table-of-contents) ### sortedLastIndexBy Returns the highest index at which value should be inserted into array in order to maintain its sort order, based on a provided iterator function. Check if the array is sorted in descending order (loosely). Use `Array.prototype.map()` to apply the iterator function to all elements of the array. Use `Array.prototype.reverse()` and `Array.prototype.findIndex()` to find the appropriate last index where the element should be inserted, based on the provided iterator function. ```js const sortedLastIndexBy = (arr, n, fn) => { const isDescending = fn(arr[0]) > fn(arr[arr.length - 1]); const val = fn(n); const index = arr .map(fn) .reverse() .findIndex(el => (isDescending ? val <= el : val >= el)); return index === -1 ? 0 : arr.length - index; }; ```
Examples ```js sortedLastIndexBy([{ x: 4 }, { x: 5 }], { x: 4 }, o => o.x); // 1 ```

[⬆ Back to top](#table-of-contents) ### stableSort ![advanced](/advanced.svg) Performs stable sorting of an array, preserving the initial indexes of items when their values are the same. Does not mutate the original array, but returns a new array instead. Use `Array.prototype.map()` to pair each element of the input array with its corresponding index. Use `Array.prototype.sort()` and a `compare` function to sort the list, preserving their initial order if the items compared are equal. Use `Array.prototype.map()` to convert back to the initial array items. ```js const stableSort = (arr, compare) => arr .map((item, index) => ({ item, index })) .sort((a, b) => compare(a.item, b.item) || a.index - b.index) .map(({ item }) => item); ```
Examples ```js const arr = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10]; const stable = stableSort(arr, () => 0); // [0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10] ```

[⬆ Back to top](#table-of-contents) ### symmetricDifference Returns the symmetric difference between two arrays, without filtering out duplicate values. Create a `Set` from each array, then use `Array.prototype.filter()` on each of them to only keep values not contained in the other. ```js const symmetricDifference = (a, b) => { const sA = new Set(a), sB = new Set(b); return [...a.filter(x => !sB.has(x)), ...b.filter(x => !sA.has(x))]; }; ```
Examples ```js symmetricDifference([1, 2, 3], [1, 2, 4]); // [3, 4] symmetricDifference([1, 2, 2], [1, 3, 1]); // [2, 2, 3] ```

[⬆ Back to top](#table-of-contents) ### symmetricDifferenceBy Returns the symmetric difference between two arrays, after applying the provided function to each array element of both. Create a `Set` by applying `fn` to each array's elements, then use `Array.prototype.filter()` on each of them to only keep values not contained in the other. ```js const symmetricDifferenceBy = (a, b, fn) => { const sA = new Set(a.map(v => fn(v))), sB = new Set(b.map(v => fn(v))); return [...a.filter(x => !sB.has(fn(x))), ...b.filter(x => !sA.has(fn(x)))]; }; ```
Examples ```js symmetricDifferenceBy([2.1, 1.2], [2.3, 3.4], Math.floor); // [ 1.2, 3.4 ] ```

[⬆ Back to top](#table-of-contents) ### symmetricDifferenceWith Returns the symmetric difference between two arrays, using a provided function as a comparator. Use `Array.prototype.filter()` and `Array.prototype.findIndex()` to find the appropriate values. ```js const symmetricDifferenceWith = (arr, val, comp) => [ ...arr.filter(a => val.findIndex(b => comp(a, b)) === -1), ...val.filter(a => arr.findIndex(b => comp(a, b)) === -1) ]; ```
Examples ```js symmetricDifferenceWith( [1, 1.2, 1.5, 3, 0], [1.9, 3, 0, 3.9], (a, b) => Math.round(a) === Math.round(b) ); // [1, 1.2, 3.9] ```

[⬆ Back to top](#table-of-contents) ### tail Returns all elements in an array except for the first one. Return `Array.prototype.slice(1)` if the array's `length` is more than `1`, otherwise, return the whole array. ```js const tail = arr => (arr.length > 1 ? arr.slice(1) : arr); ```
Examples ```js tail([1, 2, 3]); // [2,3] tail([1]); // [1] ```

[⬆ Back to top](#table-of-contents) ### take Returns an array with n elements removed from the beginning. Use `Array.prototype.slice()` to create a slice of the array with `n` elements taken from the beginning. ```js const take = (arr, n = 1) => arr.slice(0, n); ```
Examples ```js take([1, 2, 3], 5); // [1, 2, 3] take([1, 2, 3], 0); // [] ```

[⬆ Back to top](#table-of-contents) ### takeRight Returns an array with n elements removed from the end. Use `Array.prototype.slice()` to create a slice of the array with `n` elements taken from the end. ```js const takeRight = (arr, n = 1) => arr.slice(arr.length - n, arr.length); ```
Examples ```js takeRight([1, 2, 3], 2); // [ 2, 3 ] takeRight([1, 2, 3]); // [3] ```

[⬆ Back to top](#table-of-contents) ### takeRightWhile Removes elements from the end of an array until the passed function returns `true`. Returns the removed elements. Loop through the array, using a `Array.prototype.reduceRight()` and accumulating elements while the function returns falsy value. ```js const takeRightWhile = (arr, func) => arr.reduceRight((acc, el) => (func(el) ? acc : [el, ...acc]), []); ```
Examples ```js takeRightWhile([1, 2, 3, 4], n => n < 3); // [3, 4] ```

[⬆ Back to top](#table-of-contents) ### takeWhile Removes elements in an array until the passed function returns `true`. Returns the removed elements. Loop through the array, using a `for...of` loop over `Array.prototype.entries()` until the returned value from the function is `true`. Return the removed elements, using `Array.prototype.slice()`. ```js const takeWhile = (arr, func) => { for (const [i, val] of arr.entries()) if (func(val)) return arr.slice(0, i); return arr; }; ```
Examples ```js takeWhile([1, 2, 3, 4], n => n >= 3); // [1, 2] ```

[⬆ Back to top](#table-of-contents) ### toHash Reduces a given Array-like into a value hash (keyed data store). Given an Iterable or Array-like structure, call `Array.prototype.prototype.reduce.call()` on the provided object to step over it and return an Object, keyed by the reference value. ```js const toHash = (object, key) => Array.prototype.reduce.call( object, (acc, data, index) => ((acc[!key ? index : data[key]] = data), acc), {} ); ```
Examples ```js toHash([4, 3, 2, 1]); // { 0: 4, 1: 3, 2: 2, 1: 1 } toHash([{ a: 'label' }], 'a'); // { label: { a: 'label' } } // A more in depth example: let users = [{ id: 1, first: 'Jon' }, { id: 2, first: 'Joe' }, { id: 3, first: 'Moe' }]; let managers = [{ manager: 1, employees: [2, 3] }]; // We use function here because we want a bindable reference, but a closure referencing the hash would work, too. managers.forEach( manager => (manager.employees = manager.employees.map(function(id) { return this[id]; }, toHash(users, 'id'))) ); managers; // [ { manager:1, employees: [ { id: 2, first: "Joe" }, { id: 3, first: "Moe" } ] } ] ```

[⬆ Back to top](#table-of-contents) ### union Returns every element that exists in any of the two arrays once. Create a `Set` with all values of `a` and `b` and convert to an array. ```js const union = (a, b) => Array.from(new Set([...a, ...b])); ```
Examples ```js union([1, 2, 3], [4, 3, 2]); // [1,2,3,4] ```

[⬆ Back to top](#table-of-contents) ### unionBy Returns every element that exists in any of the two arrays once, after applying the provided function to each array element of both. Create a `Set` by applying all `fn` to all values of `a`. Create a `Set` from `a` and all elements in `b` whose value, after applying `fn` does not match a value in the previously created set. Return the last set converted to an array. ```js const unionBy = (a, b, fn) => { const s = new Set(a.map(v => fn(v))); return Array.from(new Set([...a, ...b.filter(x => !s.has(fn(x)))])); }; ```
Examples ```js unionBy([2.1], [1.2, 2.3], Math.floor); // [2.1, 1.2] ```

[⬆ Back to top](#table-of-contents) ### unionWith Returns every element that exists in any of the two arrays once, using a provided comparator function. Create a `Set` with all values of `a` and values in `b` for which the comparator finds no matches in `a`, using `Array.prototype.findIndex()`. ```js const unionWith = (a, b, comp) => Array.from(new Set([...a, ...b.filter(x => a.findIndex(y => comp(x, y)) === -1)])); ```
Examples ```js unionWith([1, 1.2, 1.5, 3, 0], [1.9, 3, 0, 3.9], (a, b) => Math.round(a) === Math.round(b)); // [1, 1.2, 1.5, 3, 0, 3.9] ```

[⬆ Back to top](#table-of-contents) ### uniqueElements Returns all unique values of an array. Use ES6 `Set` and the `...rest` operator to discard all duplicated values. ```js const uniqueElements = arr => [...new Set(arr)]; ```
Examples ```js uniqueElements([1, 2, 2, 3, 4, 4, 5]); // [1, 2, 3, 4, 5] ```

[⬆ Back to top](#table-of-contents) ### uniqueElementsBy Returns all unique values of an array, based on a provided comparator function. Use `Array.prototype.reduce()` and `Array.prototype.some()` for an array containing only the first unique occurence of each value, based on the comparator function, `fn`. The comparator function takes two arguments: the values of the two elements being compared. ```js const uniqueElementsBy = (arr, fn) => arr.reduce((acc, v) => { if (!acc.some(x => fn(v, x))) acc.push(v); return acc; }, []); ```
Examples ```js uniqueElementsBy( [ { id: 0, value: 'a' }, { id: 1, value: 'b' }, { id: 2, value: 'c' }, { id: 1, value: 'd' }, { id: 0, value: 'e' } ], (a, b) => a.id == b.id ); // [ { id: 0, value: 'a' }, { id: 1, value: 'b' }, { id: 2, value: 'c' } ] ```

[⬆ Back to top](#table-of-contents) ### uniqueElementsByRight Returns all unique values of an array, based on a provided comparator function. Use `Array.prototype.reduce()` and `Array.prototype.some()` for an array containing only the last unique occurence of each value, based on the comparator function, `fn`. The comparator function takes two arguments: the values of the two elements being compared. ```js const uniqueElementsByRight = (arr, fn) => arr.reduceRight((acc, v) => { if (!acc.some(x => fn(v, x))) acc.push(v); return acc; }, []); ```
Examples ```js uniqueElementsByRight( [ { id: 0, value: 'a' }, { id: 1, value: 'b' }, { id: 2, value: 'c' }, { id: 1, value: 'd' }, { id: 0, value: 'e' } ], (a, b) => a.id == b.id ); // [ { id: 0, value: 'e' }, { id: 1, value: 'd' }, { id: 2, value: 'c' } ] ```

[⬆ Back to top](#table-of-contents) ### uniqueSymmetricDifference Returns the unique symmetric difference between two arrays, not containing duplicate values from either array. Use `Array.prototype.filter()` and `Array.prototype.includes()` on each array to remove values contained in the other, then create a `Set` from the results, removing duplicate values. ```js const uniqueSymmetricDifference = (a, b) => [ ...new Set([...a.filter(v => !b.includes(v)), ...b.filter(v => !a.includes(v))]) ]; ```
Examples ```js uniqueSymmetricDifference([1, 2, 3], [1, 2, 4]); // [3, 4] uniqueSymmetricDifference([1, 2, 2], [1, 3, 1]); // [2, 3] ```

[⬆ Back to top](#table-of-contents) ### unzip Creates an array of arrays, ungrouping the elements in an array produced by [zip](#zip). Use `Math.max.apply()` to get the longest subarray in the array, `Array.prototype.map()` to make each element an array. Use `Array.prototype.reduce()` and `Array.prototype.forEach()` to map grouped values to individual arrays. ```js const unzip = arr => arr.reduce( (acc, val) => (val.forEach((v, i) => acc[i].push(v)), acc), Array.from({ length: Math.max(...arr.map(x => x.length)) }).map(x => []) ); ```
Examples ```js unzip([['a', 1, true], ['b', 2, false]]); //[['a', 'b'], [1, 2], [true, false]] unzip([['a', 1, true], ['b', 2]]); //[['a', 'b'], [1, 2], [true]] ```

[⬆ Back to top](#table-of-contents) ### unzipWith ![advanced](/advanced.svg) Creates an array of elements, ungrouping the elements in an array produced by [zip](#zip) and applying the provided function. Use `Math.max.apply()` to get the longest subarray in the array, `Array.prototype.map()` to make each element an array. Use `Array.prototype.reduce()` and `Array.prototype.forEach()` to map grouped values to individual arrays. Use `Array.prototype.map()` and the spread operator (`...`) to apply `fn` to each individual group of elements. ```js const unzipWith = (arr, fn) => arr .reduce( (acc, val) => (val.forEach((v, i) => acc[i].push(v)), acc), Array.from({ length: Math.max(...arr.map(x => x.length)) }).map(x => []) ) .map(val => fn(...val)); ```
Examples ```js unzipWith([[1, 10, 100], [2, 20, 200]], (...args) => args.reduce((acc, v) => acc + v, 0)); // [3, 30, 300] ```

[⬆ Back to top](#table-of-contents) ### without Filters out the elements of an array, that have one of the specified values. Use `Array.prototype.filter()` to create an array excluding(using `!Array.includes()`) all given values. _(For a snippet that mutates the original array see [`pull`](#pull))_ ```js const without = (arr, ...args) => arr.filter(v => !args.includes(v)); ```
Examples ```js without([2, 1, 2, 3], 1, 2); // [3] ```

[⬆ Back to top](#table-of-contents) ### xProd Creates a new array out of the two supplied by creating each possible pair from the arrays. Use `Array.prototype.reduce()`, `Array.prototype.map()` and `Array.prototype.concat()` to produce every possible pair from the elements of the two arrays and save them in an array. ```js const xProd = (a, b) => a.reduce((acc, x) => acc.concat(b.map(y => [x, y])), []); ```
Examples ```js xProd([1, 2], ['a', 'b']); // [[1, 'a'], [1, 'b'], [2, 'a'], [2, 'b']] ```

[⬆ Back to top](#table-of-contents) ### zip Creates an array of elements, grouped based on the position in the original arrays. Use `Math.max.apply()` to get the longest array in the arguments. Creates an array with that length as return value and use `Array.from()` with a map-function to create an array of grouped elements. If lengths of the argument-arrays vary, `undefined` is used where no value could be found. ```js const zip = (...arrays) => { const maxLength = Math.max(...arrays.map(x => x.length)); return Array.from({ length: maxLength }).map((_, i) => { return Array.from({ length: arrays.length }, (_, k) => arrays[k][i]); }); }; ```
Examples ```js zip(['a', 'b'], [1, 2], [true, false]); // [['a', 1, true], ['b', 2, false]] zip(['a'], [1, 2], [true, false]); // [['a', 1, true], [undefined, 2, false]] ```

[⬆ Back to top](#table-of-contents) ### zipObject Given an array of valid property identifiers and an array of values, return an object associating the properties to the values. Since an object can have undefined values but not undefined property pointers, the array of properties is used to decide the structure of the resulting object using `Array.prototype.reduce()`. ```js const zipObject = (props, values) => props.reduce((obj, prop, index) => ((obj[prop] = values[index]), obj), {}); ```
Examples ```js zipObject(['a', 'b', 'c'], [1, 2]); // {a: 1, b: 2, c: undefined} zipObject(['a', 'b'], [1, 2, 3]); // {a: 1, b: 2} ```

[⬆ Back to top](#table-of-contents) ### zipWith ![advanced](/advanced.svg) Creates an array of elements, grouped based on the position in the original arrays and using function as the last value to specify how grouped values should be combined. Check if the last argument provided is a function. Use `Math.max()` to get the longest array in the arguments. Creates an array with that length as return value and use `Array.from()` with a map-function to create an array of grouped elements. If lengths of the argument-arrays vary, `undefined` is used where no value could be found. The function is invoked with the elements of each group `(...group)`. ```js const zipWith = (...array) => { const fn = typeof array[array.length - 1] === 'function' ? array.pop() : undefined; return Array.from( { length: Math.max(...array.map(a => a.length)) }, (_, i) => (fn ? fn(...array.map(a => a[i])) : array.map(a => a[i])) ); }; ```
Examples ```js zipWith([1, 2], [10, 20], [100, 200], (a, b, c) => a + b + c); // [111,222] zipWith( [1, 2, 3], [10, 20], [100, 200], (a, b, c) => (a != null ? a : 'a') + (b != null ? b : 'b') + (c != null ? c : 'c') ); // [111, 222, '3bc'] ```

[⬆ Back to top](#table-of-contents) --- ## 🌐 Browser ### arrayToHtmlList Converts the given array elements into `
  • ` tags and appends them to the list of the given id. Use `Array.prototype.map()`, `document.querySelector()`, and an anonymous inner closure to create a list of html tags. ```js const arrayToHtmlList = (arr, listID) => (el => ( (el = document.querySelector('#' + listID)), (el.innerHTML += arr.map(item => `
  • ${item}
    • `).join('')) ))(); ```
    Examples ```js arrayToHtmlList(['item 1', 'item 2'], 'myListID'); ```

    [⬆ Back to top](#table-of-contents) ### bottomVisible Returns `true` if the bottom of the page is visible, `false` otherwise. Use `scrollY`, `scrollHeight` and `clientHeight` to determine if the bottom of the page is visible. ```js const bottomVisible = () => document.documentElement.clientHeight + window.scrollY >= (document.documentElement.scrollHeight || document.documentElement.clientHeight); ```
    Examples ```js bottomVisible(); // true ```

    [⬆ Back to top](#table-of-contents) ### copyToClipboard ![advanced](/advanced.svg) ⚠️ **NOTICE:** The same functionality can be easily implemented by using the new asynchronous Clipboard API, which is still experimental but should be used in the future instead of this snippet. Find out more about it [here](https://github.com/w3c/clipboard-apis/blob/master/explainer.adoc#writing-to-the-clipboard). Copy a string to the clipboard. Only works as a result of user action (i.e. inside a `click` event listener). Create a new `