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VS Code - unified checking of javascript, html, and CSS code as a whole - prior to running on a browser?

Forgive me if this is a really stupid question, but I haven't found any answers yet - or maybe I don't know the correct thing to ask for.
Given the following files that are part of the same project:
MyProject.html
MyProject.css
MyProject.js
(and a MyProject.py that runs on the server to make things happen)
. . . where all three of these items are related and are actually part of a single project and they need to integrate together.
The "html" part of VS code makes sure the html is correct.
The "css" part of VS code makes sure the css is correct.
the JavaScript part of VS code makes sure the javascript is correct.
However, they may not be correct together as a unified whole - I may have changed something in the javascript that references something in the html that may not yet exist - because I forgot to write it, and I don't discover this until I launch things and watch the web-page go all pear-shaped in ways I've never heard of before.
Is there something that will take all these pieces and say "Hey! You changed the definition of this element here in the Javascript but not in the HTML (or the CSS or whatever)
In other words, not only do I want to know if the individual files are syntactically correct, but do they agree with each other?
If there is a "something" that does this, what is it called?

That tool will never exist and for good reasons, it'd slow the living hell out of your computer when programming and wouldn't fair well as a best practice. Though it's cool, it's cooler to write code effectively and not have a slow code editor. So to that suggestion is write your JavaScript and HTML together hand in hand; split view and you won't ever have an issue. CSS can come into play any time.
Your best option for knowing if code is correct, would be a linter but that won't help you with the issues you face if you're calling elements that don't exist or did you'll want to improve how you code these functions/events.
As requested submitted as an answer for the OP.

Is it a bad practice to add large parts of HTML that's not required to be visible when the site loads using javascript? [closed]

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I need to have 2 popup modals in each HTML file to allow users to login and sign up, and was wondering if it was possible to simple have a common js file to add it using innerHTML or a similar method instead of typing it out in every HTML file. Would this cause any performance issues?

Neither should cause major issues with performance, especially since you're not trying to show these models until the user has interacted with the page.
You're mentioning that you would have to copy-paste this HTML chunk into every file. This would violate the DRY principle, and cause you to be left maintaining copy-pasted HTML in many different files, which is never a good thing. In this scenario, I would go for dynamically generated HTML.
If you want to dynamically generate the HTML yourself (without a templating library, etc), then I would avoid .innerHTML as much as possible - it's convenient, but it's also easy to fall into security pitfalls with it. Prefer using the Javascript built-in DOM APIs.
I personally like to use this helper function to make the DOM APIs easier to use.
function el(tagName, attrs = {}, children = []) {
const newElement = document.createElement(tagName);
for (const [key, value] of Object.entries(attrs)) {
newElement.setAttribute(key, value);
}
newElement.append(...children);
return newElement;
}
// USAGE
const customElement = el('div', { id: 'myId', class: 'myClass' }, [
el('p', {}, ['Some Text']),
el('br')
])
// The above element will get added to the body, and has the following shape:
// <div id="myId" class="myClass">
// <p>Some Text</p>
// <br>
// </div>
document.body.appendChild(customElement)
You certainly don't have to use it, do whatever floats your boat, but I just find that a helper function like that makes it easy to write HTML-looking Javascript code, without the temptation to actually use .innerHTML.

Just use it. It won't affect the performance.

it shouldn't effect performance. If anything, if the scripts loading is deferred it might improve the performance.
As for bad practice, I don't think it would be. I mean how is it any different to something like React that has a html file with a div with an id and injects the bundled react app (webpack js files) into

What are the arguments against the inclusion of server side scripting in JavaScript code blocks?

I've been arguing for some time against embedding server-side tags in JavaScript code, but was put on the spot today by a developer who seemed unconvinced
The code in question was a legacy ASP application, although this is largely unimportant as it could equally apply to ASP.NET or PHP (for example).
The example in question revolved around the use of a constant that they had defined in ServerSide code.
'VB
Const MY_CONST: MY_CONST = 1
If sMyVbVar = MY_CONST Then
'Do Something
End If
//JavaScript
if (sMyJsVar === "<%= MY_CONST%>"){
//DoSomething
}
My standard arguments against this are:
Script injection: The server-side tag could include code that can break the JavaScript code
Unit testing. Harder to isolate units of code for testing
Code Separation : We should keep web page technologies apart as much as possible.
The reason for doing this was so that the developer did not have to define the constant in two places. They reasoned that as it was a value that they controlled, that it wasn't subject to script injection. This reduced my justification for (1) to "We're trying to keep the standards simple, and defining exception cases would confuse people"
The unit testing and code separation arguments did not hold water either, as the page itself was a horrible amalgam of HTML, JavaScript, ASP.NET, CSS, XML....you name it, it was there. No code that was every going to be included in this page could possibly be unit tested.
So I found myself feeling like a bit of a pedant insisting that the code was changed, given the circumstances.
Are there any further arguments that might support my reasoning, or am I, in fact being a bit pedantic in this insistence?

Script injection: The server-side tag could include code that can break the JavaScript code
So write the code properly and make sure that values are correctly escaped when introduced into the JavaScript context. If your framework doesn't include a JavaScript "quoter" tool (hint: the JSON support is probably all you need), write one.
Unit testing. Harder to isolate units of code for testing
This is a good point, but if it's necessary for the server to drop things into the page for code to use, then it's necessary. I mean, there are times when this simply has to be done. A good way to do it is for the page to contain some sort of minimal block of data. Thus the server-munged JavaScript on the page really isn't "code" to be tested, it's just data. The real client code included from .js files can find the data and use it.
Thus, the page may contain:
<script>
(function(window) {
window['pageData'] = {
companyName: '<%= company.name %>',
// etc
};
})(this);
</script>
Now your nicely-encapsulated pure JavaScript code in ".js" files just has to check for window.pageData, and it's good to go.
Code Separation : We should keep web page technologies apart as much as possible.
Agreed, but it's simply a fact that sometimes server-side data needs to drive client-side behavior. To create hidden DOM nodes solely for the purpose of storing data and satisfying your rules is itself a pretty ugly practice.
Coding rules and aesthetics are Good Things. However, one should be pragmatic and take everything in perspective. It's important to remember that the context of such rules is not always a Perfect Divine Creation, and in the case of HTML, CSS, and JavaScript I think that fact is glaringly clear. In such an imperfect environment, hard-line rules can force you into unnecessary work and code that's actually harder to maintain.
edit — oh here's something else I just thought of; sort-of a compromise. A "trick" popularized (in part) by the jQuery gang with their "micro template" facility (apologies to the web genius who actually hit upon this first) is to use <script> tags that are sort-of "neutered":
<script id='pageData' type='text/plain'>
{
'companyName': '<%= company.name %>',
'accountType': '<%= user.primaryAccount.type %>',
// etc
}
</script>
Now the browser itself will not even execute that script - the "type" attribute isn't something it understands as being code, so it just ignores it. However, browsers do make the content of such scripts available, so your code can find the script by "id" value and then, via some safe JSON library or a native browser API if available, parse the notation and extract what it needs. The values still have to be properly quoted etc, but you're somewhat safer from XSS holes because it's being parsed as JSON and not as "live" full-blown JavaScript.

The reason for doing this was so that the developer did not have to define the constant in two places.
To me, this is a better argument than any argument you can make against it. It is the DRY principle. And it greatly enhances code maintainability.
Every style guide/rule taken to extreme leads to an anti-pattern. In this case your insistence of separation of technology breaks the DRY principle and can potentially make code harder to maintain. Even DRY itself if taken to extreme can lead to an anti-pattern: softcoding.
Code maintainability is a fine balance. Style guides are there to help maintain that balance. But you have to know when those very guides help and when they themselves become a problem.
Note that in the example you have given the code would not break syntax hilighting or parsing (even stackoverflow hilights it correctly) so the IDE argument would not work since the IDE can still parse that code correctly.

it simply gets unreadable. You have to take a closer look to divide the different languages. If JavaScript and the mixed-in language use the same variable names, things are getting even worse. This is especially hard for people that have to look at others people code.
Many IDEs have problems with syntax highlighting of heavily mixed documents, which can lead to the loss of Auto-Completion, proper Syntax Highlighting and so on.
It makes the code less re-usable. Think of a JavaScript function that does a common task, like echoing an array of things. If you separate the JavaScript-logic from the data it's iterating over, you can use the same function all over your application, and changes to this function have to be done only once. If the data it's iterating over is mixed with the JavaScript output loop you probably end up repeating the JavaScript code just because the mixed in language has an additional if-statement before each loop.

What software/webapp can I use to edit HTML pages?

Ok, my question's not as broad as it seems, to summarize 8 months effort on my part:
I create chunks of re-usable, extensible XHTML which degrades gracefully and is all kinds of awesome. Most of my code chunks are provided a Javascript interaction layer, and are styled with CSS. I set to work pulling my code chunks into Dreamweaver as 'Snippets' but they're unintelligent chunks of text. Also, once inserted, my beautiful code chunks get mangled by the non-techies who are the ones actually using Dreamweaver.
Also, because they're unintelligent snippets, I have a line of Javascript which configures the code chunks when initialised - see this post for further detail on my approach. But currently I have to replicate a single code chunk as many times as there are configuration options (so each 'snippet' may only differ from another of the same type by ONE config value). This is incredibly lame, it works, but its lame and time-consuming for me to re-deploy a bunch of snippets and hard for my team to remember all the variations.
So I have a series of requirements, to my mind, as the most likely things to solve in any system I put my chunks into:
The inserted code is not modified at insertion time, by the system
The code to be inserted needs to allow config options
I'd be overjoyed if, once inserted, the only editable parts are text nodes
Copy and pasting these whole objects
A clean interface from which to choose from my range of code chunks
It's a serious list of requirements I presume, much searching led me to Kompoze and its 'Smart widgets' which, according to a random post from 2004, suggests XUL files can be created and extensions can be made which sounds vaguely like what I want. The text editor itself was less prone to destruction, when compared to Dreamweaver.
So yeah, I've chased too many rabbits on this one, keen as for a solution whether Software+extension, or Webapp.
EDIT:
Btw, it did occur to me to investigate a highly customised TinyMCE instance, but I don't know feasible that is, and unless there's some sweet backend available, I'm stuck with local editing of files for now - not even on a web server...
To my mind the best answer to this question will solve most of the above, and provide some general workflow advice alongside the suggestion(s).

I would go with a solution based around the excellent markItUp! editor. It's very simple to extend it to cope with the requirements you have. You can add sophisticated logic, and it's nice and shiny.
I'd probably combine it with Jeditable for the inline node editing, and build the whole thing on top of Django, for ease and convenience. Completely customisable, surprisingly easy to work with, portable and cross-platform, and easy to set-up for off-line use. Oh, and all free and open-source.

What do you think of this approach:
<div class="thing">
<elements... />
<script type="text/javascript">
document.write('<span id="thing' + thingNo + '"></span>')
new Thing().init({ id:'thing'+thingNo; });
thingNo += 1;
</script>
</div>
Of course, you'll have to change Thing().init so that it would initialize the parent (instead of current) node.

Have you considered server-side includes where the directive is either a generated page or a shell command? E.g.:
<!--#include virtual="./activePage.aspx?withParam1=something&param2=somethingelse" -->
or
<!--#exec cmd="shellCommand -withParams" -->
You can reuse the same page or command, and provide parameters specific to each usage in each XHTML page.

How do you organize your Javascript code?

When I first started with Javascript, I usually just put whatever I needed into functions and called them when I needed them. That was then.
Now, as I am building more and more complex web applications with Javascript; taking advantage of its more responsive user interaction, I am realizing that I need to make my code more readable - not only by me, but anyone who replaces me. Besides that, I would like the reduce the moments of 'what the heck, why did I do this' when I read my own code months later (yes, I am being honest here, I do have what the heck was I thinking moments myself, although I try to avoid such cases)
A couple weeks ago, I got into Joose, and so far, it has been good, but I am wondering what the rest do to make their chunk their codes into meaningful segments and readable by the next programmer.
Besides making it readable, what are your steps in making your HTML separated from your code logic? Say you need to create dynamic table rows with data. Do you include that in your Javascript code, appending the td element to the string or do you do anything else. I am looking for real world solutions and ideas, not some theoretical ideas posed by some expert.
So, in case you didnt't understand the above, do you use OOP practices. If you don't what do you use?

For really JS-heavy applications, you should try to mimic Java.
Have as little JS in your HTML as possible (preferably - just the call to the bootstrap function)
Break the code into logical units, keep them all in separate files
Use a script to concatenate/minify the files into a single bundle which you will serve as part of your app
Use JS namespaces to avoid cluttering up the global namespace:
var myapp = {};
myapp.FirstClass = function() { ... };
myapp.FirstClass.prototype.method = function() { ... };
myapp.SecondClass = function() { ... };
Using all these techniques together will yield a very manageable project, even if you are not using any frameworks.

I use unobtrusive javascript, so, outside of the script tags I don't keep any javascript in the html.
The two are completely separated.
A javascript function will start when the DOM tree is completed, which will go through the html and add the javascript events, and whatever else needs to be changed.
In order to organize, I tend to have some javascript files that are named similar to the html pages that they use, and then for common functions I tend to group them by what they do, and pick a name that explains that.
So, for example, if I have UI functions then I may call them: myapp_ui_functions.js
I try to put the name of the application in the filename, unless there is some javascript that is common to several projects, such as strings.js.

I have (usually) one file that contains a bunch of functions and that's it. That is included in every page that uses Javascript. In the pages themselves, I'll make the calls to the functions like:
$(function() {
$("#delete").click(delete_user);
$("#new").click(new_user);
});
where delete_user() and new_user() are defined in the external file.
I too use unobtrusive Javascript, which for me means jQuery (there are other libraries that are unobtrusive).
You don't want a separate file for each page. That just means more unnecessary external HTTP requests. With one file—assuming you've cached it effectively—it'll be downloaded once and that's it (until it changes).
If I have a large amount of Javascript or the site is effectively split into multiple areas then I may split the Javascript but that's not often the case.
Also, in terms of my source code, I may have multiple JS files but I'll often end up combining them into one download for the client (to reduce HTTP requests).
More at Multiple javascript/css files: best practices? and Supercharging Javascript in PHP.

I've been rewriting a lot of my reusable code as jQuery plugins. I moved to jQuery from Prototype when I started doing ASP.NET MVC. Overtime I've migrated a lot my reusable code, or at least the ideas, from Prototype-based OO to jQuery-style plugins. Most of these are stored in their own JS files (mainly intranet apps so page load speed is pretty high anyway despite the extra requests). I suppose I could add a build step that coalesces these if I needed to.
I've also settled on a MasterPage approach that uses a ContentPlaceHolder for scripts that is right before the closing body tag. The standard jQuery/jQuery UI loads, and any other common JS goes right before the script placeholder in the MasterPage. I have tiny bit of JS at the top of the MasterPage that defines an array that holds any functions that partial views need to run on page load. These functions are run from the base document.ready() function in the MasterPage.
All of my JS is completely separate from my mark up. Some JS may exist in partial views -- these are encapsulated when the partial may be included more than once to make it specific to that instance of the view -- but generally not. Typically only included in the placeholders so that it's loaded at the bottom of the page.

Also, if you want to go OO heavy, check out mochikit: http://www.mochikit.com/

I find that developing your javascript using OO methodology is the way to go if you want it to be clean, readable and even somewhat secure. I posted the following question
Cleanest format for writing javascript objects
And got some fantastic responses on how to write my javascript code well. If you follow these basic principles you can use almost any library, such as yui, jquery and prototype, with ease.