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Showing posts with label JAVASCRIPT. Show all posts
Showing posts with label JAVASCRIPT. Show all posts

JavaScript Variables

Posted by champtutorial On Monday, 5 September 2011 0 comments


Variables are "containers" for storing information.

Do You Remember Algebra From School?

Do you remember algebra 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?
These letters are called variables, and variables can be used to hold values (x=5) or expressions (z=x+y).

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.

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.

Declaring (Creating) JavaScript Variables

Creating variables in JavaScript is most often referred to as "declaring" variables.
You declare JavaScript variables with the var keyword:

EXAMPLE:

var x;
var carname;

After the declaration shown above, the variables are empty (they have no values yet).
However, you can also assign values to the variables when you declare them:

EXAMPLE:



var x=5;
var carname="Volvo";

After the execution of the statements above, the variable x will hold the value 5, and carname will hold the value Volvo.
Note: When you assign a text value to a variable, use quotes around the value.
Note: If you redeclare a JavaScript variable, it will not lose its value.

Local JavaScript Variables

A variable declared within a JavaScript function becomes LOCAL and can only be accessed within that function. (the variable has local scope).
You can have local variables with the same name in different functions, because local variables are only recognized by the function in which they are declared.
Local variables are destroyed when you exit the function.
You will learn more about functions in a later chapter of this tutorial.

Global JavaScript Variables

Variables declared outside a function become GLOBAL, and all scripts and functions on the web page can access it.
Global variables are destroyed when you close the page.
If you declare a variable, without using "var", the variable always becomes GLOBAL.

Assigning Values to Undeclared JavaScript Variables

If you assign values to variables that have not yet been declared, the variables will automatically be declared as global variables.
These statements:

EXAMPLE:



x=5;
carname="Volvo";

will declare the variables x and carname as global variables (if they don't already exist).

JavaScript Arithmetic

As with algebra, you can do arithmetic operations with JavaScript variables:

EXAMPLE:



y=x-5;
z=y+5;






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JavaScript Comments

Posted by champtutorial On 0 comments


JavaScript Comments

Comments can be added to explain the JavaScript, or to make the code more readable.
Single line comments start with //.
The following example uses single line comments to explain the code:

Example:



<script type="text/javascript">
// Write a heading
document.write("<h1>This is a heading</h1>");
// Write two paragraphs:
document.write("<p>This is a paragraph.</p>");
document.write("<p>This is another paragraph.</p>");
</script>



JavaScript Multi-Line Comments

Multi line comments start with /* and end with */.
The following example uses a multi line comment to explain the code:

Example:



<script type="text/javascript">
/*
The code below will write
one heading and two paragraphs
*/
document.write("<h1>This is a heading</h1>");
document.write("<p>This is a paragraph.</p>");
document.write("<p>This is another paragraph.</p>");
</script>




Using Comments to Prevent Execution

In the following example the comment is used to prevent the execution of a single code line (can be suitable for debugging):

Example:



<script type="text/javascript">
//document.write("<h1>This is a heading</h1>");
document.write("<p>This is a paragraph.</p>");
document.write("<p>This is another paragraph.</p>");
</script>


in the following example the comment is used to prevent the execution of a code block (can be suitable for debugging):

Example:



<script type="text/javascript">
/*
document.write("<h1>This is a heading</h1>");
document.write("<p>This is a paragraph.</p>");
document.write("<p>This is another paragraph.</p>");
*/
</script>



Using Comments at the End of a Line

In the following example the comment is placed at the end of a code line:

Example:



<script type="text/javascript">
document.write("Hello"); // Write "Hello"
document.write(" Dolly!"); // Write " Dolly!"
</script>


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JavaScript Statements

Posted by champtutorial On 0 comments


JavaScript is a sequence of statements to be executed by the browser.

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.

JavaScript Statements

A JavaScript statement is a command to a 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:

EXAMPLE;

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.

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 heading and two paragraphs to a web page:

EXAMPLE;

<script type="text/javascript">
document.write("<h1>This is a heading</h1>");
document.write("<p>This is a paragraph.</p>");
document.write("<p>This is another paragraph.</p>");
</script>


JavaScript Blocks

JavaScript statements can be grouped together in blocks.
Blocks start with a left curly bracket {, and end with a right curly bracket }.
The purpose of a block is to make the sequence of statements execute together.
This example will write a heading and two paragraphs to a web page:

EXAMPLE;

<script type="text/javascript">
{
document.write("<h1>This is a heading</h1>");
document.write("<p>This is a paragraph.</p>");
document.write("<p>This is another paragraph.</p>");
}
</script>

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.



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JavaScript How To

Posted by champtutorial On 0 comments


The HTML <script> tag is used to insert a JavaScript into an HTML page.

Writing to The HTML Document

The example below writes a <p> element with current date information to the HTML document:

Example:

<html>
<body>

<h1>My First blog</h1>

<script type="text/javascript">
document.write("<p>" + Date() + "</p>");
</script>

</body>
</html>

Note: Try to avoid using document.write() in real life JavaScript code. The entire HTML page will be overwritten if document.write() is used inside a function, or after the page is loaded. However, document.write() is an easy way to demonstrate JavaScript output in a tutorial.

Changing HTML Elements

The example below writes the current date into an existing <p> element:

Example:



<html>
<body>

<h1>My First Web Page</h1>

<p id="demo"></p>

<script type="text/javascript">
document.getElementById("demo").innerHTML=Date();
</script>

</body>
</html>

Note: To manipulate HTML elements JavaScript uses the DOM method getElementById(). This method accesses the element with the specified id.

Examples Explained

To insert a JavaScript into an HTML page, use the <script> tag.
Inside the <script> tag use the type attribute to define the scripting language.
The <script> and </script> tells where the JavaScript starts and ends:

Example:



<html>
<body>
<h1>My First Web Page</h1>

<p id="demo">This is a paragraph.</p>

<script type="text/javascript">
... some JavaScript code ...
</script>
</body>
</html>

The lines between the <script> and </script> contain the JavaScript and are executed by the browser.
In this case the browser will replace the content of the HTML element with id="demo", with the current date:

Example:



<html>
<body>
<h1>My First Web Page</h1>

<p id="demo">This is a paragraph.</p>

<script type="text/javascript">
document.getElementById("demo").innerHTML=Date();
</script>
</body>
</html>

Some Browsers do Not Support JavaScript

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 should 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, like this:

Example:



<html>
<body>
<script type="text/javascript">
<!--
document.getElementById("demo").innerHTML=Date();
//-->
</script></body>
</html>





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JavaScript Introduction

Posted by champtutorial On 0 comments


JavaScript is the most popular scripting language on the internet, and works in all major browsers, such as Internet Explorer, Firefox, Chrome, Opera, and Safari.

What You Should Already Know

Before you continue you should have a basic understanding of the following:
  • HTML and CSS
If you want to study these subjects first, find the tutorials on our Home page.

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 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 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

JavaScript = ECMAScript

JavaScript is an implementation of the ECMAScript language standard. ECMA-262 is the official JavaScript standard.
JavaScript was invented by Brendan Eich at Netscape (with Navigator 2.0), and has appeared in all browsers since 1996.
The official standardization was adopted by the ECMA organization (an industry standardization association) in 1997.
The ECMA standard (called ECMAScript-262) was approved as an international ISO (ISO/IEC 16262) standard in 1998.
The development is still in progress.

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