Generate all permutations of an array. Alternative to the many variants of heap's algorithm I keep finding on the interweb. Every single algorithm I found produced incorrect results. This one is correct.

permutations-with-max-depth.js

/**
 * This variant takes a max depth as the second argument.
 */

const permutations = (value, max = value.length) => {
  let depth = Math.min(max, value.length);
  let results = [];

  const permute = (queue = []) => {
    if (queue.length === depth) {
      results.push(queue);
    } else {
      for (let ele of value) {
        permute(queue.concat(ele));
      }
    }
  };

  permute();
  return results;
};

console.log(permutations(['a', 'b', 'c', 'd', 'e']).length); // 3125
console.log(permutations(['a', 'b', 'c', 'd', 'e'], 4).length); // 625
console.log(permutations(['a', 'b', 'c', 'd', 'e'], 3).length); // 125
console.log(permutations(['a', 'b', 'c', 'd', 'e'], 2).length); // 25
console.log(permutations(['a', 'b', 'c', 'd', 'e'], 1).length); // 5

permutations-with-transform-fn.js

/**
 * This variant takes a transform function to modify each set
 * before it's pushed onto the results array
 */

const identity = ele => ele;
const permutations = (arr, max = arr.length, fn = identity) => {
  if (typeof max === 'function') return permutations(arr, arr.length, max);

  let depth = Math.min(max, arr.length);
  let results = [];

  const permute = (queue = []) => {
    if (queue.length === depth) {
      results.push(fn(queue));
    } else {
      for (let ele of arr) {
        permute(queue.concat(ele));
      }
    }
  };

  permute();
  return results;
};

console.log(permutations(['a', 'b', 'c'], 2, ele => ele.join('.')));

permutations.js

/**
 * Generates all permutations of an array, including duplicate
 * character sequences, like "aaa", "aab", and so on.
 */

const permutations = (array = []) => {
  let len = array.length;
  let results = [];

  const permute = (queue = []) => {
    if (queue.length === len) {
      results.push(queue);
    } else {
      for (let ele of array) {
        permute(queue.concat(ele));
      }
    }
  };

  permute();
  return results;
};

console.log(permutations(['a', 'b', 'c', 'd', 'e']).length); // 3125