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Achieving UI/logic separation when the logic requires callback functions

As far as I understand, in good practice, the UI code should invoke the logic whenever needed, but the logic should know nothing about the GUI ("loose coupling", see for example How can I separate the user interface from the business logic while still maintaining efficiency?).
I am currently writing a chrome web app that uses the chrome.serial api. Most functions from this api are non-blocking and instead invoke a callback function when their work is done. For example
chrome.serial.getDevices(callback)
searches for devices and than calls callback with a list of found devices.
Now, after chrome.serial.getDevices is called from the logic part of my code, its results ultimately have to be communicated back to the UI code.
How do I achieve clean UI/logic separation in this case? Does my UI need to register callback funcions with my logic code for every call it makes? That seems to violate the above principle of loose coupling and feels like it becomes very confusing very quickly.

You can use Promises. Initiate them in your controller code and pass them to the view. The view will then call its .then() method and display the result.
For example:
//controller.js
myAsyncTask = new Promise(resolve,reject=>{
chrome.serial.getDevices(resolve)
})
view(myAsyncTask);
//view.js
function view(myAsyncTask){
myAsyncTask.then(render);
}

If you are using build tools, such as Webpack or Browserify, then you can have your "logic object" extend Node's EventEmitter (there are other implementations that work in-browser, such as https://github.com/Olical/EventEmitter, if you don't want to bundle Node APIs in with a build tool).
Your "logic object", which is a specialized EventEmitter, operates the chrome async API, which contacts the serial devices, then processing the results according to your data layer rules, and then emits its own events when it has something useful for the UI.
The UI listens both listens to, and emits, events on your "logic object", depending on what's happening. Bonus: this event emitter can also be used by separate UI objects to communicate to each other, via events.
EventEmitter is the key that will make this kind of separation feel clean, simple, and extendable.

What's the idiomatic way to keep UI in sync with ajax calls when using Flux?

The general problem: Let's say I have a button with an onClick handler calling an action creator. The action does an ajax call which dispatches a message when ajax responds, and this in some way affects the UI. Given this basic pattern there's nothing stopping the user from clicking this button multiple times, and thus running the ajax call multiple times.
This is something that doesn't seem to be touched upon in the React or Flux documentation (as far as I have seen), so I've tried to come up with some methods on my own.
Here are those methods
Use lodash.throttle on a method which does an ajax call so that multiple clicks in quick succession don't create multiple calls.
Use lodash.debounce on a method so that ajax is only called once a user hasn't done any activity for a bit. This is how I'm doing semi-realtime updates of text fields on change.
Dispatch an "is updating" message to stores when the action is first called and then dispatch a "done" message when the ajax call returns. Do stuff like disabling input on the initial message and then re-enable on the second.
The third method seems to be the best in terms of functionality since it allows you to make the user interface reflect exactly what's going on, but it's also incredibly verbose. It clutters absolutely everything up with tons of extra state, handler methods, etc...
I don't feel like any of these methods are really idiomatic. What is?

Hal is pretty much correct. Dispatching multiple messages is the Fluxiest way to go.
However, I would be wary of dispatching an IS_UPDATING message. This makes reasoning about your code harder because for each AJAX action you're dispatching several actions at once.
The idiomatic solution is to split your AJAX "actions" (action-creator-actions) into three dispatched actions: MY_ACTION, MY_ACTION_SUCCESS, MY_ACTION_FAILURE, handling each instance appropriately, and tracking "pending-ness" along the way.
For example:
// MyActionCreator.js
// because this is in a closure, you can even use the promise
// or whatever you want as a sort of "ID" to handle multiple
// requests at one time.
postMessage() {
dispatch('POST_MESSAGE', { ... } );
api.slowMessagePostingAjaxThingy().then(
(success) => { dispatch('POST_MESSAGE_SUCCESS', { ... }); },
(failure) => { dispatch('POST_MESSAGE_FAILURE', { ... }); }
);
}
// MyStore.js
on('POST_MESSAGE', (payload) => { /* do stuff */ });
on('POST_MESSAGE_SUCCESS', (payload) => { /* handle success */ });
on('POST_MESSAGE_FAILURE', (payload) => { /* handle failure */ });
This gives you several benefits over your alternate solutions:
Your store is exclusively in control of whether an item is pending or not. You don't have to worry about changing UI state on actions in your UI code: you can have your UI look exclusively to a pending property of your store for truth. This is probably the biggest reason for using Flux over MVC systems.
You have a clean interface for taking your actions. It's easy to reason about and easy to attach other stores to this data (if you have a LatestMessageStore or something, it's easy to subscribe to these events). This is the benefit over using IS_UPDATING as Hal suggested.
You save your lodash calls for when they semantically make sense— like when you may be inundated with legitimate data (a text field).
You can easily switch between optimistic updates (change the store when POST_MESSAGE is called) or pessimistic updates (change the store on POST_MESSAGE_SUCCESS).

I would argue that the third method is the correct way, but I don't find it to be verbose. A lot of React code that I see written sort of misses the spirit of React with its idea of very small, composable components. When large monolithic components are created, yes, things can get very messy.
But if the button in question is its own component, then it can take care of rendering based on its state. When a user clicks the button, the state of just that component changes -- and it renders it in a way that it can't be clicked again.
Once the store has notified that component that it has changed, the component can set its state back -- and with it, re-render itself.
It's a pretty straight-forward process; it just requires thinking about pages as a collection of small, composable units.

Forever loop while waiting for asynchronous task?

I'm wondering if there's a way to cause JavaScript to wait for some variable-length code execution to finish before continuing using events and loops. Before answering with using timeouts, callbacks or referencing this as a duplicate, hear me out.
I want to expose a large API to a web worker. I want this API to feel 'native' in the sense that you can access each member using a getter which gets the information from the other thread. My initial idea was to compile the API and rebuild the entire object on the worker. While this works (and was a really fun project), it's slow at startup and cannot show changes made to the API without it being sent to the worker again after modification. Observers would solve part of this, and web workers transferrable objects would solve all, but they aren't adopted widely yet.
Since worker round-trip calls happen in a matter of milliseconds, I think stalling the thread for a few milliseconds may be an alright solution. Of course I would think about terminating in cases where calls take too long, but I'm trying to create a proof of concept first.
Let's say I want to expose the api object to the worker. I would define a getter for self.api which would fetch the first layer of properties. Each property would then be another getter and the process would continue until the final object is found.
worker.js
self.addEventListener('message', function(event) {
self.dataRecieved = true;
self.data = event.data; // would actually build new getters here
});
Object.defineProperty(self, 'api', {
get: function() {
self.dataRecieved = false;
self.postMessage('request api first-layer properties');
while(!self.dataRecieved);
return self.data; // whatever properties were received from host
}
});
For experimentation, we'll do a simple round-trip with no data processing:
index.html (only JS part)
var worker = new Worker("worker.js");
worker.onmessage = function() {
worker.postMessage();
};
If onmessage would interrupt the loop, the script should theoretically work. Then the worker could access objects like window.document.body.style on the fly.
My question really boils down to: is there a way to guarantee that an event will interrupt an executing code block?
From my understanding of events in JavaScript, I thought they did interrupt the current thread. Does it not because it's executing a blank statement over and over? What if I generated code to be executed and kept doing that until the data returned?

is there a way to guarantee that an event will interrupt an executing code block
As #slebetman suggests in comments, no, not in Javascript running in a browser's web-worker (with one possible exception that I can think of, see suggestion 3. below).
My suggestions, in decreasing order of preference:
Give up the desire to feel "native" (or maybe "local" might be a better term). Something like the infinite while loop that you suggest also seems to be very much fighting agains the cooperative multitasking environment offered by Javascript, including when thinking about a single web worker.
Communication between workers in Javascript is asynchronous. Perhaps it can fail, take longer than just a few milliseconds. I'm not sure what your use case is, but my feeling is that when the project grows, you might want to use those milliseconds for something else.
You could change your defined property to return a promise, and then the caller would do a .then on the response to retrieve the value, just like any other asynchronous API.
Angular Protractor/Webdriver has an API that uses a control flow to simulate a synchronous environment using promises, by always passing promises about. Taking the code from https://stackoverflow.com/a/22697369/1319998
browser.get(url);
var title = browser.getTitle();
expect(title).toEqual('My Title');
By my understanding, each line above adds a promise to the control flow to execute asynchronously. title isn't actually the title, but a promise that resolves to the title for example. While it looks like synchronous code, the getting and testing all happens asynchronously later.
You could implement something similar in the web worker. However, I do wonder whether it will be worth the effort. There would be a lot of code to do this, and I can't help feeling that the main consequence would be that it would end up harder to write code using this, and not easier, as there would be a lot of hidden behaviour.
The only thing that I know of that can be made synchronous in Javascript, is XMLHttpRequest when setting the async parameter to false https://developer.mozilla.org/en-US/docs/Web/API/XMLHttpRequest#Parameters. I wonder if you could come up with some sort of way to request to the server that maintains a connection with the main thread and pass data along that way. I have to say, my instinct is that this is quite an awful idea, and would be much slower than just requesting data from the main thread.

For what I know, there is not something native in JS to do this but it is relatively easy to do something similar. I made one some time ago for myself: https://github.com/xpy/whener/blob/master/whener.js .
You use it like when( condition, callback ) where condition is a function that should return true when your condition is met, and callback is the function that you want to execute at that time.

Prevent return until condition is met

I know these types of question come up fairly often, but I need help with a wait-like mechanism in JavaScript. I know setTimeout-based solutions are going to come up, but I'm not sure how to pull it off in my case.
I'm writing an API that uses a WebSocket internally. There's a connect() method that sets up the WebSocket, and I need to make it not return until after the WebSocket is set up. I'd like it to return a value for whether or not the connection was successful, but that's not the main problem.
The issue I'm hitting is that after a user calls connect(), they may call another method that relies on the WebSocket to be properly set up. If it's called too early, an error is thrown stating that the object is not usable.
My current solution is setting a "connected" flag when I've determined a successful connection and in each method checking for it in each method. If it's not connected, I add the method call to a queue that is ran through by the same code that sets the flag. This works, but it introduces that style of code all over my methods and also seems misleading from the user-perspective, since the call of those functions is deferred. Also, if there is other user code that relies on those calls being completed before it gets to them, it won't behave as expected.
I've been racking my brain with how to handle this case. The easiest solution is to just find a way to block returning from connect until after the WebSocket is set up, but that's not really the JavaScript way. The other option was to make them provide the rest of their code in a callback, but that seems like a weird thing to do in this case. Maybe I'm over-thinking it?
Edit: To better illustrate my problem, here's a example of what the user could do:
var client = new Client(options);
client.connect();
client.getServerStatus();
The getServerStatus() method would be using the WebSocket internally. If the WebSocket is not set up yet, the user will get that not usable error.

Todays Javascript does not really work like that unfortunately. In the future (ECMA6) there may be new language features that address this issue more directly. However for now you are stuck with the currently accepted method of handling asynchronous events, which is limited to callbacks. You may also want to explore 'promises' to handle 'callback hell' however you will need a library for this.
And yes it does seem strange to have callbacks everywhere, especially for someone new to web programming, however it is really the only way to go about it at this stage (assuming you want a cross-browser friendly solution).

"Wait" is almost the keyword you are looking for. Actually, it's yield that does this. See e.g. MDN's documentation.

There's a connect() method that sets up the WebSocket, and I need to make it not return until after the WebSocket is set up
That isn't going to happen unless you rewrite the javascript execution engine.
Either the code trying to send data will need to check the socket state (I'd go with encapsulating the socket in a object, supplying a method which sets a member variable on the open/close events and poll the state of that member variable from the external code). Alternatively you could add messages and call backs to a queue and process the queue when the socket connects.

Are Mutexes needed in javascript?

I have seen this link: Implementing Mutual Exclusion in JavaScript.
On the other hand, I have read that there are no threads in javascript, but what exactly does that mean?
When events occur, where in the code can they interrupt?
And if there are no threads in JS, do I need to use mutexes in JS or not?
Specifically, I am wondering about the effects of using functions called by setTimeout() and XmlHttpRequest's onreadystatechange on globally accessible variables.

Javascript is defined as a reentrant language which means there is no threading exposed to the user, there may be threads in the implementation. Functions like setTimeout() and asynchronous callbacks need to wait for the script engine to sleep before they're able to run.
That means that everything that happens in an event must be finished before the next event will be processed.
That being said, you may need a mutex if your code does something where it expects a value not to change between when the asynchronous event was fired and when the callback was called.
For example if you have a data structure where you click one button and it sends an XmlHttpRequest which calls a callback the changes the data structure in a destructive way, and you have another button that changes the same data structure directly, between when the event was fired and when the call back was executed the user could have clicked and updated the data structure before the callback which could then lose the value.
While you could create a race condition like that it's very easy to prevent that in your code since each function will be atomic. It would be a lot of work and take some odd coding patterns to create the race condition in fact.

The answers to this question are a bit outdated though correct at the time they were given. And still correct if looking at a client-side javascript application that does NOT use webworkers.
Articles on web-workers:
multithreading in javascript using webworkers
Mozilla on webworkers
This clearly shows that javascript via web-workers has multithreading capabilities. As concerning to the question are mutexes needed in javascript? I am unsure of this. But this stackoverflow post seems relevant:
Mutual Exclusion for N Asynchronous Threads

Yes, mutexes can be required in Javascript when accessing resources that are shared between tabs/windows, like localStorage.
For example, if a user has two tabs open, simple code like the following is unsafe:
function appendToList(item) {
var list = localStorage["myKey"];
if (list) {
list += "," + item;
}
else {
list = item;
}
localStorage["myKey"] = list;
}
Between the time that the localStorage item is 'got' and 'set', another tab could have modified the value. It's generally unlikely, but possible - you'd need to judge for yourself the likelihood and risk associated with any contention in your particular circumstances.
See the following articles for a more detail:
Wait, Don't Touch That: Mutual Exclusion Locks & JavaScript - Medium Engineering
JavaScript concurrency and locking the HTML5 localStorage - Benjamin Dumke-von der Eh, Stackoverflow

As #william points out,
you may need a mutex if your code does something where it expects a
value not to change between when the asynchronous event was fired and
when the callback was called.
This can be generalised further - if your code does something where it expects exclusive control of a resource until an asynchronous request resolves, you may need a mutex.
A simple example is where you have a button that fires an ajax call to create a record in the back end. You might need a bit of code to protect you from trigger happy users clicking away and thereby creating multiple records. there are a number of approaches to this problem (e.g. disable the button, enable on ajax success). You could also use a simple lock:
var save_lock = false;
$('#save_button').click(function(){
if(!save_lock){
//lock
save_lock=true;
$.ajax({
success:function()
//unlock
save_lock = false;
}
});
}
}
I'm not sure if that's the best approach and I would be interested to see how others handle mutual exclusion in javascript, but as far as i'm aware that's a simple mutex and it is handy.

JavaScript is single threaded... though Chrome may be a new beast (I think it is also single threaded, but each tab has it's own JavaScript thread... I haven't looked into it in detail, so don't quote me there).
However, one thing you DO need to worry about is how your JavaScript will handle multiple ajax requests coming back in not the same order you send them. So, all you really need to worry about is make sure your ajax calls are handled in a way that they won't step on eachother's feet if the results come back in a different order than you sent them.
This goes for timeouts too...
When JavaScript grows multithreading, then maybe worry about mutexes and the like....

JavaScript, the language, can be as multithreaded as you want, but browser embeddings of the javascript engine only runs one callback (onload, onfocus, <script>, etc...) at a time (per tab, presumably). William's suggestion of using a Mutex for changes between registering and receiving a callback should not be taken too literally because of this, as you wouldn't want to block in the intervening callback since the callback that will unlock it will be blocked behind the current callback! (Wow, English sucks for talking about threading.) In this case, you probably want to do something along the lines of redispatching the current event if a flag is set, either literally or with the likes of setTimeout().
If you are using a different embedding of JS, and that executes multiple threads at once, it can get a bit more dicey, but due to the way JS can use callbacks so easily and locks objects on property access explicit locking is not nearly as necessary. However, I would be surprised if an embedding designed for general code (eg, game scripting) that used multi threading didn't also give some explicit locking primitives as well.
Sorry for the wall of text!

Events are signaled, but JavaScript execution is still single-threaded.
My understanding is that when event is signaled the engine stops what it is executing at the moment to run event handler. After the handler is finished, script execution is resumed. If event handler changed some shared variables then resumed code will see these changes appearing "out of the blue".
If you want to "protect" shared data, simple boolean flag should be sufficient.