| 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 into the first one. This |
| 14 | * is used to conserve memory by avoiding array allocations, and thus sacrifices |
| 15 | * API cleanness. Since `current` can be null before being passed in and not |
| 16 | * null after this function, make sure to assign it back to `current`: |
| 17 | * |
| 18 | * `a = accumulateInto(a, b);` |
| 19 | * |
| 20 | * This API should be sparingly used. Try `accumulate` for something cleaner. |
| 21 | * |
| 22 | * @return {*|array<*>} An accumulation of items. |
| 23 | */ |
| 24 | |
| 25 | function accumulateInto<T>( |
| 26 | current: ?(Array<T> | T), |
| 27 | next: T | Array<T>, |
| 28 | ): T | Array<T> { |
| 29 | if (next == null) { |
| 30 | throw new Error('Accumulated items must not be null or undefined.'); |
| 31 | } |
| 32 | |
| 33 | if (current == null) { |
| 34 | return next; |
| 35 | } |
| 36 | |
| 37 | // Both are not empty. Warning: Never call x.concat(y) when you are not |
| 38 | // certain that x is an Array (x could be a string with concat method). |
| 39 | if (isArray(current)) { |
| 40 | if (isArray(next)) { |
| 41 | // $FlowFixMe[incompatible-use] `isArray` does not ensure array is mutable |
| 42 | // $FlowFixMe[method-unbinding] |
| 43 | current.push.apply(current, next); |
| 44 | return current; |
| 45 | } |
| 46 | // $FlowFixMe[incompatible-use] `isArray` does not ensure array is mutable |
| 47 | current.push(next); |
| 48 | return current; |
| 49 | } |
| 50 | |
| 51 | if (isArray(next)) { |
| 52 | // A bit too dangerous to mutate `next`. |
| 53 | /* $FlowFixMe[incompatible-return] unsound if `next` is `T` and `T` an array, |
| 54 | * `isArray` might refine to the array element type of `T` */ |
| 55 | return [current].concat(next); |
| 56 | } |
| 57 | |
| 58 | return [current, next]; |
| 59 | } |
| 60 | |
| 61 | export default accumulateInto; |