How to shuffle an array

Short answer

The Fisher-Yates algorithm goes from the end and swaps each element with a random one before it:

const shuffle = (array) => {
  const result = [...array];
  for (let i = result.length - 1; i > 0; i--) {
    const j = Math.floor(Math.random() * (i + 1));
    [result[i], result[j]] = [result[j], result[i]];
  }
  return result;
};
 
const cards = ['A', 'K', 'Q', 'J', '10'];
const shuffled = shuffle(cards);
 
console.log(shuffled.length, [...shuffled].sort().join(''));

Output:

5 10AJKQ

Common mistake

The popular sort(() => Math.random() - 0.5) looks shorter, but it mixes unevenly: some orders come out much more often than others. Count how often the first element stays in place:

let stays = 0;
for (let i = 0; i < 10000; i++) {
  const result = [1, 2, 3, 4, 5].sort(() => Math.random() - 0.5);
  if (result[0] === 1) stays++;
}
 
console.log(stays > 2000 ? 'more often than 1 in 5' : 'about 1 in 5');

Output:

more often than 1 in 5

With an even shuffle the first element stays in place in about one case out of five.

Thanks for reading!

Short answer

Common mistake

Functions used in this recipe

Thanks for reading!