How to get a random value from a JavaScript array?

Query explained:

Consider:

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

How can I select a random value from this array using JavaScript?

How to get a random value from a JavaScript array?

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 #2:

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 the 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);

Custom function to get a random value from an array in JavaScript:

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

How to get a random value from an array in JavaScript?

Use underscore (or loDash :)):

var randomArray = [
   '#cc0000','#00cc00', '#0000cc'
];

// use _.sample
var randomElement = _.sample(randomArray);

// manually use _.random
var randomElement = randomArray[_.random(randomArray.length-1)];

Or to shuffle an entire array:

// use underscore's shuffle function
var firstRandomElement = _.shuffle(randomArray)[0];

Answer #3:

1. solution: define Array prototype

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

that will work on inline arrays

[2,3,5].random()

and of course predefined arrays

var list = [2,3,5]
list.random()

2. solution: define custom function that accepts list and returns element

function get_random (list) {
  return list[Math.floor((Math.random()*list.length))];
}


get_random([2,3,5])

Answer #4:

~~ is much faster than Math.Floor(), so when it comes to performance optimization while producing output using UI elements, ~~ wins the game.

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 the example explained with the output.

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

Code examples to get a random value from a JavaScript array:

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

Using jQuery:

If you really must use jQuery to solve this problem (NB: you shouldn’t):

(function($) {
    $.rand = function(arg) {
        if ($.isArray(arg)) {
            return arg[$.rand(arg.length)];
        } else if (typeof arg === "number") {
            return Math.floor(Math.random() * arg);
        } else {
            return 4;  // chosen by fair dice roll
        }
    };
})(jQuery);

var items = [523, 3452, 334, 31, ..., 5346];
var item = jQuery.rand(items);

This plugin will return a random element if given an array, or a value from [0 .. n) given a number, or given anything else, a guaranteed random value!

For extra fun, the array return is generated by calling the function recursively based on the array’s length 🙂

Example to get a random element from an array in JavaScript using function:

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;
  } 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.

Hope you learned something from this post.

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