How to send value from Promise 'then' to 'catch'?

Question

I just want to ask how should I pass the resolve promise to catch if the value on the resolve is not intended.

e.g.

let prom = getPromise();

prom.then(value => {
    if (value.notIWant) {
        // Send to catch <-- my question is here, I want to pass it on the catch.
    }

    // Process data.
}).catch(err => {
    // Pass the error through ipc using json, for logging.
});

I tried to using throw but the object cant be parsed to json and just got an empty object.

ANSWER:

@BohdanKhodakivskyi first comment below is the answer I want.

@31py answer is also correct but the @BohdanKhodakivskyi solution is much simplier and will render the same result.

Also check this question: https://stackoverflow.com/questions/33445415/javascript-promises-reject-vs-throw — Bohdan Khodakivskyi, Dec 28 '17 at 07:59

score 6 · Accepted Answer · answered Dec 28 '17 at 08:51

6

Simply use throw value;. In your case:

prom.then(value => {
    if (value.notIWant) {
        // Send to catch
        throw value;
    }

    // Process data.
}).catch(err => {
    // Pass the error through ipc using json, for logging.
});

Please also note the difference and limitations between using Promise.reject() and throw which is perfectly described in this question. For example, throw will not work in some async scenarios.

answered Dec 28 '17 at 08:51

Bohdan Khodakivskyi

634
5
14

Thanks, actually as long as possible I can omit using callback, I wont use it. – zer09 Dec 28 '17 at 12:15

score 4 · Answer 2 · answered Dec 28 '17 at 07:57

4

You can simply return a rejected promise:

prom.then(value => {
    if (value.notIWant) {
        return Promise.reject('your custom error or object');
    }

    // Process data.
}).catch(err => {
    console.log(err); // prints 'your custom error or object'
});

.catch actually handles any promise rejection in the chain, so if you're returning a rejected promise, the control automatically flows to catch.

answered Dec 28 '17 at 07:57

31piy

23,323
6
47
67

Thanks, it works but @BohdanKhodakivskyi comment is much simpler. – zer09 Dec 28 '17 at 08:09

score 1 · Answer 3 · answered Dec 28 '17 at 07:59

1

why you just not rethrow the error? throw new Error("something");

answered Dec 28 '17 at 07:59

Jovica Madzevic

41
3

Doing this the ```JSON.stringify``` will result an empty object. – zer09 Dec 28 '17 at 08:10

score 0 · Answer 4 · answered Dec 28 '17 at 08:12

0

You can use outside functions to do it:

var processData = function(data) {
   // process data here
}

var logIt = function(data) {
   // do logging here..
}

let prom = getPromise();

prom.then(value => {
    if (value.notIWant) {
        // Send to catch <-- my question is here, I want to pass it on the catch.
        logIt(/*pass any thing*/);
    }

    // Process data.
    processData(data);

}).catch(err => {
      logIt(/*pass any thing*/);
});

answered Dec 28 '17 at 08:12

Dabbas

3,112
7
42
75

Thanks, actually this is what I am doing, but I am trying not to do things twice. I mean writing the ```logIt``` twice. – zer09 Dec 28 '17 at 08:16
you gonna write something some where twice, it's either `reject`, or this method `logIt` – Dabbas Dec 28 '17 at 08:27
Actually this is what DRY is about, you move the code to shared method and call it many times in different places. – Dabbas Dec 28 '17 at 08:29
Yup, I understand you, and I also use that, but there is some scenario that the ```logIt``` cant handle, and need some processing for special cases before it will passed to ```logIt``` method, so I don't want to write does special process in two blocks or create a new method just for that. – zer09 Dec 28 '17 at 08:38

How to send value from Promise 'then' to 'catch'?

4 Answers4