JavaScript Primer

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# JavaScript Primer

An HTML file can contain text, HTML tags and scripts. Scripts in an HTML file can be executed by the browser.

## What is JavaScript?

- JavaScript is a scripting language
- A scripting language is a lightweight programming language
- A JavaScript consists of lines of executable computer code
- A JavaScript is usually embedded directly into HTML pages
- JavaScript was designed to add interactivity to HTML pages
- Everyone can use JavaScript without purchasing a license

JavaScript is used in millions of Web pages to improve the design, validate forms, detect browsers, create cookies, and much more.

JavaScript is the most popular scripting language on the internet, and works in all major browsers.

## Client-Side Scripting

JavaScript is about "programming" the behavior of the browser. This is called client-side scripting or browser scripting.

Server-side scripting is about "programming" the behavior of the server (see Web Scripting chapter).

## What can a JavaScript Do?

- JavaScript gives HTML designers a programming tool - HTML authors are normally not programmers, but JavaScript is a scripting language with a very simple syntax! Almost anyone can put small "snippets" of code into their HTML pages
- JavaScript can put dynamic text into an HTML page - A JavaScript statement like this: document.write("<h1>" + name + "</h1>") can write a variable text into an HTML page
- JavaScript can react to events - A JavaScript can be set to execute when something happens, like when a page has finished loading or when a user clicks on an HTML element
- JavaScript can read and write HTML elements - A JavaScript can read and change the content of an HTML element
- JavaScript can be used to validate data - A JavaScript can be used to validate form data before it is submitted to a server. This saves the server from extra processing
- JavaScript can be used to detect the visitor's browser - A JavaScript can be used to detect the visitor's browser, and - depending on the browser - load another page specifically designed for that browser
- JavaScript can be used to create cookies - A JavaScript can be used to store and retrieve information on the visitor's computer

## How to Put a JavaScript Into an HTML Page

    <html>
    <body>
    <script type="text/javascript">
    document.write("Hello World!")
    </script>
    </body>
    </html>

The code above will produce this output on an HTML page:

Hello World!

### Example Explained

To insert a JavaScript into an HTML page, we use the <script> tag. The type attribute defines the scripting language.

So, the <script type="text/javascript"> tells where the JavaScript starts:

    <script type="text/javascript">

The JavaScript command for writing some output to a page is document.write:

    document.write("Hello World!")

Then, the script ends:

    </script>

## Try-It-Yourself Examples

Write text
How to write text on a page.

Write text with formatting
How to format the text on your page with HTML tags.

## JavaScript Tutorial

Study our Complete JavaScript tutorial

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