javascript - Promise.resolve() 是如何工作的？具体来说，它究竟是如何打开 thenables 的？

Question

根据 MDN:

...if the value is a thenable (i.e. has a "then" method), the returned promise will "follow" that thenable, adopting its eventual state;

这激起了我的兴趣，所以我想看看是否有可能用纯 Javascript 重新实现这个功能。我最终得到的是:

function resolve(p){
    if (
        p !== null &&
        (
            typeof p === "object" ||
            typeof p === "function"
        ) &&
        typeof p.then === "function"
    ) {
        return p.then(
            function resolved(value){
                return new Promise( function (resolve, reject) {
                    resolve(value);
                });
            },
            function rejected(error){
                return new Promise( function (resolve, reject) {
                    reject(error);
                });
            }
        );
    }
    else {
        return new Promise( function (resolve, reject) {
            resolve(p);
        });
    }
}

所以我用各种值对此进行了测试:

var a = new Promise (resolve => resolve("hello"));
var b = "hello";
var c = {
  then: onFulfilled => onFulfilled("hello")
};
var d = {
  then: onFulfilled => {
    onFulfilled("hello")
  }
}

Promise.resolve(a); //Promise {<resolved>: "hello"}
Promise.resolve(b); //Promise {<resolved>: "hello"}
Promise.resolve(c); //Promise {<resolved>: "hello"}
Promise.resolve(d); //Promise {<resolved>: "hello"}

resolve(a); //Promise {<resolved>: "hello"}
resolve(b); //Promise {<resolved>: "hello"}
resolve(c); //Promise {<resolved>: "hello"}
resolve(d); //undefined -- uh oh.

很明显，因为 d 的 then 方法没有返回任何东西，所以结果是未定义的……但是如果我正确地看这个，Promise.resolve () 正在以某种方式提取这个值？是不是自动把onFulfilled函数调用的返回值拉出来了？还是我完全看错了？

Answer 1

Promise.resolve 不依赖于 .then() 对 thenable 值调用的返回值。它看起来类似于:

function resolve(v) {
    return new Promise((resolve, reject) => {
        if (Object(v) === v && typeof v.then === "function") {
            v.then(resolve, reject);
        } else {
            resolve(v); // fulfill (no recursive resolution)
        }
    });
}

这通常称为 Promise constructor antipattern , 但当我们不能信任 then 方法时，这是有保证的。我建议看看如何 Promises/A+指定处理 thenables。