CODEWITHSUNDEEP
javascriptnode.jserror-handlingpromisebluebird
Kirk Ouimet
Kirk Ouimet

Reputation: 28374

Catching errors generated in Promises within Promises in JavaScript

Is it possible to have errors bubble up in Promises?

See the code below for reference, I'd like to get promise1.catch to catch the error generated in promise2 (which current does not work with this code):

function test() {
    var promise1 = new Promise(function(resolve1) {
        var promise2 = new Promise(function(resolve2) {
            throw new Error('Error in promise2!');
        });

        promise2.catch(function(error2) {
            console.log('Caught at error2', error2);
        });
    });

    promise1.catch(function(error1) {
        console.log('Caught at error1', error1);
    });
}

test();

Upvotes: 3

Views: 1955

Answers (1)

Benjamin Gruenbaum
Benjamin Gruenbaum

Reputation: 276386

Yes!!

Error propagation in promises is one of its strongest suits. It acts exactly like in synchronous code.

try { 
    throw new Error("Hello");
} catch (e){
    console.log('Caught here, when you catch an error it is handled');
}

Is very similar to:

Promise.try(function(){
    throw new Error("Hello");
}).catch(function(e){
    console.log('Caught here, when you catch an error it is handled');
});

Just like in sequential code, if you want to do some logic to the error but not mark it as handled - you rethrow it:

try { 
   throw new Error("Hello");
} catch (e){
    console.log('Caught here, when you catch an error it is handled');
    throw e; // mark the code as in exceptional state
}

Which becomes:

var p = Promise.try(function(){
   throw new Error("Hello");
}).catch(function(e){
    console.log('Caught here, when you catch an error it is handled');
    throw e; // mark the promise as still rejected, after handling it.
});

p.catch(function(e){
     // handle the exception
});

Note that in Bluebird you can have typed and conditional catches, so if all you're doing is an if on the type or contents of the promise to decide whether or not to handle it - you can save that.

var p = Promise.try(function(){
   throw new IOError("Hello");
}).catch(IOError, function(e){
    console.log('Only handle IOError, do not handle syntax errors');
});

You can also use .error to handle OperationalErrors which originate in promisified APIs. In general OperationalError signifies an error you can recover from (vs a programmer error). For example:

var p = Promise.try(function(){
   throw new Promise.OperationalError("Lol");
}).error(function(e){
    console.log('Only handle operational errors');
});

This has the advantage of not silencing TypeErrors or syntax errors in your code, which can be annoying in JavaScript.

Upvotes: 7

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