/**
 * Copyright (c) 2013-present, Facebook, Inc.
 *
 * This source code is licensed under the MIT license found in the
 * LICENSE file in the root directory of this source tree.
 *
 * @flow
 */

import invariant from 'fbjs/lib/invariant';

/**
 * Accumulates items that must not be null or undefined.
 *
 * This is used to conserve memory by avoiding array allocations.
 *
 * @return {*|array<*>} An accumulation of items.
 */
function accumulate<T>(
  current: ?(T | Array<T>),
  next: T | Array<T>,
): T | Array<T> {
  invariant(
    next != null,
    'accumulate(...): Accumulated items must be 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)) {
    return current.concat(next);
  }

  if (Array.isArray(next)) {
    return [current].concat(next);
  }

  return [current, next];
}

export default accumulate;
