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

But if an error is thrown by getSession, the two catch are called, as well as the second then. I'd like to stop the error propagation at the first catch, so that the second catch is only called when getUser throws, and the second then when getUser succeeds. What do?

why do you call next() if there's an error? is there an abort() like next() you can use? — dandavis
– dandavis, Commented Jul 7, 2014 at 20:54
I'm using express. next calls the next middleware. If you pass it a parameter, then it skips to the error middleware. — Alexandre Kirszenberg
– Alexandre Kirszenberg, Commented Jul 7, 2014 at 20:56

Bergi · Accepted Answer · 2014-07-07 20:58:43Z

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);

Mercury · Accepted Answer · 2014-07-07 21:07:22Z

Since you're using bluebird for promises, you actually don't need a catch statement after every function. You can chain all your thens together, and then close the whole thing off with a single catch. Something like this:

getSession(sessionId)
  .then(function (session) {
    return getUser(session.user_id);
  })
  .then(function (user) {
    req.user = user;
    next();
  })
  .catch(function(error){
    /* potentially some code for generating an error specific message here */
    next(error);
  });

Assuming the error messages tell you what the error is, it's still possible to send an error specific message like 'session not found' or 'user not found', but you'll just have to look into the error message to see what it gives you.

Note: I'm sure you probably have a reason for calling next regardless if there's an error or not, but it might be useful to throw in a console.error(error) in the case that you get an error. Alternatively, you could use some other error handling function, whether it's a console.error, or res.send(404), or something of the like.

Burak Dalkıran · Accepted Answer · 2018-11-14 11:58:53Z

2

I am using it like that:

getSession(x)
.then(function (a) {
    ...
})
.then(function (b) {
    if(err){
        throw next(new Error('err msg'))
    }
    ...
})
.then(function (c) {
    ...
})
.catch(next);

answered Nov 14, 2018 at 11:58

Burak Dalkıran

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