JavaScript Promises和setTimeout

Question

我一直在玩Promise，但我无法理解以下代码所发生的事情：

org.neo4j.ogm.model.Result

我希望这会在4.1.1.RELEASE点火后立即输出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); }); ，a() responded with: hi from a!和b() responded with: hi from b!。但是，我立即得到以下输出：

  

a（）回答：嗨来自！

     

b（）回复：undefined

     

c（）回复：undefined

我在想c() responded with: hi from c!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return b()处理程序中的return c()和then。

仅

then＆＃34;取代＆＃34;第一个承诺，其后续承诺是从其回调中返回。

如果您的then回调没有return承诺，则then适用于原始承诺，无论内容/结果如何，都将立即执行之前的then回调。

基本上...

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

反过来说：

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

Answer 2

您需要return promises将它们链接起来：

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

你忘了从函数调用返回。 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);
});