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JavaScript A high-level definition JavaScript is a scripting or programming language that allows you to implement complex features on web pages — every time a web page does more than just sit there and display static information for you to look at — displaying timely content updates, interactive maps, animated 2D/3D graphics, scrolling video jukeboxes, etc. — you can bet that JavaScript is probably involved. It is the third layer of the layer cake of standard web technologies, two of which (HTML and CSS) we have covered in much more detail in other parts of the Learning Area.

JavaScript was invented by Brendan Eich in 1995. It was developed for Netscape 2, and became the ECMA-262 standard in 1997. After Netscape handed JavaScript over to ECMA, the Mozilla foundation continued to develop JavaScript for the Firefox browser. Mozilla's latest version was 1.8.

HTML is the markup language that we use to structure and give meaning to our web content, for example defining paragraphs, headings, and data tables, or embedding images and videos in the page.

CSS is a language of style rules that we use to apply styling to our HTML content, for example setting background colors and fonts, and laying out our content in multiple columns.

JavaScript is a scripting language that enables you to create dynamically updating content, control multimedia, animate images, and pretty much everything else. (Okay, not everything, but it is amazing what you can achieve with a few lines of JavaScript code.)

The three layers build on top of one another nicely. Let's take a simple text label as an example. We can mark it up using HTML to give it structure and purpose:

<p>Player 1: Chris</p>

Then we can add some CSS into the mix to get it looking nice:

<style>
p {
  font-family: 'helvetica neue', helvetica, sans-serif;
  letter-spacing: 1px;
  text-transform: uppercase;
  text-align: center;
  border: 2px solid rgba(0,0,200,0.6);
  background: rgba(0,0,200,0.3);
  color: rgba(0,0,200,0.6);
  box-shadow: 1px 1px 2px rgba(0,0,200,0.4);
  border-radius: 10px;
  padding: 3px 10px;
  display: inline-block;
  cursor: pointer;
}
</style>

And finally, we can add some JavaScript to implement dynamic behavior:

<script>
const para = document.querySelector('p');

para.addEventListener('click', updateName);

       function updateName() {
          const name = prompt('Enter a new name');
              para.textContent = `Player 1: ${name}`;
               }

</script>

Try clicking on this last version of the text label to see what happens (note also that you can find this demo on GitHub — see the source code, or run it live)!

JavaScript can do a lot more than that — let's explore what in more detail.

So what can it really do?

The core client-side JavaScript language consists of some common programming features that allow you to do things like:

Store useful values inside variables. In the above example for instance, we ask for a new name to be entered then store that name in a variable called name.

Operations on pieces of text (known as "strings" in programming). In the above example, we take the string "Player 1: " and join it to the name variable to create the complete text label, e.g. "Player 1: Chris".

Running code in response to certain events occurring on a web page. We used a click event in our example above to detect when the label is clicked and then run the code that updates the text label.

And much more!

What is even more exciting however is the functionality built on top of the client-side JavaScript language. So-called Application Programming Interfaces (APIs) provide you with extra superpowers to use in your JavaScript code.

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