recursive flatten array in Javascript

flatten.js

// see this javascript working at http://jsbin.com/mehemepume/edit?js,console
// the function calls itself recursivly, so there can be any level of nesting and it will still flatten.

var nested = [[1, 2, [3]], 4],
  nestedDeep = [1, 2, 3, [4, 5, 6, [[7, [8]], 9, 10]]];

function flatten(arr) {
  var flat = [];

  arr.forEach(function (item) {
    if (Array.isArray(item)) {
      // recursive, will work with any level of nesting
      flatten(item).forEach(function (child) {
        return flat.push(child);
      });
      
    } else {
      flat.push(item);
    }
  });

  return flat;
}

console.log(JSON.stringify(flatten(nested)));
// [1,2,3,4]
console.log(JSON.stringify(flatten(nestedDeep)));
// [1,2,3,4,5,6,7,8,9,10]