ES6 permutations calculator

perm.js

function * permutations(str) {
  var length = str.length;
  if (length <= 1) {
    yield str;
  } else {
    for (var i = 0; i < length; i++) {
      var ch = str[i], substr = str.substr(0, i) + str.substr(i + 1);
      for (var permutation of permutations(substr)) {
        yield ch + permutation;
      }
    }
  }
}