承诺：拒绝的一些问题

Question

I have an issue with promise in 'Angular 2'.

Please see my code below:

getPromise1().then((result) => { 
        console.log("promise1 result"); 
    }, (error) => { 
        console.log("promise1 error"); 
    });

function getPromise1() {
    return getPromise2().then((result) => {
        console.log("promise2 result");
    }, (error) => {
        console.log("promise2 error");
    });

}

function getPromise2() {
    return new Promise((resolve, reject) => {
        reject("error");
    });
}

And the result is: promise2 error and promise1 result .
I don't know why not promise2 error and promise1 error
Any problem and the solution for this case?
This is preview link: http://plnkr.co/edit/RTc1wYfO8e1YPUrXM6GN

Answer 1

When a promise rejects, the control jumps to the closest rejection handler down the chain .

so, Here the .catch block finishes normally. So the next successful handler is called. Or it could return something, that would be the same.

Hence the result

And the result is: promise2 error and promise1 result .

So you could have as many .then as we want, and then use a single .catch at the end to handle errors in all of them.

But to get the following result

I don't know why not promise2 error and promise1 error

you need to re-throw the error

throw error;

And here the .catch block analyzes the error and throws it again:

function getPromise1() {
    return getPromise2().then((result) => {
        console.log("promise2 result");
    }, (error) => {
        console.log("promise2 error");
        throw error;
    });

}

Answer 2

If you handle a rejection in any promise chain then that's going to make the resulting chain to be fulfilled until an error is found then it goes back to being rejected. Think about it in terms of sync code, would you have expected this to print error A and error B ?

function myFnA(){
    throw new Error('A is bad');
}

function myFnB(){
   try {
      myFnA();
      console.log('good A');
   } catch(e){
      console.log('error A');
   }
}

function myFnC(){
   try {
      myFnB();
      console.log('good B');
   }
   catch(e){
      console.log('error B');
   }
}

myFnC();

myFnB fixes myFnA error. So myFnB is doing it's job right.