再帰を使わないflatten.js

function flatten (array) {
  var newArray = [];
  var locals = {
    i: 0,
    len: array.length,
    array: array,
  };
  var stack = [];
  var elem;

  while (locals) {
    if (locals.i >= locals.len) {
      locals = stack.pop();
      continue;
    }

    elem = locals.array[locals.i];
    locals.i += 1;
    if (Array.isArray(elem)) {
      stack.push(locals = {
        i: 0,
        len: elem.length,
        array: elem,
      });
    } else {
      newArray.push(elem);
    }
  }

  return newArray;
}