JavaScript is used in millions of Web pages to improve the design, validate forms, detect browsers, create cookies, and much more.
JavaScript is the most popular scripting language on the internet, and works in all major browsers, such as Internet Explorer, Firefox, and Opera.
What You Should Already Know
Before you continue you should have a basic understanding of the following:
HTML / XHTML
What is JavaScript?
JavaScript was designed to add interactivity to HTML pages
JavaScript is a scripting language
A scripting language is a lightweight programming language
JavaScript is usually embedded directly into HTML pages
JavaScript is an interpreted language (means that scripts execute without preliminary compilation)
Everyone can use JavaScript without purchasing a license
Are Java and JavaScript the Same?
NO!
Java and JavaScript are two completely different languages in both concept and design!
Java (developed by Sun Microsystems) is a powerful and much more complex programming language - in the same category as C and C++.
What can a JavaScript Do?
JavaScript gives HTML designers a programming tool - HTML authors are normally not programmers, but JavaScript is a scripting language with a very simple syntax! Almost anyone can put small "snippets" of code into their HTML pages
JavaScript can put dynamic text into an HTML page - A JavaScript statement like this: document.write("<h1>" + name + "</h1>") can write a variable text into an HTML page
JavaScript can react to events - A JavaScript can be set to execute when something happens, like when a page has finished loading or when a user clicks on an HTML element
JavaScript can read and write HTML elements - A JavaScript can read and change the content of an HTML element
JavaScript can be used to validate data - A JavaScript can be used to validate form data before it is submitted to a server. This saves the server from extra processing
JavaScript can be used to detect the visitor's browser - A JavaScript can be used to detect the visitor's browser, and - depending on the browser - load another page specifically designed for that browser
JavaScript can be used to create cookies - A JavaScript can be used to store and retrieve information on the visitor's computer
The Real Name is ECMAScript
JavaScript's official name is "ECMAScript". The standard is developed and maintained by the ECMA organisation.
ECMA-262 is the official JavaScript standard. The standard is based on JavaScript (Netscape) and JScript (Microsoft).
The language was invented by Brendan Eich at Netscape (with Navigator 2.0), and has appeared in all Netscape and Microsoft browsers since 1996.
The development of ECMA-262 started in 1996, and the first edition of was adopted by the ECMA General Assembly in June 1997.
The standard was approved as an international ISO (ISO/IEC 16262) standard in 1998.
The development of the standard is still in progress.
How to Put a JavaScript Into an HTML Page
<html>
<body>
<script type="text/javascript">
document.write("Hello World!");
</script>
</body>
</html>
The code above will produce this output on an HTML page:
The code above will produce this output on an HTML page:
Hello World!
Example Explained
To insert a JavaScript into an HTML page, we use the <script> tag. Inside the <script> tag we use the type attribute to define the scripting language.
So, the <script type="text/javascript"> and </script> tells where the JavaScript starts and ends:
<html>
<body>
<script type="text/javascript">
...
</script>
</body>
</html>
The word document.write is a standard JavaScript command for writing output to a page.
By entering the document.write command between the <script> and </script> tags, the browser will recognize it as a JavaScript command and execute the code line. In this case the browser will write Hello World! to the page:
<html>
<body>
<script type="text/javascript">
document.write("Hello World!");
</script>
</body>
</html>
Try it yourself.
Note: If we had not entered the <script> tag, the browser would have treated the document.write("Hello World!") command as pure text, and just write the entire line on the page.
Try it yourself.
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HTML Comments to Handle Simple Browsers
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 can be used to "hide" the JavaScript. Just add an HTML comment tag <!-- before the first JavaScript statement, and a --> (end of comment) after the last JavaScript statement.
<html>
<body>
<script type="text/javascript">
<!--
document.write("Hello World!");
//-->
</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.
Where to Put the JavaScript
JavaScripts in a page will be executed immediately while the page loads into the browser. This is not always what we want. Sometimes we want to execute a script when a page loads, other times when a user triggers an event.
Scripts in the head section: Scripts to be executed when they are called, or when an event is triggered, go in the head section. When you place a script in the head section, you will ensure that the script is loaded before anyone uses it.
<html>
<head>
<script type="text/javascript">
....
</script>
</head>
Scripts in the body section: Scripts to be executed when the page loads go in the body section. When you place a script in the body section it generates the content of the page.
<html>
<head>
</head>
<body>
<script type="text/javascript">
....
</script>
</body>
Scripts in both the body and the head section: You can place an unlimited number of scripts in your document, so you can have scripts in both the body and the head section.
<html>
<head>
<script type="text/javascript">
....
</script>
</head>
<body>
<script type="text/javascript">
....
</script>
</body>
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Using an External JavaScript
Sometimes you might want to run the same JavaScript on several pages, without having to write the same script on every page.
To simplify this, you can write a JavaScript in an external file. Save the external JavaScript file with a .js file extension.
Note: The external script cannot contain the <script> tag!
To use the external script, point to the .js file in the "src" attribute of the <script> tag:
<html>
<head>
<script src="xxx.js"></script>
</head>
<body>
</body>
</html>
Note: Remember to place the script exactly where you normally would write the script!
JavaScript is Case Sensitive
Unlike HTML, JavaScript is case sensitive - therefore watch your capitalization closely when you write JavaScript statements, create or call variables, objects and functions.
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JavaScript Statements
A JavaScript statement is a command to the browser. The purpose of the command is to tell the browser what to do.
This JavaScript statement tells the browser to write "Hello Dolly" to the web page:
document.write("Hello Dolly");
It is normal to add a semicolon at the end of each executable statement. Most people think this is a good programming practice, and most often you will see this in JavaScript examples on the web.
The semicolon is optional (according to the JavaScript standard), and the browser is supposed to interpret the end of the line as the end of the statement. Because of this you will often see examples without the semicolon at the end.
Note: Using semicolons makes it possible to write multiple statements on one line.
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JavaScript Code
JavaScript code (or just JavaScript) is a sequence of JavaScript statements.
Each statement is executed by the browser in the sequence they are written.
This example will write a header and two paragraphs to a web page:
<script type="text/javascript">
document.write("<h1>This is a header</h1>");
document.write("<p>This is a paragraph</p>");
document.write("<p>This is another paragraph</p>");
</script>
Try it yourself.
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JavaScript Blocks
JavaScript statements can be grouped together in blocks.
Blocks start with a left curly bracket {, and ends with a right curly bracket }.
The purpose of a block is to make the sequence of statements execute together.
This example will write a header and two paragraphs to a web page:
<script type="text/javascript">
{
document.write("<h1>This is a header</h1>");
document.write("<p>This is a paragraph</p>");
document.write("<p>This is another paragraph</p>");
}
</script>
Try it yourself.
The example above is not very useful. It just demonstrates the use of a block. Normally a block is used to group statements together in a function or in a condition (where a group of statements should be executed if a condition is met).
You will learn more about functions and conditions in later chapters.
JavaScript Comments
Comments can be added to explain the JavaScript, or to make it more readable.
Single line comments start with //.
This example uses single line comments to explain the code:
<script type="text/javascript">
// This will write a header:
document.write("<h1>This is a header</h1>");
// This will write two paragraphs:
document.write("<p>This is a paragraph</p>");
document.write("<p>This is another paragraph</p>");
</script>
Try it yourself.
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JavaScript Multi-Line Comments
Multi line comments start with /* and end with */.
This example uses a multi line comment to explain the code:
<script type="text/javascript">
/*
The code below will write
one header and two paragraphs
*/
document.write("<h1>This is a header</h1>");
document.write("<p>This is a paragraph</p>");
document.write("<p>This is another paragraph</p>");
</script>
Try it yourself.
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Using Comments to Prevent Execution
In this example the comment is used to prevent the execution of a single code line:
<script type="text/javascript">
document.write("<h1>This is a header</h1>");
document.write("<p>This is a paragraph</p>");
//document.write("<p>This is another paragraph</p>");
</script>
Try it yourself.
In this example the comments is used to prevent the execution of multiple code lines:
<script type="text/javascript">
/*
document.write("<h1>This is a header</h1>");
document.write("<p>This is a paragraph</p>");
document.write("<p>This is another paragraph</p>");
*/
</script>
Try it yourself.
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Using Comments at the End of a Line
In this example the comment is placed at the end of a line:
<script type="text/javascript">
document.write("Hello"); // This will write "Hello"
document.write("Dolly"); // This will write "Dolly"
</script>
Do You Remember Algebra From School?
Hopefully you remember algebra like this from school: x=5, y=6, z=x+y.
Do you remember that a letter (like x) could be used to hold a value (like 5), and that you could use the information above to calculate the value of z to be 11?
Sure you do!
These letters are called variables, and variables can be used to hold values (x=5) or expressions (z=x+y).
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JavaScript Variables
As with algebra, JavaScript variables are used to hold values or expressions.
A variable can have a short name, like x, or a more descriptive name like carname.
Rules for JavaScript variable names:
Variable names are case sensitive (y and Y are two different variables)
Variable names must begin with a letter or the underscore character
NOTE: Because JavaScript is case-sensitive, variable names are case-sensitive.
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Example
A variable's value can change during the execution of a script. You can refer to a variable by its name to display or change its value.
This example will show you how
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Declaring (Creating) JavaScript Variables
Creating variables in JavaScript is most often referred to as "declaring" variables.
You can declare JavaScript variables with the var statement:
var x;
var carname;
After the declaration shown above, the variables have no values, but you can assign values to the variables while you declare them:
var x=5;
var carname="Volvo";
Note: When you assign a text value to a variable, you use quotes around the value.
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Assigning Values to JavaScript Variables
You assign values to JavaScript variables with assignment statements:
x=5;
carname="Volvo";
The variable name is on the left side of the = sign, and the value you want to assign to the variable is on the right.
After the execution of the statements above, the variable x will hold the value 5, and carname will hold the value Volvo.
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Assigning Values to Undeclared JavaScript Variables
If you assign values to variables that have not yet been declared, the variables will automatically be declared.
These statements:
x=5;
carname="Volvo";
have the same effect as:
var x=5;
var carname="Volvo";
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Redeclaring JavaScript Variables
If you redeclare a JavaScript variable, it will not lose its original value.
var x=5;
var x;
After the execution of the statements above, the variable x will still have the value of 5. The value of x is not reset (or cleared) when you redeclare it.
