为什么后续的链式承诺得到解决？

Question

我正在努力了解 Promises/A+，但我对几点感到困惑。考虑一下：

function foo() {
    console.log("In 'foo' function: --> which returns a Promise");
    return new Promise(function (resolve, reject) {
        resolve()
    });
}

var promise1 = foo();

var promise2 = promise1.then(function () {
    console.log("Promise 1 has been resolved --> onFulfilled() returns a value");
    return 1;
});

var promise3 = promise2.then(function () {
    console.log("Promise 2 has been resolved --> onFulfilled() is not returning anything from inFulfilled()");
});

var promise4 = promise3.then(function () {
    console.log("Promise 3 has been resolved");
});

/* Console:
In 'foo' function: --> which returns a Promise
Promise 1 has been resolved --> onFulfilled() returns a value
Promise 2 has been resolved --> onFulfilled() is not returning anything from inFulfilled()
Promise 3 has been resolved
*/

Promise 1 必须通过调用 resolve() 明确解决，否则执行将停止。为什么使用 Promises 2 和 3，执行认为这些已解决（不需要 resolve()）并进入链中的下一个 .then？
链接到 promise2 的“onFullfilled()”函数没有 return 值。我找不到任何明确的 reference in the spec what is supposed to happen in this case. Is this scenario treated as if the function returned undefined？这取决于所使用的库的实际实现吗？

Answer 1

承诺处理程序（成功或错误处理程序）具有 return 值。每次 then 调用都会创建一个新的承诺。

如果一个值被 returned，那么承诺是自动解决的。如果另一个 promise 被 returned，那么 promise 将等到它被解决或拒绝，然后再继续 promise 链中的下一个 then 调用。

我认为这可以解释您所看到的行为。

Answer 2

您要查找的行为位于 promise/A+ 规范中的 #7 (https://github.com/promises-aplus/promises-spec#the-then-method)。

promise2 = promise1.then(onFulfilled, onRejected);

"If either onFulfilled or onRejected returns a value x, run the Promise Resolution Procedure [[Resolve]](promise2, x)."

我从这里得到的是，当第一个承诺是 failed/resolved 时，从它创建的任何承诺（包括新承诺 promise2）都是 failed/resolved。

例如： 当您在您的示例中解决 promise1 时，所有后续承诺也会解决。

Answer 3

我对您的代码做了一些小调整：

将参数传递给 resolve(data) 例如resolve(1001)
在.then检索并显示数据
同样在.thenreturn链中的下一个Promise与return new Promise

这是修改后的示例：

var promise1 = new Promise(function (resolve, reject) {
    console.log("Creating Promise 1");
    resolve( 1001 );
} );

var promise2 = promise1.then(function (data1) {
    console.log("Promise 1 has been resolved --> ", data1);
    return new Promise(function (resolve, reject) {
        console.log("Creating Promise 2");
        resolve( 1002 );
    } );
} );

var promise3 = promise2.then(function (data2) {
    console.log("Promise 2 has been resolved --> ", data2);
    return new Promise(function (resolve, reject) {
        console.log("Creating Promise 3");
        resolve( 1003 );
    } );
} );

promise3.then(function (data3) {
    console.log("Promise 3 has been resolved --> ", data3);
} );

为什么后续的链式承诺得到解决？

Why are consequent chained promises resolved?

javascript

node.js

promise

bluebird