Getting a random value from a JavaScript array

Question

Consider   var myArray     January    February    March          How can I select a random value from this array using JavaScript

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This is similar to  but more general than   Jacob Relkin s solution   This is ES2015   const randomChoice   arr   gt        const randIndex   Math floor Math random     arr length       return arr randIndex        The code works by selecting a random number between 0 and the length of the array  then returning the item at that index

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It s a simple one-liner  const randomElement   array Math floor Math random     array length     For example   x000D   x000D  const months     January    February    March    April    May    June    July     const random   Math floor Math random     months length   console log random  months random    x000D   x000D   x000D

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x000D   x000D  static generateMonth      x000D  const theDate     January    February    March     x000D  const randomNumber   Math floor Math random   3   x000D  return theDate randomNumber   x000D     x000D   x000D   x000D    You set a constant variable to the array  you then have another constant that chooses randomly between the three objects in the array and then the function simply returns the results

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By adding a method on prototype of array you can get random values easly  In this example you can get single or multiple random values from array  You can run to test code by clicking snippet button   x000D   x000D  Array prototype random   function n     if n amp  amp n gt 1       const a           for let i   0 i lt n i           a push this Math floor Math random   this length               return a      else       return this Math floor Math random   this length           const mySampleArray      a   b   c   d   e   f   g   h     mySampleArray random       return any random value etc   a    b  mySampleArray random 3     retun an array with random values etc    b   f   a       d   b   d    alert mySampleArray random     alert mySampleArray random 3    x000D   x000D   x000D

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Create one random value and pass to array  Please try following code      For Search textbox random value var myPlaceHolderArray     Hotels in New York       Hotels in San Francisco       Hotels Near Disney World       Hotels in Atlanta       var rand   Math floor Math random     myPlaceHolderArray length   var Placeholdervalue   myPlaceHolderArray rand    alert Placeholdervalue

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I ve found a way around the top answer s complications  just by concatenating the variable rand to another variable that allows that number to be displayed inside the calling of myArray     By deleting the new array created and toying around with it s complications  I ve come up with a working solution    lt  DOCTYPE html gt   lt html gt   lt body gt    lt p id  demo  gt  lt  p gt    lt script gt   var myArray     January    February    March    April    May         var rand   Math floor Math random     myArray length    var concat   myArray rand    function random        document getElementById  demo   innerHTML    concat      lt  script gt    lt button onClick  random     gt  Working Random Array generator  lt  button gt    lt  body gt   lt  html gt

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To get crypto-strong random item form array use   x000D   x000D  let rndItem   a  gt  a rnd   a length 0   x000D  let rnd       gt  crypto getRandomValues new Uint32Array 1   0  2  32  x000D   x000D  var myArray     January    February    March    x000D   x000D  console log  rndItem myArray    x000D   x000D   x000D

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If you want to write it on one line  like Pascual s solution  another solution would be to write it using ES6 s find function  based on the fact  that the probability of randomly selecting one out of n items is 1 n     x000D   x000D  var item     A    B    C    D   find     i  ar    gt  Math random    lt  1    ar length - i    x000D  console log item   x000D   x000D   x000D    Use that approach for testing purposes and if there is a good reason to not save the array in a seperate variable only  Otherwise the other answers  floor random   length and using a seperate function  are your way to go

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Looking for a true one-liner I came to this    January    February    March   reduce  a  c  i  o    gt    return o Math floor Math random     Math floor o length

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A generic way to get random element s     x000D   x000D  let some array     Jan    Feb    Mar    Apr    May    x000D  let months   random elems some array  3   x000D   x000D  console log months   x000D   x000D  function random elems arr  count    x000D    let len   arr length  x000D    let lookup       x000D    let tmp       x000D   x000D    if  count  gt  len  x000D      count   len  x000D   x000D    for  let i   0  i  lt  count  i      x000D      let index  x000D      do   x000D        index      Math random     len   x000D        while  index in lookup   x000D      lookup index    null  x000D      tmp push arr index    x000D      x000D   x000D    return tmp  x000D    x000D   x000D   x000D

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If you have fixed values  like a month name list  and want a one-line solution  var result     January    February    March   Math floor Math random     3     The second part of the array is an access operation as described in Why does  5 6 8 7  1 2    8 in JavaScript

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Simple Function    var myArray     January    February    March    function random array         return array Math floor Math random     array length     random myArray     OR  var myArray     January    February    March    function random          return myArray Math floor Math random     myArray length     random      OR  var myArray     January    February    March    function random          return myArray Math floor Math random     myArray length     random

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If you ve already got underscore or lodash included in your project you can use   sample      will return one item randomly from the array   sample   January    February    March       If you need to get more than one item randomly  you can pass that as a second argument in underscore      will return two items randomly from the array using underscore   sample   January    February    March    2     or use the   sampleSize method in lodash      will return two items randomly from the array using lodash   sampleSize   January    February    March    2

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Recursive  standalone function which can return any number of items  identical to lodash sampleSize    function getRandomElementsFromArray array  numberOfRandomElementsToExtract   1        const elements            function getRandomElement arr            if  elements length  lt  numberOfRandomElementsToExtract                const index   Math floor Math random     arr length              const element   arr splice index  1  0                elements push element               return getRandomElement arr            else               return elements                      return getRandomElement     array

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Prototype Method If you plan on getting a random value a lot  you might want to define a function for it  First  put this in your code somewhere  Array prototype sample   function      return this Math floor Math random   this length       Now   1 2 3 4  sample       gt  a random element   Code released into the public domain under the terms of the CC0 1 0 license

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Here is an example of how to do it    scope ctx skills   data result skills       scope praiseTextArray          Hooray        You  re ready to move to a new skill         Yahoo  You completed a problem         You  re doing great          You succeeded         That was a brave effort trying new problems         Your brain was working hard        All your hard work is paying off        Very nice job   Let  s see what you can do next        Well done        That was excellent work        Awesome job        You must feel good about doing such a great job        Right on        Great thinking        Wonderful work        You were right on top of that one        Beautiful job        Way to go        Sensational effort           scope praiseTextWord    scope praiseTextArray Math floor Math random    scope praiseTextArray length

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var item   myArray Math floor Math random   myArray length     or equivalent shorter version   var item   myArray  Math random   myArray length  0    Sample code    x000D   x000D  var myArray     January    February    March        x000D  var item   myArray  Math random   myArray length  0   x000D  console log  item    item   x000D   x000D   x000D

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is much faster than Math Floor    so when it comes to performance optimization while producing output using UI elements     wins the game  MORE INFO var rand   myArray    Math random     myArray length     But if you know that the array is going to have millions of elements than you might want to reconsider between Bitwise Operator and Math Floor    as bitwise operator behave weirdly with large numbers  See below example explained with the output  MORE INFO deadlink  var number   Math floor 14444323231 2        gt  14444323231 var number   14444323231 2   0       gt  1559421343

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In my opinion  better than messing around with prototypes   or declaring it just in time  I prefer exposing it to window   window choice   function       if   this length    this length    0  return    if  this length    1  return this 0     return this Math floor Math random   this length        Now anywhere on your app you call it like   var rand   window choice call array    This way you can still use for x in array  loop properly

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Editing Array prototype can be harmful  Here it is a simple function to do the job   function getArrayRandomElement  arr      if  arr  amp  amp  arr length        return arr Math floor Math random     arr length             The undefined will be returned if the empty array was passed     Usage      Example 1 var item   getArrayRandomElement   January    February    March         Example 2 var myArray     January    February    March    var item   getArrayRandomElement myArray

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Say you want to choose a random item that is different from the last time  not really random  but still a common requirement            Return a random element from an array that is    different than  last   as long as the array has  gt  1 items       Return null if the array is empty     function getRandomDifferent arr  last   undefined      if  arr length     0        return      else if  arr length     1        return arr 0       else       let num   0      do         num   Math floor Math random     arr length         while  arr num      last       return arr num          Implement like this  const arr    1 2 3   const r1   getRandomDifferent arr   const r2   getRandomDifferent arr  r1      r2 is different than r1

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The shortest version   var myArray     January    February    March     var rand   myArray  Math random     myArray length    0

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Faker js has many utility functions for generating random test data  It is a good option in the context of a test suite   const Faker   require  faker    Faker random arrayElement   January    February    March       As commenters have mentioned  you generally should not use this library in production code

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randojs makes this a little more simple and readable    x000D   x000D  console log  rando   January    February    March    value    x000D   lt script src  https   randojs com 1 0 0 js  gt  lt  script gt  x000D   x000D   x000D

[javascript] Getting a random value from a JavaScript array

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