Develop Reference

Loading external HTML files synchronously

We have a locally run HTML app, generated from a CMS, for a 3rd party system running in a native IOS app.
Each main page needs to be an individual, standalone HTML file.
Shared resources (common html partials, js css etc) used across all pages are in a shared folder.
Loading the CSS and JS is fine, but we need a way to load the shared HTML in BEFORE the first render of the page, as though it were part of the page (else you see the page without the shared content for a split second / a white flash).
We have come up with a few approaches, none of them are very nice, so any thoughts ?
1) document.write
We wrap each shared html partial in JS:
document.write('<div class=\'title\'>The Title</div>');
Then in the main HTML page, we include it with a JS tag..
<script src="../shared/template.html"></script>
This works, but uses document.write which gets lots of warnings in the console, and we will have to do some processing of the .html files to escape them and wrap them in JS.
2) Synchronous Ajax - (Sjax?)
Load the file with XMLHttpRequest BEFORE the closing body tag, but with the sync flag set to false, and then onload insert it into a placeholder node in the main page.
const xhr = new XMLHttpRequest();
xhr.addEventListener('load', () => {
document.getElementsByClassName('template-container')[0].innerHTML = xhr.responseText;
});
xhr.open('GET', '../shared/template.html', false);
xhr.send();
This is much better as we don't have to touch the .html pages at all, and there is no document.write going on.
However, Sync on the main thread is supposedly deprecated - however, when/if IOS ever remove this I don't know.
3) iframe
Include the partials as an iframe.
<iframe src="../shared/template.html"></iframe>
Issue with this is that we again have to modify the partials so they include the relevant CSS to render, we need to modify the parent file/css to render the iFrame correctly, and has more roundtrips to achieve the same idea as point 1.
4) Ajax / fetch
Both are async, so will always have a slight delay between main render and then loading the external html.
Currently I'm thinking 2, as its the cleanest implementation, but worried if it will really be deprecated.
Any other approaches?

Execute a page script, which was inserted in the page from a content script, prior to other JavaScript in the page

I am trying to analyze some JavaScript code for which I make use of function rewriting so that calls to a JavaScript library go through my JavaScript code. My JavaScript code is part of a Chrome Extension. From a Chrome extension content script, I install/inject the code into the target page's DOM.
This works fine for functions that are induced after the load of page. The library calls go through my function. But, there's JavaScript code that runs while the page is actually loading (probably while the DOM is being rendered). This happens before my custom script is injected. This way, the function calls before the custom script is injected are lost to me, or those JavaScript calls do not go through my function.
I make use of Content Script to actually inject other JavaScript by appending to the DOM as mentioned in the following Stack Exchange question:
Insert code into the page context using a content script
I know I can cause the loading time of Content Script to be at the start/end of the DOM but this is another script file that I append to the DOM of the target page. I do not seem to understand how to control it.
The problem explained in Is it possible to run a script in context of a webpage, before any of the webpage's scripts run, using a chrome extension?
is exactly the same, but the solution does not seem to work. My intention is to make the injected script execute before any JavaScript code executes from the webpage. By specifying document_start in manifest.json, content script execution can be made to run before the webpage, but not the script that I inject through the content script (injecting script as explained in first link). This injected script is not running in any specific manner with respect to the webpage
Manifest.json:
Manifest file has the content script content.js added at document_start, so content.js is run before the target webpage (underlying page) runs.
"content_scripts":[
{
"matches":["<all_urls>"],
"js":["content.js"],
"run_at":"document_start",
"all_frames":false
}
],
content.js:
content.js has the below code with which I add the main.js to the DOM, so that I am actually able to interact with the JavaScript that is in the target page's environment. I do this from a different file and attach it to the DOM because I cannot interact with the target page's JavaScript through the Content Scripts, since they both do not interfere with each other.
To explain further, main.js has some JavaScript that intercepts JavaScript calls during the execution of JavaScript in target page. JavaScript in target page makes calls to a library and I intend just to write a wrapper on those library functions.
var u = document.createElement('script');
u.src = chrome.extension.getURL('main.js');
(document.head||document.documentElement).appendChild(u);
u.onload = function() {
u.parentNode.removeChild(u);
};
I expect that main.js is available in the target page's domain and any of the scripts in the target page, since I inject it through the content script that is run at document_start.
Assume I have a call to some JavaScript function like this in my target page HTML, someJSCall() is defined by the target page's domain.
<html onLoad="someJSCall( )">
In this scenario, main.js (code injected through my Chrome extension) is already available. So calls to the JavaScript library from someJSCall() function go through main.js wrapper functions.
This works fine.
The problem is when there are IIFE (immediately invoked function expressions) defined in the target page's JavaScript. If these IIFE calls make library calls, this does not go through my main.js interceptions. If I look at the files loaded in the browser through Chrome Dev Tools, I see that main.js is still not loaded while IIFE calls are executing.
I hope I have explained the problem in detail.

Based on the additional information you added to the question about 2.5 weeks after I answered, you are adding code to the page context by including a "main.js", which is a separate file in your extension, using a <script> that looks something like:
<script src="URL_to_file_in_extension/main.js"/>
However, when you do that you introduce an asynchronous delay between when the <script> is inserted into the page and when the "main.js" is fetched from the extension and executed in the page context. You will not be able to control how long this delay is and it may, or may not, result in your code running prior to any particular code in the page. It will probably run prior to code that has to be fetched from external URLs, but may not.
In order to guarantee that your code runs synchronously, you must insert it in a <script> tag as actual code, not using the src attribute to pull in another file. That means the code which you want to execute in the page must exist within the content script file you are loading into the page.
Needing to execute code in the page context is a fairly common requirement. I've needed to do so in browser extensions (e.g. Chrome, Firefox, Edge, etc.) and in userscripts. I've also wanted to be able to pass data to such code, so I wrote a function called executeInPage(), which will take a function defined in the current context, convert it to text, insert it into the page context and execute it while passing any arguments you have for it (of most types). If interested, you can find executeInPage() in my answer to Calling webpage JavaScript methods from browser extension and my answer to How to use cloneInto in a Firefox web extension?
The following is my original answer based on the original version of the question, which did not show when the content script was being executed, or explain that the code being added to the page was in a separate file, not in the actual content script.
You state in your question that you "can handle the loading time of Content Script to be at the start/end of the DOM", but you don't make clear why you are unable to resolve your issue by executing your content script at document_start.
You can have your script injected prior to the page you are injecting into being built by specifying document_start for the run_at property in your manifest.json content_scripts entry, or for the runAt option passed to chrome.tabs.executeScript(). If you do this, then your script will start running when document.head and document.body are both null. You can then control what gets added to the page.
For chrome.tabs.executeScript() exactly when your script runs depends on when you execute chrome.tabs.executeScript() in relation to the process of loading the page. Due to the asynchronous nature of the processing (your background script is usually running in a different process), it is difficult to get your script consistently injected when document.head and document.body are both null. The best I've accomplished is to have the script injected sometimes when that is the case, and sometimes after the page is populated, but prior to any other resources being fetched. This timing will work for most things, but if you really need to have your script run prior to the page existing, then you should use a manifest.json content_scripts entry.
With your content script running prior to the existence of the head and body, you can control what gets inserted first. Thus, you can insert your <script> prior to anything else on the page. This should make your script execute prior to any other script in the page context.

Is it possible to locally replace a website's Javascript when the page loads?

I am not talking about using an add-on such as greasemonkey to inject new javascript. I'm talking about providing (possibly a browser extension of some sort) a custom js script to be loaded instead of the one that the page provides.
Example:
Page has <script src="script.js"></script>
I would like to tell my browser to run X code instead of what was in script.js
Thank you. Sorry if the question is not very clear.

There are a few ways to programmatically execute Javascript on a web page.
Add Another Script Tag
If you can save your script on the web somewhere, then you can just insert a new script tag referencing it.
var script = document.createElement('script');
script.setAttribute('src', 'http://yourscript.com/script.js');
document.body.appendChild(script);
Alternatively, if you can find the existing script tag, you can just change the source.
var script = document.getElementById('my-script');
script.setAttribute('src', 'http://yourscript.com/script.js');
Eval the Code
If you already have the code you want to execute, then you can write a script that uses Javascript's eval function to run it.
var code = '...';
eval(code);
Warning! Unless you absolutely trust the source of the code, then using eval can be dangerous.

Try this, this will inject a script element into the page. I am finding the way to remove the existing element.
<script type="text/javascript">
document.write("<script src='codex.js'></script>");
</script>

Open the developer tools in chrome (F12 on windows), navigate to your .js source file, then proceed to promptly modify what you see in the editor that is presented. Chrome only persists these changes as long as the page doesnt get refreshed. Once the page is refreshed it drags files from the server again and your local changes will be lost.
You can do a replace of the code in that file.
Hope this was what you were looking for!

HTML content loaded via AJAX loads external JavaScript out of order

Here's the scenario, not sure what I'm missing.
Page A.htm makes an ajax request for page B.htm, and inserts the response into the page.
Page B.htm contains links to several other JS files, many of which contain a document.ready() function to initialize them.
This works fine when A.htm and B.htm are on the same server but not when they are on different servers.
What I think I'm seeing here, is that when page A and B are on different servers (cross domain ajax), the external resources are being returned asynchronously, or at least out of order, so scripts are executing expecting JQuery.UI to be loaded already, when it is not.
Appreciate any pointers or advice. Apologies for the poor explanation.

You are injecting HTML + script tags via jQuery. In this case *:
HTML content except scripts are injected in the document
Then all scripts are executed one by one
If a script is external then it is downloaded and executed asynchronously
Therefore an external or inline script that depends on jQuery UI might execute before jQuery UI.
One possible solution is to change the way your pages work:
Get rid of external scripts in pageb.html but keep inline scripts
Load the required scripts in pagea.html
Load pageb.html
Another solution is to roll your own jQuery function that will:
Strip all <script src> elements from HTML
Download and execute those scripts in order
Inject the remaining HTML
* The exact behavior is not documented. I had to look into the source code to infer the details.

you are correct in your impression that the issue is a difference in how the requests are handled cross-domain.
Here is a link to get you on the right track : How to make synchronous JSONP crossdomain call
However, you will have to actually re-achitect your solution somewhat to check if the resource has been loaded before moving on. There are many solutions (see the link)
You can set a timer interval and check for something in the dom, or another reasonable solution (despite it's lack of efficiency) is to create a "proxy" serverside (eg php) file on your server and have that file do the cross-domain request, then spit out the result.
Note that since jquery UI is a rather large file, it's conceivable that the cross-domain request finishes first, and executes immediately, even though jqueryUI is not loaded yet. In any case, you're going to have to start thinking about having your app react rather than follow a sequence.

Can I load JavaScript code after the rest of page loads?

I have a section of a webpage that loads a JavaScript file from an external source and then kicks off an Ajax query.
When I load the page, I see the browser saying "waiting for example.com" a lot, so I think the dependency on this external JavaScript is slowing my initial page load.
Is there a way I can load this external JavaScript asynchronously so it doesn't slow the loading of the rest of my page at all?

It's good practice to put JS at the bottom, right above the closing body tag. In addition, use load events window.onload or $(document).ready() to fire your JavaScript after the page has loaded.
As far as loading JavaScript files themself asynchronously or on demand, you could inject it from another JavaScript function or event. But really you are doing the same thing as placing it at the bottom.
Check out the YSlow Guidelines for front-end optimizations.

You could use jQuery's .getScript() method, which is simply a wrapper for an AJAX call.
http://api.jquery.com/jquery.getscript/
This makes the request asynchronous, and gives you a callback that runs after the script has loaded.

You can see my answer here: Dynamic (2 levels) Javascript/CSS Loading
And grab the script from here (see the source). Use it at the bottom, and your scripts will not block other resources (and if you got more than one they will be downloaded in parallel cross-browser).

I wrote a library to asynchronously load javascript files with callbacks for when it loads:
https://github.com/ssoroka/sigma
Sigma.async_script_load('http://example.com/underscore/underscore-min.js', '_', function() {
_([1,2,3,2,3,1]).uniq();
});