javascript - Promise.then 绑定(bind)问题

Question

我在将值绑定(bind)到下一个 Promise 时遇到了问题。看下面的代码，它会更好地解释情况。

'use strict';

function FindEvent(eventId) {
    console.log('Event:', eventId);
    return Promise.resolve({title: 'MyEvent'});
}

function FindUser(userId) {
    console.log('User:', userId);
    return Promise.resolve({username: 'MyUser'});
}

function MyServiceProblem(payload) {
    payload.userId = 'blah';
    return FindEvent(payload.eventId)
        .then((event) => {
            payload.userId = 'test';
            // setting a breakpoint here shows
            // that the value to payload.userId has been
            // assigned, i.e. 'test'

            return Promise.resolve(payload);
        })
        .then(FindUser.bind(this, payload.userId));
        // But new value doesn't reaches inside FindUser
}

MyServiceProblem({ // userId is 'blah', why not 'test'
    eventId: '456'
});

function MyServiceWorks(payload) {
    payload.userId = 'blah';
    return new Promise((resolve) => {
        payload.userId = 'test';

        return resolve(payload);
    })
        .then(FindUser.bind(this, payload.userId));
        // From here, the 'test' value reaches FindUser
}

MyServiceWorks({ // userId is 'test'
    eventId: '456'
});

问题是，为什么这两种情况它绑定(bind)的值不同。两者完全相同，只是这个首先解析 Promise 部分，然后为 payload.userId 赋值。

Answer 1

让我们稍微分解一下您的代码。你有

function MyServiceProblem(payload) {
    payload.userId = 'blah';
    return FindEvent(payload.eventId)
        .then((event) => {
            payload.userId = 'test';

            return Promise.resolve(payload);
        })
        .then(FindUser.bind(this, payload.userId));
}

问题是您的 .bind 将在回调之前运行。这段代码也可以写成

function MyServiceProblem(payload) {
    payload.userId = 'blah';

    var firstThenCallback = (event) => {
        payload.userId = 'test';
        return Promise.resolve(payload);
    };
    var secondThenCallback = FindUser.bind(this, payload.userId);

    return FindEvent(payload.eventId)
        .then(firstThenCallback)
        .then(secondThenCallback);
}

payload 对象在所有对象之间共享，但 payload.userId 在 firstThenCallback 之前传递到 .bind > 有机会执行。

与其使用 .bind 并立即传递值，最简单的解决方案似乎是使用匿名函数，以便稍后读取 userId 值。

function MyServiceProblem(payload) {
    payload.userId = 'blah';
    return FindEvent(payload.eventId)
        .then((event) => {
            payload.userId = 'test';

            return Promise.resolve(payload);
        })
        .then(() => FindUser(payload.userId));
}

也不清楚为什么你要编写如此迂回的 promise 代码，但我认为这是一个精简的示例？为什么要使用 payload 进行解析，而不是仅在该函数中调用 FindUser，例如

function MyServiceProblem(payload) {
    payload.userId = 'blah';
    return FindEvent(payload.eventId)
        .then((event) => {
            payload.userId = 'test';

            return FindUser(payload.userId);
        });
}