Lecture Slides for Java Script | CMSC 102, Study notes of Computer Science

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JavaScript – programming language that can appear in html pages.
It allow us to:  To dynamically create web pages  To control a browser application
Open and create new browser windows
Download and display contents of any URL  To interact with the user  Ability to interact with HTML forms
You can process values provided by checkbox, text, textarea buttons 2 JavaScript
What is not possible with JavaScript  It is not possible to read and write files (security reasons)  The only networking support it provides is:
It can send the contents of forms to a server and e-mail addresses
It can cause the browser to load a web page
JavaScript is not Java, however …  JavaScript program constructs are similar to Java’s constructs (in many cases identical)  JavaScript can interact with java programs 5 Embedding JavaScript in HTML
Different ways to embed JavaScript in HTML  By using <script> and </script> tags in the html document  From an external file (specified via URL) using the src attribute of the script tag
<script src=“demo.js”></script>
script src behaves as if contents of file appears directly between the tags  As event handler
Example2 (See Example2.html)  JavaScript URLs
javascript:alert(“Welcome”) 6 <script> Embedding
You may place any number of JavaScript statements between <script> and </script>
Statements are executed as the document is loaded
<script> may appear in the head or body section of an html document
Several pairs of nonoverlapping <script></script> blocks can appear in a document  All are consider part of the same program  If you define a value in a block it can be referred from another block 7 <script> Embedding
language attribute (e.g., <script language=“JavaScript”></script>)  JavaScript is not the only language out there
Visual Basic Scripting Language (language = “VBScript”)  IE and Netscape will assume JavaScript if language is not specified.  The language attribute allows you to specify a JavaScript version to use (e.g., <script language=“JavaScript1.5”>  If a browser doesn’t understand a language it will ignore the statements
type (e.g, <script type=“text/javascript”>  HTML 4 deprecates the language attribute (although language is widely use) using instead the type attribute 10 Execution of JavaScript Programs
HTML parser – Takes care of processing an html document
JavaScript interpreter – Takes care of processing JavaScript code
HTML parser – must stop processing an html file when JavaScript code is found (JavaScript interpreter will then be running)  This implies a page with JavaScript code that is computationally intensive can take a long time to load 11 JavaScript
Let’s go over several basic constructs that allow us to define JavaScript programs.
Some definitions  string – Any set of characters in double quotes (“ “)  function/method – An entity that completes a particular task for us. It can takes values necessary to complete the particular task and it can return values.
Generating Output with the document.writeln method  Allow us to add text to the html file (see Example1) by providing the required text in “ “  You can specify html code and results of JavaScript constructs 12 JavaScript (Output)
Example 4 (See Example4.html)  Illustrates how we can create a table using document.writeln  Notice how we can use the Date() to specify a particular date format. Date() is part of JavaScript  The + allow us to concatenate strings  Notice how we have specified the border size. If you use “ “ then the table will not be generated. You need to use single quotes.  Keep in mind that this example could have been written without using JavaScript. However you will see how by extending code similar to the one provided you can dynamically decide what your final html look like