errors in transition handlers are swallowed

[ryanflorence]

I hate promises ... can we not use promises?

willTransitionFrom: function() {
  undefined(); // ignored :(
}

[KidkArolis]

well.. the rule is quite simple - promises should always be returned

here many promises are being created in a loop, but only the last one is getting returned
https://github.com/rpflorence/react-router/blob/7fc55834e9f607f553c96774c99c8cc7785b1c26/lib/router.js#L522

replace with smth like

return when.map(reversedArray(matches), function () {
  ... make your promise ...
  return promise;
});

[andreypopp]

I'd also suggest using bluebird promise library which logs uncaught errors in browser console.

[KidkArolis]

Or when.

[ryanflorence]

"just do promises right" :P

Anyway, not actually interested in a promise conversation (I was just bugging Michael about them before this commit, talking about how people always mess them up and then app code errors get swallowed, so it was fun for me to come back and say "see ... pain in the neck").

I have two requirements though:

1. We don't swallow user's code in our promise usage
2. We support a simple callback in transition hooks so people don't have to use promises in application code because half of us have had nothing but bad experiences with them.

I do not want to constantly be having promise conversations with people using this library.

[mjackson]

I'll take a look at this today. @rpflorence I agree that the last thing I want is to have endless conversations with people trying to track down swallowed errors, so we'll just fix it and not swallow them. If we can't, we'll remove support for them.

[mjackson]

BTW, I'd be happy to discuss support for callbacks in transition hooks in a separate issue. Promises give us guarantees that callbacks don't, and they present a really clean interface for dealing with asynchronous code.

@andreypopp I used es6-promise because it's just a shim instead of a full-blown library with lots of extra bells and whistles. Using es6-promise today means we can easily remove it at some point in the future when more runtimes support a native Promise.