Return all permutations of a string

permute.js

/**
 * Return all permutations of given string.
 *
 * @example
 *   permute('abc');
 *   //=> ['abc', 'acb', 'bac', 'bca', 'cab', 'cba']
 * @param str {String}
 * @return {Array}
 */
function permute(str) {

  var ret = [];

  // permutation for one or two characters string is easy:
  // 'a'  -> ['a']
  // 'ab' -> ['ab', 'ba']
  if (str.length == 1) return [str];
  if (str.length == 2) return [str, str[1]+str[0]];

  // otherwise combine each character with a permutation
  // of a subset of the string. e.g. 'abc':
  //
  // 'a' + permutation of 'bc'
  // 'b' + permutation of 'ac'
  // 'c' + permutation of 'ab'
  str.split('').forEach(function (chr, idx, arr) {
    var sub = [].concat(arr); // "clone" arr
    sub.splice(idx, 1);
    permute(sub.join('')).forEach(function (perm) {
      ret.push(chr+perm);
    });
  });

  return ret;
}