Recursive-based, dependency-free, vanilla array flattening

flatten.js

function flatten (arr, deep) {
  if (deep) arr = arr.map(function(e){
    return Object.prototype.toString.call(e) ===
      '[object Array]' ? flatten(e, true) : e;
  });
  arr = Array.prototype.concat.apply([], arr || []);
  return arr;
}