我的承诺链中的承诺被拒绝 "reject is not defined"

Question

我有以下承诺链：

let promise         = new Promise((resolve, reject) => {
    imgToDisplay.onload = () => {
        resolve(imgToDisplay.width);
    }
})
.then((result) => {
    window.URL.revokeObjectURL(imgToDisplay.src);
    if (result >= 20)
        reject('Image width too large');
})
.then(() => {   
    //I do something with the image file here if it is not rejected
})
.catch((e) => {
    alert(e.message); //This happened, however "reject is not defined"
});

我没有 reject 也没有 resolve 我在第二个 then() 中的承诺。我这里做错了什么？

Answer 1

除非您直接在 Promise 构造函数中，否则您不能调用reject，因为它不在范围内。但是，如果您在 .then 中抛出错误，您可以强制控制流转到链中的下一个 .catch（跳过中间的 .then）：

let promise         = new Promise((resolve, reject) => {
  imgToDisplay.onload = () => {
    resolve(imgToDisplay.width);
  }
})
.then((result) => {
  window.URL.revokeObjectURL(imgToDisplay.src);
  if (result >= 20)
    // This will result in the Promise being rejected:
    throw new Error('Image width too large');
})
.then(() => {   
  //I do something with the image file here if it is not rejected
})
.catch((e) => {
  alert(e.message); //This happened, however "reject is not defined"
});

但是在这种情况下，由于导致被拒绝的 Promise 的测试可以在 Promise 构造函数中完成，您可以改为在构造函数中调用 reject，如果你想要：

let promise         = new Promise((resolve, reject) => {
  imgToDisplay.onload = () => {
    if (imgToDisplay.width >= 20) {
      reject('Image width too large');
    }
    resolve(imgToDisplay.width);
  }
})

Answer 2

在 then 回调中使用 throw 而不是拒绝

let promise         = new Promise((resolve, reject) => {
    imgToDisplay.onload = () => {
        resolve(imgToDisplay.width);
    }
})
.then((result) => {
    window.URL.revokeObjectURL(imgToDisplay.src);
    if (result >= 20)
        throw 'Image width too large';
})
.then(() => {   
    //I do something with the image file here if it is not rejected
})
.catch((e) => {
    alert(e.message); //This happened, however "reject is not defined"
});

我的承诺链中的承诺被拒绝 "reject is not defined"

Promise rejected "reject is not defined" in my chain of Promises

javascript

promise

es6-promise