处理Js承诺拒绝

Question

How do you handle an error (eg. "new error" below) that is outside of the promise?

function testError() {
    throw new Error("new error") // how to handle this?
    var p123 = new Promise(function(resolve, reject) {
         resolve(123)
    });
    return p123
};

testError().catch(err => {
        return err;  // code doesn't come here
    })
    .then(ok => {
        console.log(ok)
    });

Answer 1

If you're not sure whether a function will throw (or return a value) synchronously, you can call it using .then() . This will wrap it so that the result will be handled consistently no matter how it is produced:

 function testError() { throw new Error("new error") // how to handle this? var p123 = new Promise(function(resolve, reject) { resolve(123) }); return p123 }; Promise.resolve() .then(testError) .catch(err => { console.error(err); return err; }) .then(ok => { console.log(ok.message) }); 

Answer 2

Since the error doesn't involve the async code, a regular try-catch should do fine here:

try {
  testError().catch(err => {
    return err;  // code doesn't come here
  })
  .then(ok => {
     console.log(ok)
  });
}
catch(e) {
   // 
}

Note that when the async-await pattern finally becomes the native way of resolving promises, the try-catch will also become the native way of handling errors:

try {
    var ok = await testError();
    console.log(ok)
}
catch(e) {
    console.log('e:' +e);
}

As one can easily verify, this one correctly handles both the sync and the async error and is much cleaner than then-catch .

Answer 3

Since the error is thrown outside of the promises, you cannot catch it using a promise catch statement.

You can use a try/catch to catch the error instead.

function testError() {
    throw new Error("new error") // how to handle this?
    var p123 = new Promise(function(resolve, reject) {
         resolve(123)
    });
    return p123
};

try {
  testError().then(ok => {
    console.log(ok)
  });
} catch (err) {
  console.log(err.message);
}

Answer 4

If you can, rewrite your testError function like so

 function testError () { return new Promise(function (resolve, reject) { throw new Error('new error') resolve(123) }) } testError().then(ok => console.log(ok), err => console.error(err.message)) 

1. Run it once to see it throw the error in console.error
2. Comment out the throw line to see the promise resolve successfully

Answer 5

You rewrite it, because making a caller check for both exceptions and rejections is an anti-pattern:

 function testError() { return Promise.resolve().then(() => { throw new Error("new error"); // rejects returned promise return new Promise(function(resolve) { resolve(123); }); }); } testError().catch(err => console.log("Caught " + err)); 

This is implicit with async functions; they always return a promise:

 async function testError() { throw new Error("new error"); // rejects implicit promise return await new Promise(function(resolve) { resolve(123); }); } testError().catch(err => console.log("Caught " + err));