code: function flatMap(iterator) {
      // Calling .flatMap() with no arguments is a no-op. Don't bother cloning.
      if (arguments.length === 0) {
        return this;
      }
  
      var result = [],
          length = this.length,
          index;
  
      for (index = 0; index < length; index++) {
        var iteratorResult = iterator(this[index], index, this);
  
        if (Array.isArray(iteratorResult)) {
          // Concatenate Array results into the return value we're building up.
          result.push.apply(result, iteratorResult);
        } else {
          // Handle non-Array results the same way map() does.
          result.push(iteratorResult);
        }
      }
  
      return makeImmutableArray(result);
    }
docs: Effectively performs a map() over the elements in the array, using the
  provided iterator, except that whenever the iterator returns an array, that
  array's elements are added to the final result instead of the array itself.
  
  @param {function} iterator - The iterator function that will be invoked on each element in the array. It will receive three arguments: the current value, the current index, and the current object.