flatten an arbitrary integer array

flatten.js

function flatten(a) {
  if (!Array.isArray(a)) {
    throw("Not an array: "+a);
  }

  let b = new Array();
  append(a);
  return b;

  function append(x) {
    if (Array.isArray(x)) {
      for (let i = 0; i < x.length; i++) {
        append(x[i]);
      }
    } else {
      b.push(x);
    }
  }
}