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What are standard JS practices about when it's OK *not* to await for an async task?

Coming from a python async background, in python it's very important to always keep track of async tasks (promises). The runtime gives errors for "floating" async tasks with no references. In Javascript, though, in some cases it seems perfectly OK to just start an async task and not await it or even remember its Promise, for instance if you don't care about its return value and just want it to get executed "later".
Are there guidelines or best practices about when in JS/TS, either in browser or in node.js, it's acceptable to just let an async task go, and not keep a reference to it? Clearly if you care about its return value in the mainline code you have to await it, and similarly if you care that errors are reported before the main function completes. What are other cases and considerations?
To be clear I'm not asking about opinions, I'm asking what are the important things to keep in mind when "firing and forgetting" async tasks (if that's the right term).

Whether and when to do this will be a matter of opinion, so that part is off-topic for Stack Overflow.
But the concrete part is the question of: if you do this, are there any precautions you have to take? And the answer there is: Yes. You need to catch and handle errors.
Consider this fire-and-forget async call that returns a promise:
doSomethingAsync();
If the thing being done can fail, that promise can be rejected. If it's rejected and nothing handles that rejection, then:
In a browser, you get an error written to the browser console. 99.9999999% of users won't notice.
In Node.js, you get this error written to the console:
(node:26477) [DEP0018] DeprecationWarning: Unhandled promise rejections are deprecated. In the future, promise rejections that are not handled will terminate the Node.js process with a non-zero exit code.
Note that warning: At some point, a future version of Node.js may start *terminating the process when you allow a promise rejection to go unhandled.
So if you fire-and-forget, be sure to catch errors, even if you just silently swallow them:
doSomethingAsync().catch(error => {}); // Obviously only do this if you really don't
// care about the error!
Fun side note: In JavaScript, you have unhandled promise fulfillment all the time. Consider:
doSomethingAsync()
.then(result => {
// ...do something with result...
})
.catch(error => {
// ...handle/report the error...
});
Are all the promises handled there?
No. And that's just fine.
Remember that then and catch create and return a promise. The promise returned by catch in the above has no handlers attached to it. That's fine, provided it is only ever fulfilled, not rejected. Nothing ever does anything with that fulfillment. :-)

I think the general concern I would have is, do you have a mechanism to handle errors?
eslint has a no-floating-promises rule that at the very least forces you to add a .catch(), which I think is good.

Static/dynamic way to detect dangling promises

I know this problem has been dealt with so many times, but none of them seem to solve the issue of reliably detecting dangling promises (even those that resolve properly).
So I want to be able to figure out a way (whether at runtime or better at static time) to root out "dangling promises", especially of this class:
async function a() {
... do some async operation ...
}
async function b() {
a(); // forgot to call await on it
}
where I have accidentally forget to await on one function and some task that executes asynchronously doesn't get awaited on. Often times these types of bugs don't throw exceptions so I can't just use "unhandledRejection" and call it a day.
At this point after lots of desperate attempts I just need a way to detect this kinda faulty pattern at either (optimally) static/compile/lint time or at runtime. Runtime for me I think should work assuming I have good test coverage.
tl;dr basically, I'm searching for some code that would do something like the following:
Error: Potential dangling promise detected!
...
at b (/dangling.js:5:3)
For each dangling promise
My thought process
I first tried finding some static analysis library that would help to detect these things (theoretically it should be possible but I had no luck with finding such a thing). A while back I remember finding something in the depths of stackoverflow that talked about using some typescript checker to check for dangling promises, though now I can't find it anymore :( . Though at the time changing the entire codebase to typescript was a no-go. (Later I learned you can actually use tsc to typecheck javascript (given that you do some type annotation in comments))
Currently I was previously using node version 11, so I thought about using the node.js's async_hooks API and try to listen on events (apparently simply monkey patching the Promise constructor wouldn't work because node.js bypasses the Promise constructor when creating a Promise object to return from an async function). Using node v11, after a bit of code hackery here it seemed to work (though it's not very efficient cause it throws away a lot of promise optimization in the v8 engine, but it does the job). There was a small hiccup in this entire operation in that I had to still monkey patch the then/catch/finally functions of the Promise API to check if we are currently calling that function (somehow that worked with detecting some dangling promises).
Now enter node v12 (apparently I need this for certain other things that are breaking), and now that hackery (unsurprisingly) completely breaks. After scrutinizing the version diffs, seems like they optimized the await/async implementation. After narrowing down the reason, it seems like await no longer calls the then function of the promise and just directly does some native shenanigans (idk what really).
Now I'm actually kinda desperate for some solution (and maybe if Typescript has some way of type-checking these dangling promises, what was the option to enable this check? I know (tested) that tsc doesn't do this by default).

You are most likely looking for the no-floating-promises rule of typescript-eslint

How to deal with dangling promises

Oftentimes I want to call a promise and just let it run asynchronously instead of waiting for it. Like:
... some code ...
fetchMyCount().then(count => {
updateTheUI(count);
});
... more code ...
Now this works great, but oftentimes I don't put a failure handler on the promise because that is pretty onerous for each one. That would be like putting a try {} catch {} on every one of my data fetches.
The problem is, if it fails, it does so silently. It doesn't throw an exception that my window.onerror can get, it doesn't log, it doesn't do anything.
Now I can make a nice wrapper like:
Async(fetchMycount().then(count => {
updateTheUI(count);
}));
which can generically deal with promises, but then I have to remember to wrap each and every async call.
Is there a way to have a top-level handler like window.onerror or a big try {} catch {} wrapper that can catch all un-dealt-with rejected promises so I can log them and make sure they get fixed?

This is a common use case. Your problem is real. In Node, you have process.on("unhandledRejection", ... events which you can use. Sadly, these are not available in browsers yet. Some libraries which are compatible with ES2015 promises offer them and you can use those until native promises get support.
The upside is that support in browsers is coming and browsers will eventually support these hooks.
Currently, I've convinced the polyfill to add it, so if you're using the core-js polyfill you can do:
window.onunhandledrejection = function(e){
// handle here e.promise e.reason
};
Note that this assumes that something is an unhandled rejection if: It's unhandled, and an error handler was not attached synchronously (within a task to be precise).

Exactly for issues like this, I wrote my own utility for writing asynchronous code:
https://github.com/vacuumlabs/yacol/
It does not help you with your existing code, however with newly written code it provides (among many other things) quite advanced error-handling capabilities.
This said, I don't think that "unhandledRejection" / "uncaughtException" events are good (although it's the best node.js natively provides) tools to do proper error handling:
First, you can only attach it process-wide, so it forces you to handle all uncaught errors the same way (unlike with try-catch where you can react differently to errors that originate from different parts of your code)
Second, "unhandledRejection" is called if .catch is not attached to the Promise in the same run of event loop. This may result in false errors being caught; note it is pretty OK to attach error handler to the promise later!

How do I work with the Bluebird error handler?

Introduction
This question aims to, eventually, resolve a problem I'm having in development with Bluebird. However, I'm also using the opportunity to get some things clarified, so there will be side-questions. I also apologize in advance for any feelings of confusion or boredom you might experience while reading the story to come.
Questions
As far as my understanding goes, Bluebird attempts to intelligently catch ignored rejections, according to the following strategy:
The second approach, which is what bluebird by default takes, is to call a registered handler if a rejection is unhandled by the start of a second turn.
-- Bluebird Readme # Error Handling
Now herein lies the first side-question: What does "the start of a second turn" mean?
Later in the same section, the following is documented:
Of course this is not perfect, if your code for some reason needs to swoop in and attach error handler to some promise after the promise has been hanging around a while then you will see annoying messages. In that case you can use the .done() method to signal that any hanging exceptions should be thrown.
-- Bluebird Readme # Error Handling
Now, I believe that I ran in to the situation described above with my use case being as follows:
I call a function which will provide me the promise to which I attach a .catch():
lib.loadUrls()
.catch(function(e){console.log(e);});
Internally, that function loads content from URL1 and based on the content, loads content from URL2 in sequence:
lib.loadUrls =
return this.loadUrl1()
.then(this.loadUrl2.bind(this))
If the second promise in this chain is rejected the error is handled by the catch first, and then by Bluebirds Possibly unhandled error handler as well.
This last behavior is unwanted and I can not figure out why it's doing this. So question two could be: Why, despite an error handler being attached and executed, does Bluebird still consider the possibility of the error being "unhandled"?
I'm thinking, apparently the promise "has been hanging around for a while" by the time the rejection propagates to the .catch(). In which case I should solve it (according to the quoted documentation) by "using the .done()".
Now, I've tried several things, but I can't quite figure out how to "use the .done" in this scenario. (It doesn't help that .done() returns undefined, preventing me from .finally-ing.)
So this introduces my third, and fourth questions: How do I use .done() in this scenario, and how do I explicitly conclude a promise-chain, but still attach a .finally()
EDIT 1: I've created some JSFiddles to reproduce the bug:
Using Bluebird 1.0.5 reproduces the bug.
Using the latest Bluebird.js reproduces the bug (at this time)
Using Beta version 0.10.0-1 does not reproduce the bug.
EDIT 2: The dev fixed the bug.

This was indeed just a regression bug in bluebird and is now fixed.
The bit about needing to use .done() is pretty much theoretical, you won't run in a situation in practice where you would need to attach error handlers in such a way that would cause false positives to be reported.

It's most likely Bluebird bug, as handled error should not be reported (assuming you properly handle promises in loadUrls body). So probably you should report it to Bluebird issue tracker.
Concerning done, it's pure access function that's best if used instead of then or catch when you just process resolved value.
It's good to treat done as first choice function and use then and catch only if you really need transformation to other promise, with such approach you also don't need to rely on buggy errors monitoring (best to turn it off completely).
In your case done should be used as:
lib.loadUrls().done(); // eventual error will be thrown
and if for some reason you want to handle error specifically (e.g. in running server you don't want it throw) do:
lib.loadUrls().done(null, function (error) {
// handle error
});
EDIT:
Just noticed, that you still want to process promise returned by lib.loadUrls().catch(..) with finally. In such case done is not a solution. done should only be used as final call, but you can combine it with finally as follows:
lib.loadUrls().finally(function () {
// cleanup
}).done();

How do I debug my asynchronous, promise based code if the library is swallowing all the exceptions?

The Problem
JSFiddle:
http://jsfiddle.net/missingno/Gz8Pe/2/
I have some code that looks like this:
var d = new Deferred();
d.resolve(17);
return d.then(function(){
//do some stuff...
})
.then(function(){
var obj = a_funtion_that_returns_null_on_IE();
var x = obj.some_property; //BOOM!
});
The problem is that when I am on IE all I can see are 'obj' is null or not an object errors, without any reference to the corresponding line number and without the debugger halting at the offending line (like I wish it would).
This kind of issue is making the code a pain to debug and the only solutions I can think of right now (messing around with the control flow library or resorting to step-by-step debugging with the debugger or console.log) are things I would rather not have to do.
What I think is going on
In order to allow errbacks to be added after the chain is fired, then will preemptively catch any exceptions thrown by the callbacks. I think this is the reason for the IE debugger not halting on the error or showing the usual error message witht the line number in it.
The error messages without the line numbers are coming from the control-flow library: it provides a deferredOnError hook that is called whenever an exception is caught and saved for later and the default behaviour is console.error-ing the Error object:
dojo.config.deferredOnError = function(err){
//a chance to log the exception after it is captured by "then"
//or do other things with it
console.error(err);
}
Sadly, I could not figure out a way to get the line number or stack trace from the error object in IE and the hook is called in a way that does not allow me to just rethrow the exception and let it bubble up to the toplevel.
What I want
I want to have a better way to debug the async code then goind along with the debugger step-by-step. In the best case a way to have the debugger halt on the exceptions (as it does on unhandled exceptions) or at least a way to get line numbers or stack traces from the Error object that was thrown.

This works with any framework without prior configuration and all recent browsers support this.
Pause On Caught Exceptions: This will actually stop javascript's execution and will take you exactly where the problematic code is, as it's happening.
In Chrome:
Developer Tools,
Sources tab,
Pause on exceptions (stop-like icon) and then
the Pause On Caught Exceptions checkbox

What I ended up doing
I added a sequencing function to my mini library of async helper functions. It basically runs a sequence of "then" calls, except that it adds extra intermediate steps to rethrow any exceptions that end up being caught by the Deferreds. It also accepts an optional error handler to catch the exceptions.
When I call it it looks like this:
go([
function(){
return 17;
},
function(x){
//return some stuff
},
function(){
var obj = a_function_that_returns_null_on_IE();
var x = obj.some_property; //BOOM!
}
], function(){
//an optional error handler
});
Another reason that I did things this way is that I have lots of code that needs to work simultaneously with either sync or async code (using Deferred.when to do the chaining). Using my custom function let me use a single, unified syntax and the errors not being captured in the async case is consistent with the sync case, where there are no Deferreds involved. I also think its OK to not capture the error, since unlike in the general case, when I am using "go" I know a-priori what code will be called, so there is no need to capture exceptions for if someone needs to catch them in the future.
Also, using a custom solution gave me the freedom to enforce some personal design preferences :)
In addition to that, I ended up reducing the amount of exceptions I generate myself throughout the code base. Managing exceptions in async code is more annoying then usual and sometimes its simpler to just fallback into handling error conditions by returning null or an error code.
Also, we made sure that any exceptions that we create ourselves are instances of the builtin Error class, intead of other objects. The reason is that the builtin Error class records the line number and stack trace of where it was generated in a kind of cross-browser way.

2016 solution
If you're using native ES Promises do absolutely nothing; Chrome automatically reports uncaught promise rejections in the console.
Notice how the caught one (Second fail) doesn't show anything but the uncaught rejection appears in the console after the code is done running.