JavaScript Promise 和 setTimeout

Question

我一直在玩 Promises，但我无法理解以下代码发生了什么：

function a() { 
    return new Promise(function (resolve, reject) { 
        resolve("hi from a!");
    });
}

function b() { 
    return new Promise(function (resolve, reject) { 
        setTimeout(function () { 
            resolve("hi from b!");
        }, 5000);
    });
}

function c() { 
    return new Promise(function (resolve, reject) { 
        setTimeout(function () { 
            resolve("hi from c!");
        }, 1000);
    });
}

a().then(function (resultFromA) {
    console.log("a() responded with: " + resultFromA);
    b();
}).then(function (resultFromB) { 
    console.log("b() responded with: " + resultFromB);
    c();
}).then(function (resultFromC) { 
    console.log("c() responded with: " + resultFromC);
});

我希望这会立即输出a() responded with: hi from a!，以及b() responded with: hi from b! 和c() responded with: hi from c! 在各自的setTimeout() 触发后。但是，我立即得到以下输出：

a() 回复：hi from a!

b() 回应：未定义

c() 回应：未定义

我在想.then() 会等待这些承诺，但事实并非如此。任何帮助将不胜感激。

Answer 1

您需要在 then 处理程序中使用 return b() 和 return c()。

then 仅将第一个 Promise 替换为随后的 Promise，该 Promise 从其回调中返回。

如果你的then 回调不是return 一个promise，那么then 会应用到原来的promise，它会立即执行而不管之前的then 回调的内容/结果。

基本上...

a().then(function () {
  b()
}).then( # This "then" is chained off of a's promise

反之：

a().then(function () {
  return b()
}).then( # This "then" is chained off of b's promise

Answer 2

您需要return promises 链接它们：

a().then(function (resultFromA) {
  console.log("a() responded with: " + resultFromA);
  // return b() promise
  return b();
}).then(function (resultFromB) { 
  console.log("b() responded with: " + resultFromB);
  // return c() promise
  return c();
}).then(function (resultFromC) { 
  console.log("c() responded with: " + resultFromC);
});

Answer 3

您忘记从函数调用返回。 Javascript 函数不会隐式返回。

function a() { 
    return new Promise(function (resolve, reject) { 
        resolve("hi from a!");
    });
}

function b() { 
    return new Promise(function (resolve, reject) { 
        setTimeout(function () { 
            resolve("hi from b!");
        }, 5000);
    });
}

function c() { 
    return new Promise(function (resolve, reject) { 
        setTimeout(function () { 
            resolve("hi from c!");
        }, 1000);
    });
}

a().then(function (resultFromA) {
    console.log("a() responded with: " + resultFromA);
    return b(); // return 
}).then(function (resultFromB) { 
    console.log("b() responded with: " + resultFromB);
    return c(); // return
}).then(function (resultFromC) { 
    console.log("c() responded with: " + resultFromC);
});