How to randomize (shuffle) a JavaScript array?

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I have an array like this:

var arr1 = ["a", "b", "c", "d"];

How can I randomize / shuffle it?

The de-facto unbiased shuffle algorithm is the Fisher-Yates (aka Knuth) Shuffle.

You can see a great visualization here (and the original post linked to this)

function shuffle(array) {
var currentIndex = array.length, temporaryValue, randomIndex;

// While there remain elements to shuffle...
while (0 !== currentIndex) {

// Pick a remaining element...
randomIndex = Math.floor(Math.random() * currentIndex);
currentIndex -= 1;

// And swap it with the current element.
temporaryValue = array[currentIndex];
array[currentIndex] = array[randomIndex];
array[randomIndex] = temporaryValue;
}

return array;
}

// Used like so
var arr = [2, 11, 37, 42];
arr = shuffle(arr);
console.log(arr);

Here is a JavaScript implementation of the Durstenfeld shuffle, a computer-optimized version of Fisher-Yates:

/**
* Randomize array element order 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;
}
}

The Fisher-Yates algorithm works by picking one random element for each original array element, and then excluding it from the next draw. Just like randomly picking from a deck of cards.

This exclusion is done in a clever way (invented by Durstenfeld for use by computers) by swapping the picked element with the current element, and then picking the next random element from the remainder. For optimal efficiency, the loop runs backwards so that the random pick is simplified (it can always start at 0), and it skips the last element because there are no other choices anymore.

The running time of this algorithm is O(n). Note that the shuffle is done in-place. So if you do not want to modify the original array, make a copy of it first with .slice(0).

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]];
}
}

[community edit: This answer is incorrect; see comments. It is being left here for future reference because the idea is not that rare.]
[1,2,3,4,5,6].sort(function() {
return .5 - Math.random();
});
Reprint:https://stackoverflow.com/questions/2450954/how-to-randomize-shuffle-a-javascript-array