JavaScript Promises and setTimeout

JavaScript Promises and setTimeout

本文关键字：setTimeout and Promises JavaScript      更新时间：2024-03-12

我一直在玩Promises，但我很难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);
});

我希望它能立即输出a() responded with: hi from a!，以及b() responded with: hi from b!和c() responded with: hi from c!在它们各自的setTimeout()激发后。然而，我立即得到以下输出：

a（）回应道：你好！

b（）响应为：未定义

c（）响应为：未定义

我原以为.then()会等待这些承诺，但事实并非如此。如有任何帮助，我们将不胜感激。

您需要从then处理程序中调用return b()和return c()。

then仅用后续promise"替换"第一个promise，该后续promise从其回调中返回。

如果您的then回调没有将return作为promise，那么then将应用于原始promise，并且无论上一次then回调的内容/结果如何，它都将立即执行。

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

您需要return promises来链接它们：

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);
});

您忘记了从函数调用返回。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);
});
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