> For the complete documentation index, see [llms.txt](https://mm.ricky.moe/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://mm.ricky.moe/javascript/chang-jian-han-shu/shi-xian-promise.finnaly.md).

# 实现Promise.finnaly

```javascript
Promise.prototype.finally = function (callback) {
  let P = this.constructor
  return this.then(
    value  => P.resolve(callback()).then(() => value),
    reason => P.resolve(callback()).then(() => { throw reason })
  )
}
```