Recursively flatten an array (e.g. _.flatten but less performant probs)

reflatten.js

var assert = require('assert');

function reflatten (source, accum) {
  accum = accum || [];

  if (Array.isArray(source)) {
    source.forEach(function (child) {
      if (Array.isArray(child)) {
        reflatten(child, accum);
      } else {
        accum.push(child);
      }
    });
  }

  return accum;
}

var before = [[1,2,3], 4, [5,6 ,7, [8, 9], [10, [11, 12]]]];
var result = reflatten(before);

assert.deepEqual(result, [1,2,3,4,5,6,7,8,9,10,11,12]);