Stop execution of a Sequelize promise in Express.js

Question

I'm new to the world of promises and I'm not sure I fully understand how to use them in some cases.

Sequelize recently added support for promises, which really makes my code more readable. A typical scenario is to avoid handling errors several times in infinite callbacks.

The snippet below always return 204 , while I'd like it to return 404 when the photo cannot be found.

Is there a way to tell Sequelize to "stop" the execution of the promise chain after sending 404? Note that res.send is asynchronous so it does not stop the execution.

// Find the original photo
Photo.find(req.params.id).then(function (photo) {
    if (photo) {
        // Delete the photo in the db
        return photo.destroy();
    } else {
        res.send(404);
        // HOW TO STOP PROMISE CHAIN HERE?
    }
}).then(function () {
    res.send(204);
}).catch(function (error) {
    res.send(500, error);
});

Of course this example is trivial and could easily be written with callbacks. But in most cases the code can become way longer.

Answer 1

Your promise chains don't necessarily have to be linear. You can "branch off" and create a separate promise chain for the success case, chaining on as many .then() 's as you need to, while having a separate (shorter) promise chain for the failure case.

Conceptually, that looks like this:

         Photo.find
          /     \
         /       \
    (success)   (failure)
       /           \
      /             \
photo.destroy    res.send(404)
     |
     |
res.send(204)

And in the actual code, that looks like this:

// Find the original photo
Photo.find(req.params.id).then(function (photo) {
    if (photo) {
        // Delete the photo in the db
        return photo.destroy().then(function () {
            res.send(204);
        });
    } else {
        res.send(404);
    }
}).catch(function (error) {
    res.send(500, error);
});