Shuffle an array

Return a new array in random order, every order equally likely.

JavaScriptNode.js & browsers Version 1.0.0 · MIT · .md

function shuffle(values) {
  const result = [...values];
  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;
}

Usage

const deck = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10];
shuffle(deck).toSorted((a, b) => a - b); // → [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
deck; // → [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
shuffle(deck); // e.g. [9, 1, 10, 6, 8, 5, 2, 3, 7, 4]

Lines marked → were executed in Node.js 22 and in headless Chromium, and each result was compared with the value shown. Lines marked e.g. are illustrative.

Know the limits

• Fisher–Yates in O(n). The original sorted by random keys, which is also unbiased but O(n log n); avoid sort(() => Math.random() - 0.5), which is biased.
• Math.random is not cryptographic. For draws people could dispute, use crypto.getRandomValues, as in Generate a random string from given characters.
• The input array is unchanged.

The function above is the code the usage lines run. It does not validate its arguments; add checks where it receives untrusted input.

Keep exploring

• Clone an array — Create a new array without copying the objects inside it.
• Remove duplicate values — Keep the first occurrence of each value, in its original order.
• Flatten a nested array — Turn a nested array into one sequence of values.