javascript - 理解 Promise.all

Question

考虑以下我从 https://stackoverflow.com/a/28250704/460084 获取的代码

function getExample() {
    var a = promiseA(…);
    var b = a.then(function(resultA) {
        // some processing
        return promiseB(…);
    });
    return Promise.all([a, b]).spread(function(resultA, resultB) {
        // more processing
        return // something using both resultA and resultB
    });
}

以及我创建的代码演示 https://jsfiddle.net/Lsobypup/

这个想法是运行多个 Promise 并根据其结果返回一些复合值。

我不明白的是为什么在上面的代码中promiseA只运行一次？在我看来，使用 Promise.all([a, b]) 它应该首先在评估 a 时运行，然后在评估 b 时再次运行，因为它依赖于 a。但正如演示所示，这并没有发生。

Promise.all 中有什么魔法可以实现这一点吗？这种行为有哪些规则？

Answer 1

var b = a.then(function(resultA) {
        // some processing
        return promiseB(…);
    });

这是链接 a 的结果，这意味着如果 a 处于已完成状态，将立即调用回调。您对 Promise a 的解析首先发生，因为它是在 all() 调用中首先遇到的。一旦实现，最终值(value)将始终遵循 promise 。

据此MDN reference :

Internally, a promise can be in one of three states:

• Pending, when the final value is not available yet. This is the only state that may transition to one of the other two states.
• Fulfilled, when and if the final value becomes available. A fulfillment value becomes permanently associated with the promise. This may be any value, including undefined.
• Rejected, if an error prevented the final value from being determined. A rejection reason becomes permanently associated with the promise. This may be any value, including undefined, though it is generally an Error object, like in exception handling.