How to Put a JavaScript Into an HTML Page

Contractjack · Post by **Contractjack** » Sun Feb 04, 2007 8:43 am

How to Put a JavaScript Into an HTML Page
<html>
<body>
<script type="text/javascript">
****.write("Hello World!")
</script>
</body>
</html>

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 (also 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 ****.write is a standard JavaScript command for writing output to a page.

By entering the ****.write command between the <script type="text/javascript"> 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">
****.write("Hello World!")
</script>
</body>
</html>

Note: If we had not entered the <script> tag, the browser would have treated the ****.write("Hello World!") command as pure text, and just write the entire line on the page.

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Ending Statements With a Semicolon?
With traditional programming languages, like C++ and Java, each code statement has to end with a semicolon.

Many programmers continue this habit when writing JavaScript, but in general, semicolons are optional! However, semicolons are required if you want to put more than one statement on a single line.

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How to Handle Older Browsers
Browsers that do not support JavaScript will display the script as page content. To prevent them from doing this, we may use the HTML comment tag:

<script type="text/javascript">

</script>

The two forward slashes at the end of comment line (//) are a JavaScript comment symbol. This prevents the JavaScript compiler from compiling the line.

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 ****, 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="****.js"></script>
</head>
<body>
</body>
</html>

Note: Remember to place the script exactly where you normally would write the script!

Variables
A variable is a "container" for information you want to store. A variable's value can change during the script. You can refer to a variable by name to see its value or to change its value.

Rules for variable names:

Variable names are case sensitive
They must begin with a letter or the underscore character
IMPORTANT! JavaScript is case-sensitive! A variable named strname is not the same as a variable named STRNAME!

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Declare a Variable
You can create a variable with the var statement:

var strname = some value

You can also create a variable without the var statement:

strname = some value

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Assign a Value to a Variable
You can assign a value to a variable like this:

var strname = "Hege"

Or like this:

strname = "Hege"

The variable name is on the left side of the expression and the value you want to assign to the variable is on the right. Now the variable "strname" has the value "Hege".

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Lifetime of Variables
When you declare a variable within a function, the variable can only be accessed within that function. When you exit the function, the variable is destroyed. These variables are called local variables. You can have local variables with the same name in different functions, because each is recognized only by the function in which it is declared.

If you declare a variable outside a function, all the functions on your page can access it. The lifetime of these variables starts when they are declared, and ends when the page is closed.

Conditional Statements
Very often when you write code, you want to perform different actions for different decisions. You can use conditional statements in your code to do this.

In JavaScript we have the following conditional statements:

if statement - use this statement if you want to execute some code only if a specified condition is true
if...else statement - use this statement if you want to execute some code if the condition is true and another code if the condition is false
if...else if....else statement - use this statement if you want to select one of many blocks of code to be executed
switch statement - use this statement if you want to select one of many blocks of code to be executed

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If Statement
You should use the if statement if you want to execute some code only if a specified condition is true.

Syntax
if (condition)
{
code to be executed if condition is true
}

Note that if is written in lowercase letters. Using uppercase letters (IF) will generate a JavaScript error!

Example 1
<script type="text/javascript">
//Write a "Good morning" greeting if
//the time is less than 10var d=new Date()
var time=d.getHours()

if (time<10)
{
****.write("<b>Good morning</b>")
}
</script>

Example 2
<script type="text/javascript">
//Write "Lunch-time!" if the time is 11var d=new Date()
var time=d.getHours()

if (time==11)
{
****.write("<b>Lunch-time!</b>")
}
</script>

Note: When comparing variables you must always use two equals signs next to each other (==)!

Notice that there is no ..else.. in this syntax. You just tell the code to execute some code only if the specified condition is true.

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If...else Statement
If you want to execute some code if a condition is true and another code if the condition is not true, use the if....else statement.

Syntax
if (condition)
{
code to be executed if condition is true
}
else
{
code to be executed if condition is not true
}

Example
<script type="text/javascript">
//If the time is less than 10,
//you will get a "Good morning" greeting.
//Otherwise you will get a "Good day" greeting.var d = new Date()
var time = d.getHours()

if (time < 10)
{
****.write("Good morning!")
}
else
{
****.write("Good day!")
}
</script>

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If...else if...else Statement
You should use the if....else if...else statement if you want to select one of many sets of lines to execute.

Syntax
if (condition1)
{
code to be executed if condition1 is true
}
else if (condition2)
{
code to be executed if condition2 is true
}
else
{
code to be executed if condition1 and
condition2 are not true
}

Example
<script type="text/javascript">
var d = new Date()
var time = d.getHours()
if (time<10)
{
****.write("<b>Good morning</b>")
}
else if (time>10 && time<16)
{
****.write("<b>Good day</b>")
}
else
{
****.write("<b>Hello World!</b>")
}
</script>

The JavaScript Switch Statement
You should use the switch statement if you want to select one of many blocks of code to be executed.

Syntax
switch(n)
{
case 1:
execute code block 1
break
case 2:
execute code block 2
break
default:
code to be executed if n is
different from case 1 and 2
}

This is how it works: First we have a single expression n (most often a variable), that is evaluated once. The value of the expression is then compared with the values for each case in the structure. If there is a match, the block of code associated with that case is executed. Use break to prevent the code from running into the next case automatically.

Example
<script type="text/javascript">
//You will receive a different greeting based
//on what day it is. Note that Sunday=0,
//Monday=1, Tuesday=2, etc.var d=new Date()
theDay=d.getDay()
switch (theDay)
{
case 5:
****.write("Finally Friday")
break
case 6:
****.write("Super Saturday")
break
case 0:
****.write("Sleepy Sunday")
break
default:
****.write("I'm looking forward to this weekend!")
}
</script>

Well thats all i know for now, but you can try w3schools on learning javascript if you wish.

Post by **SHAdmin** » Mon Feb 05, 2007 5:46 am

A bonus of 30 points have been credited into your account for sharing that very usefull "How To".