Question

我在理解转发承诺结果（嵌套承诺）的工作原理方面存在问题。

此代码按预期工作（最后我得到整数值）：

＆＃13;

function opThatResolves() {
  return Promise.resolve(1);
}

function opThatWillBeForwarded(x) {
  return new Promise(function(resolve, reject) {
    resolve(yetAnotherNestedPromise(x));
  })
}

function yetAnotherNestedPromise(x) {
  return Promise.resolve(-x);
}

opThatResolves()
  .then(x => opThatWillBeForwarded(x))
  .then(x => x * 2)
  .then(x => x * 2)
  .then(x => console.log("Resolved: " + x))
  .catch(x => console.log("Rejected: " + x))

＆＃13;

所以我认为，如果我将resolve更改为reject，我会在catch块中获得类似的结果（整数值但没有double * 2乘法）。但是我得到一个完整的Promise对象：

＆＃13;

function opThatResolves() {
  return Promise.resolve(1);
}

function opThatWillBeForwarded(x) {
  return new Promise(function(resolve, reject) {
    reject(yetAnotherNestedPromise(x));
  })
}

function yetAnotherNestedPromise(x) {
  return Promise.resolve(-x);
}

opThatResolves()
  .then(x => opThatWillBeForwarded(x))
  .then(x => x * 2)
  .then(x => x * 2)
  .then(x => console.log("Resolved: " + x))
  .catch(x => console.log("Rejected: " + x))

＆＃13;

为什么reject没有＆＃34;打开＆＃34;从yetAnotherNestedPromise返回的承诺就像resolve一样？

Answer 1

因为它不应该首先收到Promise。

reject回调应包含失败的原因（即Error）。

function opThatResolves() {
  return Promise.resolve(1);
}

function opThatWillBeForwarded(x) {
  return new Promise(function(resolve, reject) {
    reject(new Error("failed"));
  })
}

opThatResolves()
  .then(x => opThatWillBeForwarded(x))
  .then(x => x * 2)
  .then(x => x * 2)
  .then(x => console.log("Resolved: " + x))
  .catch(x => console.log("Rejected: " + x));

请注意，错误可以通过后续错误回调重新抛出：

new Promise((resolve, reject) => {
    reject(new Error("failed !"));
})
    .then(null, reason => {
        console.log(1, reason);
        throw reason;
    })
    .catch(reason => {
        console.log(2, reason);
    });

另请注意，如果错误回调无法重新抛出错误（或抛出另一个错误），则后续then方法将启动其成功回调：

new Promise((resolve, reject) => {
    reject(new Error("failed !"));
})
    .then(null, reason => {
        console.log(1, reason);
    })
    .then(result => {
        console.log("Success! Let's throw something else...");
        throw new Error("Another failure");
    })
    .catch(reason => {
        console.log(2, reason);
    });

转发承诺导致Promise链 - 为什么我得到Promise对象而不是被拒绝的值

1 个答案: