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>

Warning for Non-JavaScript Browsers

If you have to do something important using JavaScript, then you can display a warning message to the user using <noscript> tags.

You can add a noscript block immediately after the script block as follows −

<html>

<body>

<script language=”javascript” type=”text/javascript”>

<!–           document.write(“Hello World!”)       //–>

</script>           <noscript>        Sorry…JavaScript is needed to go ahead.

</noscript>

</body>

</html>

Now, if the user’s browser does not support JavaScript or JavaScript is not enabled, then the message from </noscript> will be displayed on the screen.

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>

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”)}

 

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