pdbtest: Improve Promise patterns

[nolanlawson]

Excellent article on the Blackberry Dev Blog. :) I took a look at your code, though, and I see a few places for improvement.

First, I think you might not be aware that you can improve your Promise patterns to avoid the "pyramid of doom" (i.e. code marching steadily to the right, until you are a dozen indentation levels deep).

For instance, this code is needlessly pyramid-y. (It's a common mistake folks make with Promises, though!) In general, what you don't want to do is this:

// BAD pyramid code
doSomething().then(function () {
  doSomethingElse().then(function () {
    doAnotherThing().then(function () {
      // etc.
    }).catch(function (err) {
      // handle error
    });
  }).catch(function (err) {
    // handle error
  });
}).catch(function (err) {
  // handle error
});

Instead you can do:

// GOOD pyramid-less code!
doSomething().then(function () {
  return doSomethingElse();
}).then(function () {
  return doAnotherThing();
}).catch(function (err) {
  // handle errors all in one place!
});

Now let's say that you want to handle an error in a particular place. You can simply do:

doSomething().then(function () {
  return doSomethingElse();
}).catch(function (err) {
  // handle some very specific error
  return anotherPromiseOrMaybeAValue();
}).then(function () {
  return doAnotherThing();
}).catch(function (err) {
  // handle errors all in one place!
});

Also, a lot of your error handling seems to be caused by 409s and 404s, which you can now handle more elegantly with the pouchdb-upsert plugin. But it's good to get some promise practice in, so maybe you'll want to catch your own 409s and 404s using the pattern I described above. :)

Great work, looking forward to your next article!

[nolanlawson]

Oh yeah, and the PouchDB guide to async code may help you out here, and the video in that article is a must-watch! :)

[timwindsor]

Thanks Nolan - I'm not very familiar with the Promise pattern, and I don't think the student who wrote this is either. I really like the improvements to the code though, so I'm going to learn more and see if I can apply it, both here and elsewhere. Particularly I want see how APIs are written to work that way so we can apply it to our Cordova plugins where it makes sense to do so.