code: function accumulateInto(current, next) {
    !(next != null) ? invariant(false, 'accumulateInto(...): Accumulated items must not be null or undefined.') : void 0;
  
    if (current == null) {
      return next;
    }
  
    // Both are not empty. Warning: Never call x.concat(y) when you are not
    // certain that x is an Array (x could be a string with concat method).
    if (Array.isArray(current)) {
      if (Array.isArray(next)) {
        current.push.apply(current, next);
        return current;
      }
      current.push(next);
      return current;
    }
  
    if (Array.isArray(next)) {
      // A bit too dangerous to mutate `next`.
      return [current].concat(next);
    }
  
    return [current, next];
  }
docs: Accumulates items that must not be null or undefined into the first one. This
  is used to conserve memory by avoiding array allocations, and thus sacrifices
  API cleanness. Since `current` can be null before being passed in and not
  null after this function, make sure to assign it back to `current`:
  
  `a = accumulateInto(a, b);`
  
  This API should be sparingly used. Try `accumulate` for something cleaner.
  
  @return {*|array<*>} An accumulation of items.