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Sunday, June 3, 2012

Java Script Beginner Tutorial 2 : How To Use

JavaScript is typically used to manipulate existing HTML elements. The HTML "id" attribute is used to identify HTML elements. The document.getElementById() method will access the HTML element with the specified id. For Example, in following code, the browser will access the HTML element with id="content", and replace the content with "Edited using JavaScript ".


<!DOCTYPE html>
<html>
<body>
<h1>My Web Page</h1>
<p id="content">My First Paragraph</p>
<script type="text/javascript">
document.getElementById("content").innerHTML="Edited using JavaScript";
</script>
</body>
</html>


The HTML <script> tag is used to insert a JavaScript into an HTML document. Inside the <script> tag use the type attribute to define the scripting language (e.g type="text/javascript"). The <script> and </script> tells where the JavaScript starts and ends. The lines between the <script> and </script> contain the JavaScript and are executed by the browser:


Browsers that do not support JavaScript, will display JavaScript as page content. To prevent them from doing this, and as a part of the JavaScript standard, the HTML comment tag should be used to "hide" the JavaScript. Add an HTML comment tag <!-- before the first JavaScript statement, and a --> (end of comment) after the last JavaScript statement, as follows


<!DOCTYPE html>
<html>
<body>
<script type="text/javascript">
<!--
document.getElementById("content").innerHTML="Edited using JavaScript";
//-->
</script>
</body>
</html>


The two forward slashes at the end of comment line (//) is the JavaScript comment symbol. This prevents JavaScript from executing the --> tag.


The following example writes a <p> element into the HTML document:


<!DOCTYPE html>
<html>
<body>
<h1>My Web Page</h1>
<script type="text/javascript">
document.write("<p>Added using JavaScript</p>");
</script>
</body>
</html>


The problem here is that, the entire HTML page will be overwritten if document.write() is used inside a function. So we must use document.getElementById() to avoid overwritten entire HTML page.

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