Asynchronous exception handling with bluebird promises

Question

Asynchronous exception handling with bluebird promises

What is the best way to handle this scenario. I am in a controlled environment and I don't want to crash.

var Promise = require('bluebird');

function getPromise(){
    return new Promise(function(done, reject){
        setTimeout(function(){
                throw new Error("AJAJAJA");
        }, 500);
    });
}

var p = getPromise();
    p.then(function(){
        console.log("Yay");
    }).error(function(e){
        console.log("Rejected",e);
    }).catch(Error, function(e){
        console.log("Error",e);
    }).catch(function(e){
        console.log("Unknown", e);
    });

When throwing from within the setTimeout we will always get:

$ node bluebird.js

c:\blp\rplus\bbcode\scratchboard\bluebird.js:6
                throw new Error("AJAJAJA");
                      ^
Error: AJAJAJA
    at null._onTimeout (c:\blp\rplus\bbcode\scratchboard\bluebird.js:6:23)
    at Timer.listOnTimeout [as ontimeout] (timers.js:110:15)

If the throw occurs before the setTimeout then bluebirds catch will pick it up:

var Promise = require('bluebird');

function getPromise(){

    return new Promise(function(done, reject){
        throw new Error("Oh no!");
        setTimeout(function(){
            console.log("hihihihi")
        }, 500);
    });
}

var p = getPromise();
    p.then(function(){
        console.log("Yay");
    }).error(function(e){
        console.log("Rejected",e);
    }).catch(Error, function(e){
        console.log("Error",e);
    }).catch(function(e){
        console.log("Unknown", e);
    });

Results in:

$ node bluebird.js
Error [Error: Oh no!]

Which is great - but how would one handle a rogue async callback of this nature in node or the browser.

javascript
node.js
asynchronous
promise
bluebird

Answer 1

Promises are not domains, they will not catch exceptions from asynchronous callbacks. You just can't do that.

Promises do however catch exceptions that are thrown from within a then callback. So use

function getPromise(){
    return new Promise(function(done, reject){
        setTimeout(done, 500);
    }).then(function() {
        console.log("hihihihi");
        throw new Error("Oh no!");
    });
}

(or just Promise.delay) to get the desired behaviour. Never throw in custom (non-promise) async callbacks, always reject the surrounding promise. Use try-catch if it really needs to be.