Javascript Polyfill Array.shuffle #6

This problem was asked by Tencent.

How would you implement a shuffle() ?

Shuffle the array, it may be [1, 3, 2, 4], but it should modify the array inline to generate a randomly picked permutation at the same probability.

Example

[1, 2, 3, 4].shuffle()

Solution

if (!Array.prototype.shuffle) {
  Array.prototype.shuffle = function shuffle() {
    let i = this.length - 1;
    while (i) {
      const j = Math.floor(Math.random() * (i + 1));
      [this[i], this[j]] = [this[j], this[i]];
      i--;
    }
    return this;
  };
}

This works by:

• Checking if Array.prototype.shuffle() already exists
• If not, adding a shuffle() method to the Array prototype
• The shuffle() method:
• Loops from the array length down to 0
• Picks a random index between 0 and i
• Swaps the current element (i) with the random element (j)
• This shuffling procedure is performed in place on the array
• The shuffled array is returned

This polyfill implements a simple shuffle algorithm to randomly rearrange the elements of an array.