Question

我正在学习承诺，并且我尝试使用以下代码链接它们。在此，首先执行Connect Error (1044) Access denied for user 'user'@'%' to database 'name'：

.then()

这是输出：

function myPromise1()
{
  console.log("Promise", 1);
  return Promise.all([
    new Promise((resolve, reject) => {
      setTimeout(() => {
        console.log("This should be before 'Yay!' for 1");
        resolve(1);
      }, 3000);
    }),
    new Promise((resolve, reject) => {
      setTimeout(() => {
        console.log("This should be before 'Yay!' for 2");
        resolve(2);
      }, 3000);
    })
  ]);
}

function myPromise2() {
  console.log("Promise", 2);
  var arr = [1,2,3,4];
  return Promise.all(arr.map(i => {
    new Promise((resolve, reject) => {
      setTimeout(() => {
        console.log("This should be before 'Yay!' for i= " + i);
        resolve("Success!" + i);
      }, 3000)
    })
  }));
}

function anotherPromise(val) {
  return new Promise((resolve, reject) => {
    setTimeout(() => {
      resolve(val);
    }, 3000);
  });
}

myPromise2()
  .then((msg) => {
  console.log("Yay! " + msg);
  return anotherPromise("Awesome!");
})
  .then((msg2) => {
  console.log("This should be Awesome:", msg2);
})
  .catch((err) => {
  console.log("Uh no, this is a lie --->", err);
});

console.log("This will print as second line after the method name, even though its the last line.");

如您所见，Promise 2 This will print as second line after the method name, even though its the last line. Yay! ,,, This should be before 'Yay!' for i= 1 This should be before 'Yay!' for i= 2 This should be before 'Yay!' for i= 3 This should be before 'Yay!' for i= 4 This should be Awesome: Awesome! Process finished with exit code 0之前被调用，因此不会返回值。

但是，如果我改为呼叫.then()，我会得到所需的输出。这些值也会被返回，并且在两个承诺之后都会调用myPromise1()。

如何解决.then()的问题？

Answer 1

您忘记在此处使用map方法返回：

function myPromise2() {
  console.log("Promise", 2);
  var arr = [1,2,3,4];
  return Promise.all(arr.map(i => {
  /* missing return ---> */  new Promise((resolve, reject) => { 
      setTimeout(() => {
        console.log("This should be before 'Yay!' for i= " + i);
        resolve("Success!" + i);
      }, 3000)
    })
  }));
}

＆＃13;

function myPromise1() {
  console.log("Promise", 1);
  return Promise.all([
    new Promise((resolve, reject) => {
      setTimeout(() => {
        console.log("This should be before 'Yay!' for 1");
        resolve(1);
      }, 3000);
    }),
    new Promise((resolve, reject) => {
      setTimeout(() => {
        console.log("This should be before 'Yay!' for 2");
        resolve(2);
      }, 3000);
    })
  ]);
}

function myPromise2() {
  console.log("Promise", 2);
  var arr = [1, 2, 3, 4];
  return Promise.all(arr.map(i => {
    return new Promise((resolve, reject) => {
      setTimeout(() => {
        console.log("This should be before 'Yay!' for i= " + i);
        resolve("Success!" + i);
      }, 3000)
    })
  }));
}

function anotherPromise(val) {
  return new Promise((resolve, reject) => {
    setTimeout(() => {
      resolve(val);
    }, 3000);
  });
}

myPromise2()
  .then((msg) => {
    console.log("Yay! " + msg);
    return anotherPromise("Awesome!");
  })
  .then((msg2) => {
    console.log("This should be Awesome:", msg2);
  })
  .catch((err) => {
    console.log("Uh no, this is a lie --->", err);
  });

console.log("This will print as second line after the method name, even though its the last line.");

＆＃13;

Answer 2

您忘记return来自map回调的承诺，导致Promise.all被调用4 undefined个数组：

function myPromise2() {
  console.log("Promise", 2);
  var arr = [1,2,3,4];
  return Promise.all(arr.map(i => {
    return new Promise((resolve, reject) => {
//  ^^^^^^
      setTimeout(() => {
        console.log("This should be before 'Yay!' for i= " + i);
        resolve("Success!" + i);
      }, 3000)
    })
  }));
}

或者，删除大括号并使其成为一个简洁的主体，隐式返回表达式：

function myPromise2() {
  console.log("Promise", 2);
  var arr = [1,2,3,4];
  return Promise.all(arr.map(i =>
//                                ^
    new Promise((resolve, reject) => {
      setTimeout(() => {
        console.log("This should be before 'Yay!' for i= " + i);
        resolve("Success!" + i);
      }, 3000)
    })
   ));
//^
}

.then（）在Promise.all（）之前执行

2 个答案: