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Using JavaScript in Ionic 2

I have several JavaScript files (e.g., myFunctions.js). Where do I need to specify information about this file and how can I use it in a .html page? For example, if myFunctions.js contains a function called myFunction1, what do I need to do to be able to call myFunction1 in myPage.html that has myPage.ts and myPage.sccs files?
I DO NOT want to recode the JavaScript as a TypeScript file and am looking for a way to use "legacy" code.

How to call common JavaScript function to all the files without adding it in every JS file?

I have a log.js file(it contains log function along with some properties) for debugging purpose.
I have two other js file which are the controlling various behavior of the web application.
Now I need to include the log function considering not to repeat the debug function in both the js file and just calling the file name.
How do I do it?
The idea is to make my code clean and separate them in other files to limit the size of a single js file.

Include all .js-files in the html page. Include log.js first.
Call the functions all you want.

All functions in all files are included in the source and are "written on the page", any functions can be accessed from anywhere within the HTML as they all become essentially one document. Make sure you do not have duplicate functions as this could cause an issue

JavaScript use script from other JavaScript file

I have two JavaScript files in a web application project and want a function in one file to call a function from the other. How can that be done?

Refer to both files from the HTML page
<script src="js/file1.js"></script>
<script src="js/file2.js"></script>
functions in one file should be accessible from functions in another file.

When both JS files are parsed by a browser functions from one file will be visible to another and vice versa.
Please bear in mind that the JavaScript file which should be used in another JavaScript file usually needs to be declared before the other JavaScript file (using the HTML <script> tag).

Access function defined in separate .js file from a .js file

I've seen quite a few questions regarding loading a .js file into an HTML file, and I know how to do that. However, say I have the file "classlist.js." How can I go about using the classes defined in that javascript file in another javascript file? I keep seeing answers that suggest using the
<script type="text/javascript" src="filepath"></script>
syntax. When used in a .js file, though, it throws a syntax error on the "<" so I assume this code is invalid.
So, how would one utilize a function in a .js file that was defined in a separate .js file... that works, and is efficient (If there is one)?
EDIT:
I'm going to clarify some thing for the future, since I'm still fairly new to Javascript, and it looks like there were a number of other factors I didn't even know came into play.
I had two .js files, one of which declared classes that were extensions of classes in the other file. I wanted to use the extended classes in a webpage, and I thought I had to load the originial classes into the second .js file, THEN load that .js file into the HTML file. I wasn't programming completely outside of HTML.
Sorry for any misunderstanding, hopefully this thread is helpful to somebody else in the future.

Assuming you are talking about javascript in a web browser, all js files are loading in an html file, typically index.html. You need to use the script tag to load the javascript in the proper order in that html file, not in the javascript file. So if file B requires the things in file A, you need to load file A first, meaning put the script tag that loads file A before the script tag that loads file B.

Two answers:
Non Browser
If you're using JavaScript in a non-browser environment (NodeJS, RingoJS, SilkJS, Rhino, or any of a bunch of others), the answer depends on the environment — but many of these use the CommonJS require mechanism. E.g.:
// Get access to public symbols in foo.js
var foo = require("foo.js");
// Use the `bar` function exported by foo.js
foo.bar();
Browser
If you're using JavaScript in a browser, you put script tags like the one you quoted in the HTML, in the order in which they should be processed (so, scripts relying on things defined in other scripts should be included after the scripts they rely on).
If you want to maximize efficiency in terms of page load time, combine the scripts together on the server (probably also minifying/compressing/packing them) and use just the one script tag.

The answers posted above should do the trick however since you mentioned doing it efficiently you can consider taking a look at javascript module based loaders like require js( http://requirejs.org/ ) based on AMD

You have to put the reference to classlist.js in your HTML file (not your Javascript file), before any other SCRIPT element which requires it. For example, within the 'head' element:
<html>
<head>
<script src="testclass.js"></script>
<script src="file_using_testclass.js"></script>
<script>
var tc = new TestClass();
</script>
</head>

Dynamicly loading js files (during runtime)

I have been able to implant the function to load the js files, but my problem is that it seems that I can only access my variables not my functions (in the object that I load). anyone got any ideas on why this is?

You can load one JS file during runtime which will load other JS files
Suppose you have one js file say "Default.js" in this file just write following line.
It will automatically load "FileToLoad.js" file.
You can add as many lines as shown below to load more & more files-
e.g. document.write("");
document.write("");
document.write("");