Flattens an array using a stack

flatten.js

function flatten(a) {
  if (!Array.isArray(a)) throw new TypeError('argument must be array');

  let stack = [a];
  let stackIndex = [0];
  let rv = [];

  while (stack.length > 0) {
    const i = stack.length - 1;

    if (stackIndex[i] >= stack[i].length) {
      stack.pop();
      stackIndex.pop();
    }
    else {
      const x = stack[i][stackIndex[i]];

      if (Array.isArray(x)) {
        stack.push(x);
        stackIndex.push(0);
      }
      else {
        rv.push(x);
      }

      stackIndex[i] += 1;
    }
  }

  return rv;
}