Angular promises: trigger catch(...) from within a then(...) statement?

Question

Is it possible to trigger the catch(...) portion of a promise chain from within a then(...) portion?

For example, I make an $http request and chain some behavior. The $http resolves successfully, but upon processing of the data, it is clear the data is more suited for the error case, so we want to activate error handler instead.

The issue is -- I've got this promise chain being used within two locations; one is within the service itself, and the other is in the controller that activated the service. The controller is the one that handles the catch(...) portion of the promise chain, where it opens a modal that displays an error message.

So far, I've been able to chain the promise such that, whenever a then(...) or catch(...) is triggered within the service, I can return a result, and it will then be triggered within the controller -- just as expected.

But how do I have then(...) trigger within service, but return a result such that the catch(...) will be triggered instead within the controller?

I tried using $q.reject(...) to create and return a new promise, as the return value from within the then(...) function, but that didn't seem to work.

Example:

var url = 'http://google.com';
$http.get(url).then(handleFirst).then(handleSecond).catch(error);

function handleFirst (response) {
    console.log("HandleFirstCalled", response);
    return response;
}

function handleSecond (response) {
    console.log("HandleSecondCalled", response);
    return response;
}

function error (response) {
    console.log("ErrorCalled", response);
}

How do you get handleFirst(...), skip executing handleSecond(...), and execute error(...) instead? Note: you can't just call error(response) because there's no access to it from the outside promise chain.

[Edit:] Found the solution, return $q.reject(response); does work. However, the Service catch(...) function has to always return $q.reject(response). Previously, I had it just return response;, which would be continued by the Controller promise chain. But then the Controller would activate then(...) within its chain.

So with return response; it went:

Service -> $http -> then(...) -> catch(...) -> Controller -> then (...)

By changing it return $q.reject(response); it goes:

Service -> $http -> then(...) -> catch(...) -> Controller -> catch (...)

Answer 1

You have two options in Angular to reject the returned promise from within a then.

throw new Error(...); // this will reject the promise and trigger $exceptionHandler

Or:

return $q.reject(new Error(...)); // this will reject the promise

Use the first one to signal an error you can't really recover from and the latter for errors you can recover from.

Note that in standard (ES6 promises) and other compliant promises (like Bluebird) this distinction between throwing and rejecting isn't made.