How to randomize (shuffle) a JavaScript array?
The de-facto unbiased shuffle algorithm is the Fisher-Yates (aka Knuth) Shuffle.
See https://github.com/coolaj86/knuth-shuffle
You can see a great visualization here (and the original post linked to this)
function shuffle(array) { let currentIndex = array.length, randomIndex; // While there remain elements to shuffle... while (currentIndex != 0) { // Pick a remaining element... randomIndex = Math.floor(Math.random() * currentIndex); currentIndex--; // And swap it with the current element. [array[currentIndex], array[randomIndex]] = [ array[randomIndex], array[currentIndex]]; } return array;}// Used like sovar arr = [2, 11, 37, 42];shuffle(arr);console.log(arr);Some more info about the algorithm used.
Here's a JavaScript implementation of the Durstenfeld shuffle, an optimized version of Fisher-Yates:
/* Randomize array in-place using Durstenfeld shuffle algorithm */function shuffleArray(array) { for (var i = array.length - 1; i > 0; i--) { var j = Math.floor(Math.random() * (i + 1)); var temp = array[i]; array[i] = array[j]; array[j] = temp; }}It picks a random element for each original array element, and excludes it from the next draw, like picking randomly from a deck of cards.
This clever exclusion swaps the picked element with the current one, then picks the next random element from the remainder, looping backwards for optimal efficiency, ensuring the random pick is simplified (it can always start at 0), and thereby skipping the final element.
Algorithm runtime is O(n). Note that the shuffle is done in-place so if you don't want to modify the original array, first make a copy of it with .slice(0).
EDIT: Updating to ES6 / ECMAScript 2015
The new ES6 allows us to assign two variables at once. This is especially handy when we want to swap the values of two variables, as we can do it in one line of code. Here is a shorter form of the same function, using this feature.
function shuffleArray(array) { for (let i = array.length - 1; i > 0; i--) { const j = Math.floor(Math.random() * (i + 1)); [array[i], array[j]] = [array[j], array[i]]; }}
Warning!
The use of this algorithm is not recommended, because it is inefficient and strongly biased; see comments. It is being left here for future reference, because the idea is not that rare.
[1,2,3,4,5,6].sort( () => .5 - Math.random() );This https://javascript.info/array-methods#shuffle-an-array tutorial explains the differences straightforwardly.