The web browser is usually expecting HTML, so you must specifically tell the browser when JavaScript is coming by using the <script> tag.
The <script> tag is regular HTML.When the web browser encounters the closing </script> tag, it knows it's reached the end of the JavaScript program and can get back to its normal duties.
Much of the time, you'll add the <script> tag in the web page's <head> section, like this:
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN" "http://www.w3.org/TR/ html4/strict.dtd"> <html> <head> <title>My Web Page</title> <script type="text/javascript"> </script> </head> <body> </body> </html>
The <script> tag's type attribute indicates the format and the type of script that follows. In this case, type="text/javascript" means the script is regular text (just like HTML) and that it's written in JavaScript.
If you're using HTML5, life is even simpler. You can skip the type attribute entirely:
<!doctype html> <html> <head> <meta charset="UTF-8"> <title>My Web Page</title> <script> </script> </head> <body> </body> </html>
You then add your JavaScript code between the opening and closing <script> tags:
<!doctype html> <html> <head> <meta charset="UTF-8"> <title>My Web Page</title> <script> alert('hello world!'); </script> </head> <body> </body> </html>
In many cases, you'll put the <script> tags in the page's <head> in order to keep your JavaScript code neatly organized in one area of the web page.
However, it's perfectly valid to put <script> tags anywhere inside the page's HTML.
In fact, it's common to put <script> tags just below the closing </body> tag?this approach makes sure the page is loaded and the visitor sees it before running any JavaScript.
Using the <script> tag as discussed in the previous section lets you add JavaScript to a single web page. But many times you'll create scripts that you want to share with all of the pages on your site.
An external JavaScript file is a text file containing JavaScript code and ending with the file extension .js - test.js, for example. The file is linked to a web page using the <script> tag. For example, to add this JavaScript file to your home page, you might write the following:
<!doctype html> <html> <head> <meta charset="UTF-8"> <title>My Web Page</title> <script src="test.js"> </script> </head> <body> </body> </html>
The src attribute of the <script> tag works just like the src attribute of an <img> tag, or an <a> tag's href attribute.
<!doctype html> <html> <head> <meta charset="UTF-8"> <title>My Web Page</title> <script> alert("Writing Hello World Using document.write..."); document.write('<p>Hello world!</p>'); </script> </head> <body> </body> </html>
See also JavaScript Functions
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