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

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

Assign a Value to a Variable

You 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".

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.

JavaScript Operators

Operators are used to operate on values.

Arithmetic Operators

Comparison Operators

Assignment Shorthand Operators

Logical Operators

String Operator

A string is most often a text, for example "Hello World!". To stick two or more string variables together, use the + operator.

txt1="What a very"
txt2="nice day!"
txt3=txt1+txt2 

The variable txt3 now contains "What a verynice day!".

To add a space between two string variables, insert a space into the expression, OR in one of the strings.

txt1="What a very"
txt2="nice day!"
txt3=txt1+" "+txt2

or

txt1="What a very "
txt2="nice day!"
txt3=txt1+txt2

JavaScript Functions

 A function is a reusable code-block that will be executed by an event, or when the function is called.

Functions

A function contains some code that will be executed by an event or a call to that function. A function is a set of statements. You can reuse functions within the same script, or in other documents. You define functions at the beginning of a file (in the head section), and call them later in the document. It is now time to take a lesson about the alert-box:

This is JavaScript's method to alert the user.

alert("here goes the message")

How to Define a Function

To create a function you define its name, any values ("arguments"), and some statements:

function myfunction(argument1,argument2,etc)
{
some statements
}

A function with no arguments must include the parentheses:

function myfunction()
{
some statements
}

Arguments are variables that will be used in the function. The variable values will be the values passed on by the function call.

By placing functions in the head section of the document, you make sure that all the code in the function has been loaded before the function is called.

Some functions return a value to the calling expression

function result(a,b)
{
c=a+b
return c
}

How to Call a Function

A function is not executed before it is called.

You can call a function containing arguments:

myfunction(argument1,argument2,etc)

or without arguments:

myfunction()

The return Statement

Functions that will return a result must use the "return" statement. This statement specifies the value which will be returned to where the function was called from. Say you have a function that returns the sum of two numbers:

function total(a,b)
{
result=a+b
return result
}

When you call this function you must send two arguments with it:

sum=total(2,3)

The returned value from the function (5) will be stored in the variable called sum.