The tutorial that you’re reading is about core JavaScript, which is platform-independent. Further on, you will learn Node.JS and other platforms that use it.
But, we need a working environment to run our scripts, and, just because this book is online, the browser is a good choice. We’ll keep the amount of browser-specific commands (like
alert) to a minimum, so that you don’t spend time on them if you plan to concentrate on another environment like Node.JS.
So first, let’s see how to attach a script to a webpage. For server-side environments, you can just execute it with a command like
"node my.js" for Node.JS.The “script” tag
JavaScript programs can be inserted in any part of an HTML document with the help of the
<script> tag.
For instance:
<!DOCTYPE HTML> <html> <body> <p>Before the script...</p> <script> alert( 'Hello, world!' ); </script> <p>...After the script.</p> </body> </html>
You can run the example by clicking on the “Play” button in its right-top corner.
The
<script> tag contains JavaScript code which is automatically executed when the browser meets the tag.The modern markup
The
<script> tag has a few attributes that are rarely used nowadays, but we can find them in old code:- The
typeattribute:<script type=…> - The old standard HTML4 required a script to have a type. Usually it was
type="text/javascript". The modern HTML standard assumes thistypeby default. No attribute is required. - The
languageattribute:<script language=…> - This attribute was meant to show the language of the script. As of now, this attribute makes no sense, the language is JavaScript by default. No need to use it.
- Comments before and after scripts.
- In really ancient books and guides, one may find comments inside
<script>, like this:<script type="text/javascript"><!-- ... //--></script>These comments were supposed to hide the code from an old browser that didn’t know about a<script>tag. But all browsers born in the past 15+ years don’t have any issues. We mention it here, because such comments serve as a sign. If you see that somewhere – that code is probably really old and not worth looking into.
External scripts
If we have a lot of JavaScript code, we can put it into a separate file.
The script file is attached to HTML with the
src attribute:<script src="/path/to/script.js"></script>
Here
/path/to/script.js is an absolute path to the file with the script (from the site root).
It is also possible to provide a path relative to the current page. For instance,
src="script.js" would mean a file "script.js" in the current folder.
We can give a full URL as well, for instance:
<script src="https://cdnjs.cloudflare.com/ajax/libs/lodash.js/3.2.0/lodash.js"></script>
To attach several scripts, use multiple tags:
<script src="/js/script1.js"></script>
<script src="/js/script2.js"></script>
…
Please note:
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