Chaining promises with then and catch

Question

I\'m using the bluebird Promise library. I\'d like to chain promises and catch specific promises errors. Here\'s what I\'m doing :

getSession(sessionId)
  .c         


        

Answer 1

The promise that is returned by the .catch method will still be resolved with the result of the callback, it doesn't just stop the propagation of the chain. You will either need to branch the chain:

var session = getSession(sessionId);
session.catch(function (err) { next(new Error('session not found')); });
var user = session.get("user_id").then(getUser);
user.catch(function (err) { next(new Error('user not found')); })
user.then(function (user) {
    req.user = user;
    next();
});

or use the second callback to then:

getSession(sessionId).then(function(session) {
    getUser(session.user_id).then(function (user) {
        req.user = user;
        next();
    }, function (err) {
        next(new Error('user not found'));
    });
}, function (err) {
    next(new Error('session not found'));
});

---

Alternatively, the better way would to just propagate the errors through the chain, and call next only in the very end:

getSession(sessionId).catch(function (err) {
    throw new Error('session not found'));
}).then(function(session) {
    return getUser(session.user_id).catch(function (err) {
        throw new Error('user not found'));
    })
}).then(function (user) {
    req.user = user;
    return null;
}).then(next, next);