Function accumulateInto

code/composition/public/app.js:2333–2357

* 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 `c

(current, next)

 */

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];
}

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