Q:

How do I use the  tag to send data to a javascript function?

I was testing and trying to make an little form that when the user entered their name, it would take that name and display it on to the screen.
<html>
<head>
    <center><h1>Test-Page</h1></center>
</head>
<body>
    <div class="someRandomStuff">
    <h2 id="testingID">What is your first name?</h2>
    <form name="input" action="login.js" method="post">
        Name: <input type="text" name="userID"/>
        <input type="submit" value="Submit"/>
    </form>
    </div>
</body>

Here is the js file
function displaySystem(name) {
    document.getElementById("testingID").innerHTML("Ah, hello there" + name)
}

I know that I could probably do this in one HTML file, however I want to try and make the js and HTML separate. ANY help is appreciated. 

A:

You don't send data to a JavaScript function, but a JavaScript function can retrieve form data.
For example, and input of type text can be retrieved using its value property:
var input = document.getElementById("userID");
var value = input.value;

I know that I could probably do this in one HTML file, however I want
  to try and make the js and HTML separate.

Nice step. In fact, there's no practical difference in terms of retrieving form data or manipulating the document from an inline script or a script that's included using a <script src=... element. The main difference is a script embedded in the HTML document won't be cached, while a one included as a separate file will be cached (obviously, there're other reasons if we talk about good separation of concerns!).

