new Promise(function(resolve, reject) { setTimeout(() => { throw new Error("Whoops!"); }, 1000); }).catch(alert);

My gut told me calling an async function from the setTimeout callback was a bad thing. Since the setTimeout machinery ignores the return value of the function, there is no way it was awaiting on it. This means that there will be an unhandled promise. An unhandled promise could mean problems if the function called in the callback takes a long time to complete or throws an error.

The callback executed by setTimeout is not expected to return anything, it just ignores the returned value. Since once you enter the promise/async world in JavaScript you cannot escape, I was left to wonder what happens when the setTimeout callback returns a promise?

test2 being marked async does wrap your return value in a new promise:

const rejectedP = Promise.reject('-'); const finallyP = rejectedP.finally(); const result1 = rejectedP; const result2 = new Promise(resolve => { const rejectedP = Promise.reject('-'); const finallyP = rejectedP.finally(); resolve(rejectedP); }); we can see that the first snippet creates two promises (result1 and rejectedP being the same) while the second snippet creates three promises. All of these promises are rejected, but the rejectedP rejection is handled by the callbacks attached to it, both through ….finally() and resolve(…) (which internally does ….then(resolve, reject)). finallyP is the promise whose rejection is not handled in the both examples. In the second example, result2 is a promise distinct from rejectedP that is also not handled, causing the second event.

You basically did var a = promise.then(…); var b = promise.catch(…); creating a branch in the chain. If promise is getting rejected now, the catch callback will be called and b will be a fulfilled promise just fine, but the a promise is getting rejected too and nobody handles that. Instead, you should use both arguments of then and write Requirement.create({id: id, data: req.body.data, deleted: false}) .then(requirement => { res.json(requirement); }, reason => { let err = {'error': reason}; res.json(err); });

Just "ignoring" a Promise result if it is not longer needed is an antipattern in my opinion.

Generator based control flow goodness for nodejs and the browser, using promises, letting you write non-blocking code in a nice-ish way.

Your problem is that you were returning the rejected loginDaoCall, not the promise where the error was already handled. loginApi.login(user, password) did indeed return a rejected promise, and even while that was handled in another branch, the promise returned by the further .then() does also get rejected and was not handled.

// LoginApi.js return loginDao.login(username, password).then(function (res) { store.dispatch(loginSuccess()); log.log("[loginApi.login] END"); return true; }, function (err) { store.dispatch(loginFail()); errorUtils.dispatchErrorWithTimeout(errorLogin); log.log(err); return false; }); // never supposed to reject

The "invisible try..catch" around the executor automatically catches the error and turns it into rejected promise.

This treatment of thenables allows promise implementations to interoperate, as long as they expose a Promises/A+-compliant then method. It also allows Promises/A+ implementations to “assimilate” nonconformant implementations with reasonable then methods.

the promise specification explicitly does not make a distinction

This isn't really a bug, because if you have an async function that returns a function, it doesn't really return a function - it returns a promise. I don't remember whether the Svelte internals currently check for the onMount callback's return being a function or whether they check for it being truthy. Maybe there's some adjustment to be made there.

The error callback becomes a reject, while the "happy path" callback becomes a resolve.

Here we store the three Promise objects in variables, which has the effect of setting off their associated processes all running simultaneously. Next, we await their results — because the promises all started processing at essentially the same time, the promises will all fulfill at the same time

.catch(error => download_count = error) // check for error in template

Making the value change to a Promise for the duration would be more fitting for many uses.

The promises of a chain are nested like Russian dolls, but get popped like the top of a stack.

Similarly, .catch() is really just a .then() without a slot for handleFulfilled.

in the absence of an immediate need, it is simpler to leave out error handling until a final .catch() statement.

Handling a rejected promise too early has consequences further down the promise chain.   Sometimes there is no choice, because an error must be handled immediately.