Promise.finally
- 接收 onFinally cb，返回 promise
- 链式调用，用 then 模拟
- 成功或失败，都要执行
js
// 简版
Promise.prototype.finally = function(cb){
return this.then(
value => Promise.resolve(cb()).then(() => value),
reason => Promise.resolve(cb()).then(() => Promise.reject(reason))
)
}
// 为什么要用对象？
Promise.prototype.finally = Promise.prototype.finally || {
finally (fn) {
const onFinally = callback => Promise.resolve(fn()).then(callback);
return this.then(
result => onFinally(() => result),
reason => onFinally(() => Promise.reject(reason))
);
}
}.finally;