Function accumulateInto

docs/external/js/react-dom.development.js:3338–3367

* 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

(current, next)

   */

  function accumulateInto(current, next) {
    if (!(next != null)) {
      {
        throw Error( "accumulateInto(...): Accumulated items must not be null or undefined." );
      }
    }

    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];
  }

  /**
   * @param {array} arr an "accumulation" of items which is either an Array or