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How to update a document without causing a write firestore?

I am building a billing web app where users update stock of an item frequently. So it will cost me much. Is there a way to only update the stock without causing a document write?

Short answer: no
Long answer: most likely not
Each document updated will count as a document write for billing on Firestore. Even if you try to bundle multiple updates into a single batch call.
One suggestion would be to throttle updates on the client side to slow down rapid updates. This will however affect the accuracy of the data your end users will get while using the app. Example of throttling: Small article on throtting/debouncing
Another suggestion is to avoid making updates that are meaningless.
You can check if a value isn't going to change or if the value change isn't important before trying to do an update:
function updateValue(newValue) {
// Check if value has changed
if (newValue === value) {
// pass
return;
}
// make an update request
// ...
}
Any other suggested solution would require your end users sharing the same network or introducing another service outside of Firestore.
Last thing to consider is that maybe it's too soon to consider billing. Firestore is pretty cheap for what it offers. Here's a good video by Firebase about billing:
Firebase video on Getting to know pricing
If none of these suggestions has been helpful, I'd recommend adding some code examples, etc for better suggestions :)

Should updates to Firstore items in AngularFire be done through the AngularFirestoreCollection?

In my app, I have a list that requires an "or" condition. But, as the docs say:
In this case, you should create a separate query for each OR condition and merge the query results in your app.
As a result, in my service, I'm managing two queries and surfacing them as a single observable list to consumers.
The problem comes in with updating. I have the choice of doing extra work to match up the item needing update to the correct collection so I can do the following:
myCollection.doc(item.id).update(item);
or I can make this much more simple and just:
angularFirestore.doc(`path/to/${item.id}`).update(item);
I'm operating under the assumption that the first method will result in faster updates as I'm using the same reference that it would optimistically update instantly. And that the latter will be slower in that it would be more round about by updating the persistence layer and then the collection referencing getting notified about later (probably still a small time).
All of the above is assumption, however. I back this just with a few random instances where I've seen it take a second or two for an update or delete to show up in an other part of the view, but I haven't been able to actually inspect the process.
Does anyone know if the above is correct? Should I be doing the extra work to write through the collection references or does angularfire(and/or firestore) handle this and make them effectively the same operation under the hood?

AngularFire2 is a thin wrapper around RxFire, which itself is a relatively thin wrapper around the Firebase JavaScript SDK.
There should be no significant performance difference between updating a document through AngularFire or updating it directly through the JavaScript SDK. In both cases the majority of the time is spent in the JavaScript SDK, and on the wire between the client and server. For this reason I typically update directly through the JavaScript SDK, since it's often a bit more direct and the AngularFire abstraction has little advantage for me in write operations. Given that AngularFire is built on top of this SDK, it picks up the changes instantly even when they're not made through AngularFire.
If you have an instance where this does not seem to be the case, I recommend creating a question with the minimal, complete/standalone code that reproduces that problem.

Updating Dynamic HTML file

I have an application where the application's user relates to an HTML page during the application's runtime.
During runtime, the HTML file is being continuously updated. In order to handle every update, the HTML file is being refreshed at a fixed interval.
Is there any other better way to achieve this with more efficiency and/or to hide the refreshing technique from the user's perspective?
Any help will be greatly appreciated.

If what you're doing doesn't involve retrieving data from a server, and instead involves some kind of process that occurs on the page itself, like a clock, I would use setInterval
setInterval(function(){myFunction()},1000);
This would call myFunction() every second, and inside of that function you can change visual elements as appropiate.
You may also want to consider using a Javascript MVC framework such as Backbone.js or Angular.js, which handle changing the appearance of html elements mapped to javascript variables over the lifetime of an application for you.

From your question it sounds as if your updating the HTML file on the server in response to user actions. I'm guessing that's not literally what you meant. If it is, though, there are almost certainly better ways to do what you want to do -- depending on what, exactly, it is.
If you are just showing new information to the user, you will want to use Javascript to update the page's DOM. The easiest way for a beginner to do that would be to start by looking at jQuery tutorials. With Javascript (possibly aided by jQuery) you can easily update the content of any part of your page, without needing to actually change the original HTML file in any way.
If you want the page to be updated for everyone who visits it at a later date, then you need to put the new information in a database, and get the content from the database when you the page is requested or loaded.
It would greatly help if you specified exactly what you were trying to do.

How to update variable dynamically within Rails app

I am writing my first Rails app using the twitter gem. I'm simply retrieving search results and trying to cycle through them individually every 5 seconds or so.
My thought was to create a variable and have this variable represent the array index and simply update this variable dynamically with Javascript (every 5 seconds or so). What's the best way to achieve this on the client-side? AJAX? Javascript?
Does this make sense? I will be glad to provide more context if helpful. Thanks.

Sounds you're trying to build a "recent tweets" marquee of some sort. Without knowing your requirements, you could try simply loading the ten most recent tweets in Rails, putting them in ten hidden divs, and then using jQuery just to cycle through the different tweets on the page.
If it is a requirement to "update" the most recent tweets without the user refreshing the page, then yes, you'd probably need an AJAX call.

It's hard to tell what you think you're asking: by the time your JavaScript is executing the server is no longer involved.
If you want to update some sort of count on the server side and persist it in a meaningful way, you can do so via Ajax.
What are you actually trying to do, though?

Ruby runs on the server while JavaScript (usually) runs on the client.
The Ruby generates an HTML document (perhaps with embedded JS) and the server delivers it to the client.
At that stage the Ruby has finished executing. The only way to do anything further with Ruby would be to make a new HTTP request to the server. This could be done by following a link, submitting a form, setting location.href, using XMLHttpRequest or numerous other techniques.
This would cause the Ruby program to be executed again (or a different one to be executed) which would do whatever it did with the input data.
You cannot simply "set a variable" on the server from the client.

In my particular case, I used ruby's .to_json method to convert the data and then manipulated it with javascript. This gave me the flexibility to loop through the data pretty seamlessly. Atleast it seemed to work for my particular situation. Thanks for the help guys!

Is it possible to complete the loop from browser->java->c++->java->browser?

I've got a question about data flow that is summarized best by the image below:
I've got the data path from the UI (WaveMaker) down to the hardware working perfectly. The question I have is whether I'm missing something in the connection from the Java Service to Wavemaker.
I'm trying to provide information back to Wavemaker from the HW. The specifics of shared memory and semaphore signaling are worked out already. Where I'm running into a problem is how to get the data from the Java Service back to WaveMaker, when it hasn't specifically requested it. My plan was to generate events when the Java Service returned, but another engineer here insists that it won't work, since there's no direct call from Wavemaker and we don't want to poll.
What I proposed was to call the function after the page loaded, allow the blocking to occur at the .so level, as shown below, and then handle the return string when the call returned. We would then call the function again. That has the serious flaw of blocking out interaction with the user interface.
Another option put forth would be to use a hidden control, somehow pass it into Java, and invoke an event on it from Java, which could then be made to execute a script to update the UI with the HW response. That keeps the option of using threads alive, and possibly resolves the issue. Is there some more elementary way of getting information from Java->JavaScript->UI without it having been asked for?