JavaScript Promises和setTimeout

Question

我一直在玩Promises，但我無法通過以下代碼了解正在發生的事情：

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);
});

我期待這個輸出a() responded with: hi from a! 馬上，與b() responded with: hi from b! 並且c() responded with: hi from c! 在各自的setTimeout()觸發后。 但是，我立即獲得以下輸出：

a（）回復：嗨來自！

b（）回復：undefined

c（）回復：undefined

我在想.then()會等待這些承諾，但事實並非如此。 任何幫助，將不勝感激。

Answer 1

您需要return b()並從then處理程序中return c() 。

then只用從其回調返回的后續承諾“替換”第一個承諾。

如果你then回調不return一個承諾，然后then應用到原來的承諾，將立即不管前面的內容/結果的執行then回調。

基本上...

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

反之亦然：

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

Answer 2

你需要return promises鏈接它們：

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

你忘了從函數調用返回。 Javascript函數不會隱式返回。

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);
});