> For the complete documentation index, see [llms.txt](https://bvct1742.gitbook.io/one-book/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://bvct1742.gitbook.io/one-book/knowledge/js/promise.finally-shi-xian-yuan-li.md).

# Promise.finally 实现原理

```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 })
    );
};
```