code: function depthFlatten(array, i) {
    i = i || 0;
  
    if(i >= array.length)
      return array;
  
    if(isArray(array[i])) {
      return depthFlatten(array.slice(0,i)
        .concat(array[i], array.slice(i+1)), i);
    }
    return depthFlatten(array, i+1);
  }
docs: flatten nested array (the nesting can be to any depth).
  @param array {Array}
  @param i {int}
  @returns {Array}
  @private