/**

 * Copyright (c) Meta Platforms, Inc. and affiliates.

 *

 * This source code is licensed under the MIT license found in the

 * LICENSE file in the root directory of this source tree.

 *

 * @flow

 */

import isArray from 'shared/isArray';

/**

 * 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> {

  if (next == null) {

    throw new Error(

      'accumulate(...): 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 (isArray(current)) {

    /* $FlowFixMe[incompatible-return] if `current` is `T` and `T` an array,

     * `isArray` might refine to the array element type of `T` */

    return current.concat(next);

  }

  if (isArray(next)) {

    /* $FlowFixMe[incompatible-return] unsound if `next` is `T` and `T` an array,

     * `isArray` might refine to the array element type of `T` */

    return [current].concat(next);

  }

  return [current, next];

}

export default accumulate;