Names
External JavaScript files can be called almost anything as long as they end .js. Using sensible names would help later though. Try to use fairly short names but ones which mean something. They should describe what is in the file. You can collect functions into one file or split them into multiple files with functions grouped together into a number of easy-to-handle files.
Never use spaces or special symbols. You can use underscores or dashes to represent spaces to make the names more readable but that can get confusing
One other consideration is capitalisation. Some people follow the JavaScript convention of only using capital letters where a name contains more than one "word":
<script type="text/javascript" src="./javascript/telephoneNumber.js">
Another, perhaps easier, approach is to never use capitals at all.
Whatever approach you use make sure you stick to that convention.
Directories
Once you start having multiple JavaScript files on your server it can get cluttered. Therefore you should start to put JavaScript files into a separate folder/directory. You could call it "scripts" or "javascript" perhaps. With images in a folder/directory called "images" and CSS in "css" the root directory will be easier to handle.
The only other change you need to make is to change the SRC attribute to point to the file in it's new location:
<script type="text/javascript" src="./javascript/fred.js">
</script>
The dot tells the browser that the starting point for the file path is the current directory (where the Web page is). The forward slashes separate directories from each other and from the file name.
An alternative is:
<script type="text/javascript" src="/javascript/fred.js">
</script>
Starting with a slash says to start at the root directory of the site. This allows the Web page to move and the link will still point to the same location as it is not relative to the location of the Web page.
A final alternative is:
<script type="text/javascript" src="http://www.mysite.com/javascript/fred.js">
</script>
The problems with this approach are that if you move the Web page to another site the SRC still points to where the JavaScript is on the first server. That would be useful if you wanted to use a library such as Yahoo which is hosted on their server.
