Express framework and Promises - what to return from a route handler?

Question

I have been using the following promise-based template for my Express route handlers:

app.get('/accounts/:id', function(req, res) {

    AccountService.getAccount(req.params.id)
        .then(function(account) {
            res.send(account);
            //----- then must return a promise here -----
        })
        .catch(function(error) {
            res.status(500).send({'message': error.toString()});
        });
});

While this code works perfectly fine, I am uncomfortable that the onFulfilled function is not returning a promise. This is required by the Promise Specification: then must return a promise. Is there a better way to code this?

Answer 1

You've misinterpreted the spec.

then must return a promise

You are confusing the return value of then with the return value of your callback to then. They are very different.

then must return a promise, and it is. You're invoking .catch on that promise. Nothing you can do can make then not return a promise, it's part of the implementation of whatever Promise library you're using. If the library conforms to the spec, then will return a promise.

Your callback to then does not have to return a promise; whatever your callback does or does not return cannot change then's returning of a promise, it will regardless. Your callback may return a promise which causes then to behave differently, but it will still return a promise.

To summarize:

.then( //THIS must return a promise, and it's up to the implementation to do so


  function(account) { // THIS (your code) doesn't have to return a promise

      res.send(account);
       //----- then must return a promise here -----
       //       ^ NO, that is wrong
       //
       // We're inside a callback being passed to then, the return value
       // of this function is *not* then's return value
  })