JavaScript - Select a Random Element from JS Array

Selecting a random element from array in JavaScript is done by accessing an element using random index from the array. We can select the random element using the Math.random() function.

1. Using Math.random() Method

The Math.random() method is used to get the random number between 0 to 1 (1 exclusive). It can be multiplied with the size of the array to get a random index and access the respective element using their index.

let a = [10, 20, 30, 40, 50];
let i = Math.floor(Math.random() * a.length);
let r = a[i];
console.log(r);

30

2. Using Fisher-Yates Shuffle Algorithm

The Fisher-Yates Shuffle Algorithm, also known as the Knuth Shuffle is a method for randomly shuffling elements in an array. It works by iterating over the array from the last element to the first, swapping each element with a randomly chosen one that comes before it (including itself).

function shuffleA(a) {
    for (let i = a.length - 1; i > 0; i--) {
        const j = Math.floor(Math.random() * (i + 1));
        [a[i], a[j]] = [a[j], a[i]];
    }
}

let a = [10, 20, 30, 40, 50];

shuffleA(a);

let r = a[0];
console.log(r);

30

3. Using Array.prototype.sort()

This method randomly shuffles the array using the sort() function with a random comparison, and then picks the first element. While it works, it can be inefficient for large arrays as it sorts the entire array.

let colors = ["red", "green", "blue", "yellow"];
let random = colors.sort(() => 0.5 - Math.random())[0];
console.log(random);

Output

blue

In this example

• The sort() function is called with a random comparison function (() => 0.5 - Math.random()).
• This shuffles the array randomly.
• The first element of the shuffled array is selected.

Note: This method modifies the original array by shuffling it.

4. Using Array.prototype.slice()

slice() can be used to create a sub-array and select a random element. This method is typically used for copying or extracting elements from an array.

let n = [10, 20, 30, 40];
let random = n.slice(Math.floor(Math.random() * n.length), Math.floor(Math.random() * n.length) + 1)[0];
console.log(random);

20

In this example

• slice() extracts a portion of the array starting from a random index.
• The [0] selects the first element of the sliced array.

Note: This method creates a new array with one element, which may not be the most efficient for large arrays.condition.