JavaScript - Placement in HTML File library82.blogspot.com

There is a flexibility given to include JavaScript code anywhere in an HTML document. However the most preferred ways to include JavaScript in an HTML file are as follows −

Script in <head>...</head> section.

Script in <body>...</body> section.

Script in <body>...</body> and <head>...</head> sections.

Script in an external file and then include in <head>...</head> section.

In the following section, we will see how we can place JavaScript in an HTML file in different ways.

JavaScript in <head>...</head> section
If you want to have a script run on some event, such as when a user clicks somewhere, then you will place that script in the head as follows −

<html>

   <head>
 
      <script type="text/javascript">
         <!--
            function sayHello() {
               alert("Hello World")
            }
         //-->
      </script>
     
   </head>
 
   <body>
      <input type="button" onclick="sayHello()" value="Say Hello" />
   </body>
 
</html>
This code will produce the following results −


JavaScript in <body>...</body> section
If you need a script to run as the page loads so that the script generates content in the page, then the script goes in the <body> portion of the document. In this case, you would not have any function defined using JavaScript. Take a look at the following code.

<html>

   <head>
   </head>
 
   <body>
 
      <script type="text/javascript">
         <!--
            document.write("Hello World")
         //-->
      </script>
     
      <p>This is web page body </p>
     
   </body>
</html>
This code will produce the following results −


JavaScript in <body> and <head> Sections
You can put your JavaScript code in <head> and <body> section altogether as follows −

<html>
   <head>
      <script type="text/javascript">
         <!--
            function sayHello() {
               alert("Hello World")
            }
         //-->
      </script>
   </head>
 
   <body>
      <script type="text/javascript">
         <!--
            document.write("Hello World")
         //-->
      </script>
     
      <input type="button" onclick="sayHello()" value="Say Hello" />
     
   </body>
</html>
This code will produce the following result −


JavaScript in External File
As you begin to work more extensively with JavaScript, you will be likely to find that there are cases where you are reusing identical JavaScript code on multiple pages of a site.

You are not restricted to be maintaining identical code in multiple HTML files. The script tag provides a mechanism to allow you to store JavaScript in an external file and then include it into your HTML files.

Here is an example to show how you can include an external JavaScript file in your HTML code using script tag and its src attribute.

<html>

   <head>
      <script type="text/javascript" src="filename.js" ></script>
   </head>
 
   <body>
      .......
   </body>
</html>
To use JavaScript from an external file source, you need to write all your JavaScript source code in a simple text file with the extension ".js" and then include that file as shown above.

For example, you can keep the following content in filename.js file and then you can use sayHello function in your HTML file after including the filename.js file.

function sayHello() {
   alert("Hello World")
}

JavaScript - Syntax Learn Libery

JavaScript can be implemented using JavaScript statements that are placed within the <script>... </script>.

You can place the <script> tags, containing your JavaScript, anywhere within your web page, but it is normally recommended that you should keep it within the <head> tags.

The <script> tag alerts the browser program to start interpreting all the text between these tags as a script. A simple syntax of your JavaScript will appear as follows.

<script ...>
   JavaScript code
</script>
The script tag takes two important attributes −

Language − This attribute specifies what scripting language you are using. Typically, its value will be javascript. Although recent versions of HTML (and XHTML, its successor) have phased out the use of this attribute.

Type − This attribute is what is now recommended to indicate the scripting language in use and its value should be set to "text/javascript".

So your JavaScript segment will look like −

<script language="javascript" type="text/javascript">
   JavaScript code
</script>
Your First JavaScript Script
Let us take a sample example to print out "Hello World". We added an optional HTML comment that surrounds our JavaScript code. This is to save our code from a browser that does not support JavaScript. The comment ends with a "//-->". Here "//" signifies a comment in JavaScript, so we add that to prevent a browser from reading the end of the HTML comment as a piece of JavaScript code. Next, we call a function document.write which writes a string into our HTML document.

This function can be used to write text, HTML, or both. Take a look at the following code.

<html>
   <body>
      <script language="javascript" type="text/javascript">
         <!--
            document.write("Hello World!")
         //-->
      </script>
   </body>
</html>
This code will produce the following result −

Hello World!
Whitespace and Line Breaks
JavaScript ignores spaces, tabs, and newlines that appear in JavaScript programs. You can use spaces, tabs, and newlines freely in your program and you are free to format and indent your programs in a neat and consistent way that makes the code easy to read and understand.

Semicolons are Optional
Simple statements in JavaScript are generally followed by a semicolon character, just as they are in C, C++, and Java. JavaScript, however, allows you to omit this semicolon if each of your statements are placed on a separate line. For example, the following code could be written without semicolons.

<script language="javascript" type="text/javascript">
   <!--
      var1 = 10
      var2 = 20
   //-->
</script>
But when formatted in a single line as follows, you must use semicolons −

<script language="javascript" type="text/javascript">
   <!--
      var1 = 10; var2 = 20;
   //-->
</script>
Note − It is a good programming practice to use semicolons.

Case Sensitivity
JavaScript is a case-sensitive language. This means that the language keywords, variables, function names, and any other identifiers must always be typed with a consistent capitalization of letters.

So the identifiers Time and TIME will convey different meanings in JavaScript.

NOTE − Care should be taken while writing variable and function names in JavaScript.

Comments in JavaScript
JavaScript supports both C-style and C++-style comments, Thus −

Any text between a // and the end of a line is treated as a comment and is ignored by JavaScript.

Any text between the characters /* and */ is treated as a comment. This may span multiple lines.

JavaScript also recognizes the HTML comment opening sequence <!--. JavaScript treats this as a single-line comment, just as it does the // comment.

The HTML comment closing sequence --> is not recognized by JavaScript so it should be written as //-->.

Example
The following example shows how to use comments in JavaScript.

<script language="javascript" type="text/javascript">
   <!--
 
      // This is a comment. It is similar to comments in C++
 
      /*
      * This is a multiline comment in JavaScript
      * It is very similar to comments in C Programming
      */
 
   //-->
</script>