promise_finally

promise_finally.md

promise的finally 方法，有几点注意：

• resolve/reject都会被调用；
• 函数没有入参，因为它不管状态；
• 函数的返回值，除了返回一个Promise.reject 对象，不影响后续 then 的传值。
• 返回 Promise.reject(),将改变后续then方法的执行，进入到 rejected 方法里面

// output: res 2
Promise.resolve(2).finally(() => {return Promise.resolve(11)}).then(res => console.log('res',res), err => console.log('err',err))
Promise.resolve(2).finally(() => {return 11}).then(res => console.log('res',res), err => console.log('err',err))
Promise.resolve(2).finally(() => {Promise.reject(11)}).then(res => console.log('res',res), err => console.log('err',err))

// output: err 2
Promise.reject(2).finally(() => {return Promise.resolve(11)}).then(res => console.log('res',res), err => console.log('err',err))
Promise.reject(2).finally(() => {return 11}).then(res => console.log('res',res), err => console.log('err',err))
Promise.reject(2).finally(() => {Promise.reject(11)}).then(res => console.log('res',res), err => console.log('err',err))

// change chain, return the rejected value 11
Promise.resolve(2).finally(() => {return Promise.reject(11)}).then(res => console.log('res',res), err => console.log('err',err))
// err 11

Promise.reject(3).finally(() => {return Promise.reject(11)}).then(res => console.log('res',res), err => console.log('err',err))
//err 11

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Promise/finally
https://exploringjs.com/es6/ch_promises.html

Promise.prototype.finally = function (callback) {
    const P = this.constructor;
    // We don’t invoke the callback in here,
    // because we want then() to handle its exceptions
    return this.then(
        // Callback fulfills => continue with receiver’s fulfillment or rejection
        // Callback rejects => pass on that rejection (then() has no 2nd parameter!)
        value  => P.resolve(callback()).then(() => value),
        reason => P.resolve(callback()).then(() => { throw reason })
    );
};