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?

Answer 1

To get crypto-strong random item form array use

let rndItem = a=> a[rnd()*a.length|0];
let rnd = ()=> crypto.getRandomValues(new Uint32Array(1))[0]/2**32;

var myArray = ['January', 'February', 'March'];

console.log( rndItem(myArray) )

Additional info from mdn:

Note: Math.random() does not provide cryptographically secure random numbers. Do not use them for anything related to security. Use the Web Crypto API instead, and more precisely the Crypto.getRandomValues() method.

Answer 2

MyArray.at(MyArray.length*Math.random())

I find this both concise and clear. Requires ES2022.

Answer 3

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();

Answer 4

Many of the offered solutions add a method to a specific Array which restricts it's use to just that array. This solution is reusable code that works for any array and can be made type safe.

TypeScript

export function randChoice<T>(arr: Array<T>): T {
  return arr[Math.floor(Math.random() * arr.length)]
}

JavaScript

function randChoice(arr) {
  return arr[Math.floor(Math.random() * arr.length)]
}

Answer 5

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.helpers.arrayElement(['January', 'February', 'March']);

As commenters have mentioned, you generally should not use this library in production code.

Answer 6

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 > 1 items). 
 * Return null if the array is empty.
*/
function getRandomDifferent(arr, last = undefined) {
  if (arr.length === 0) {
    return null;
  } 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.

Answer 7

If you need to fetch a random item more than once, then, obviously you would use a function. One way is to make that function a method of the Array.prototype, but that will normally get you shouted down for tampering with built in prototypes.

However, you can add the method to the specific array itself:

var months = ['January', 'February', 'March'];
months.random = function() {
    return this[Math.floor(Math.random()*this.length)];
};

That way you can use months.random() as often as you like without interfering with the generic Array.prototype.

As with any random function, you run the risk of getting the same value successively. If you don’t want that, you will need to track the previous value with another property:

months.random=function() {
    var random;
    while((random=this[Math.floor(Math.random()*this.length)]) == this.previous);
    this.previous=random;
    return random;
};

If you’re going to do this sort of thing often, and you don’t want to tamper with Array.prototype, you can do something like this:

function randomValue() {
    return this[Math.floor(Math.random()*this.length)];
}

var data = [ … ];
var moreData = [ … ];

data.random=randomValue;
moreData.random=randomValue;

Answer 8

The shortest version:

var myArray = ['January', 'February', 'March']; 
var rand = myArray[(Math.random() * myArray.length) | 0]
console.log(rand)

Answer 9

You may consider defining a function on the Array prototype, in order to create a method [].sample() which returns a random element.

First, to define the prototype function, place this snippet in your code:

Array.prototype.sample = function(){
  return this[Math.floor(Math.random()*this.length)];
}

Later, to sample a random element from the array, just call .sample():

[1,2,3,4].sample() //=> a random element

_{I'm releasing these code snippets into the public domain, under the terms of the CC0 1.0 license.}

Answer 10

~~ 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 then you might want to reconsider between Bitwise Operator and Math.Floor(), as bitwise operators behave weirdly with large numbers. See below example explained with the output.

var number = Math.floor(14444323231.2); // => 14444323231
var number = 14444323231.2 | 0; // => 1559421343

Answer 11

It's a simple one-liner:

const randomElement = array[Math.floor(Math.random() * array.length)];

For example:

const months = ["January", "February", "March", "April", "May", "June", "July"];

const random = Math.floor(Math.random() * months.length);
console.log(random, months[random]);

Answer 12

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.

Array.prototype.random = function(n){
  if(n&&n>1){
    const a = [];
    for(let i = 0;i<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));

Answer 13

Editing Array prototype can be harmful. Here it is a simple function to do the job.

function getArrayRandomElement (arr) {
  if (arr && 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);

Answer 14

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 < 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])
}

Answer 15

var item = myArray[Math.floor(Math.random()*myArray.length)];

or equivalent shorter version:

var item = myArray[(Math.random()*myArray.length)|0];

Sample code:

var myArray = ['January', 'February', 'March'];    
var item = myArray[(Math.random()*myArray.length)|0];
console.log('item:', item);

Answer 16

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);

Answer 17

This is similar to, but more general than, @Jacob Relkin's solution:

This is ES2015:

const randomChoice = arr => {
    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.

Answer 18

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?