VirtualMV/JavaScript/Starting/Basics

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Introduction

Overview

VmvIcon Objectives.png

By the end of this page you will be able to:

  • Understand how JavaScript is placed in an HTML document
  • Understand how Browsers handle JavaScript

Code placement

  • JavaScript can be inserted into the HEAD or BODY section of a document.
  • If code is to be reused within a page then it is better to place the code in the HEAD section.
  • Code in the HEAD section is loaded before the page is built and any referrals within a page are already in memory.
  • Code within the page must be loaded into memory before execution.

Browser Handling

  • Older style browsers may not handle JavaScript
  • Browsers such as Internet Explorer warn you that running scripts from your local hard drive is a security risk
JavaScript security warning in Internet Explorer
  • To trick the browser we enclose the JavaScript code between a <!-- and //-->
<script type="text/javascript">
<!--     JavaScript code  
//--></script>
  • The older browser then ignores all code between the start and end tags
  • You may come across JavaScripts inserted as
  <script language="JavaScript"> 
  </script>

This has now be deprecated and the following format should be used:

  <script type="text/javascript">
  </script>


JavaScript (js1_02)

<html>
<head>
  <script type="text/javascript">
    <!-- hide from old browsers     
    document.write("<p>This is code in the HEAD!!!</p>"); 
    // -->  
  </script>
</head>
<body>
  <p>This is a normal HTML code in the BODY section.<br />  
  <script type="text/javascript">
    <!-- hide from old browsers     
    document.write("This is JavaScript in the BODY!!!");  
    // -->  
  </script>
  <br />
  Back to normal HTML again.</p>
</body>
</html>

Example :Click here to run js1_02.

JavaScript: Order of execution


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VirtualMV/JavaScript/Starting/Basics. (2017). In WikiEducator/VirtualMV wiki. Retrieved October 17, 2017, from http://wikieducator.org/VirtualMV/JavaScript/Starting/Basics    (zotero)