| 1 | /** |
| 2 | * Copyright (c) Meta Platforms, Inc. and affiliates. |
| 3 | * |
| 4 | * This source code is licensed under the MIT license found in the |
| 5 | * LICENSE file in the root directory of this source tree. |
| 6 | * |
| 7 | * @flow |
| 8 | */ |
| 9 | |
| 10 | import isArray from 'shared/isArray'; |
| 11 | |
| 12 | /** |
| 13 | * Accumulates items that must not be null or undefined. |
| 14 | * |
| 15 | * This is used to conserve memory by avoiding array allocations. |
| 16 | * |
| 17 | * @return {*|array<*>} An accumulation of items. |
| 18 | */ |
| 19 | function accumulate<T>( |
| 20 | current: ?(T | Array<T>), |
| 21 | next: T | Array<T>, |
| 22 | ): T | Array<T> { |
| 23 | if (next == null) { |
| 24 | throw new Error('Accumulated items must not be null or undefined.'); |
| 25 | } |
| 26 | |
| 27 | if (current == null) { |
| 28 | return next; |
| 29 | } |
| 30 | |
| 31 | // Both are not empty. Warning: Never call x.concat(y) when you are not |
| 32 | // certain that x is an Array (x could be a string with concat method). |
| 33 | if (isArray(current)) { |
| 34 | // $FlowFixMe[incompatible-use] `isArray` does not ensure array is mutable |
| 35 | return current.concat(next); |
| 36 | } |
| 37 | |
| 38 | if (isArray(next)) { |
| 39 | /* $FlowFixMe[incompatible-return] unsound if `next` is `T` and `T` an array, |
| 40 | * `isArray` might refine to the array element type of `T` */ |
| 41 | return [current].concat(next); |
| 42 | } |
| 43 | |
| 44 | return [current, next]; |
| 45 | } |
| 46 | |
| 47 | export default accumulate; |