permutation.js

const generatePermutations = (items, process) => {
  const mutableItems = items.slice();
  const isEven = n => (n & 1) === 0;

  const permute = (itemArray, x, last) => {
    if (isEven(last)) { x = 0; }

    const temp = itemArray[x];
    itemArray[x] = itemArray[last];
    itemArray[last] = temp;
  };

  const generate = function generateNext(last, process) {
    let i;
    if (last === 0) {
      process(mutableItems);
    }
    else {
      for (i = 0; i < last; i += 1) {
        generateNext(last - 1, process);
        permute(mutableItems, i, last);
      }
      generateNext(last - 1, process);
    }
  };

  generate(mutableItems.length - 1, process);
};