JavaScript Variables
- Variables are "containers" for storing information .
- 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 must begin with a letter or the underscore character
- Note: Because JavaScript is case-sensitive, variable names are case-sensitive.
- 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.
Example
<html>
<body>
<script type="text/javascript">
var firstname;
firstname="Hege";
document.write(firstname);
document.write("<br />");
firstname="Tove";
document.write(firstname);
</script>
<p>The script above declares a variable,
assigns a value to it, displays the value, changes the value,
and displays the value again.</p>
</body>
</html>Declaring JavaScript Variables
Creating variables in JavaScript is most often referred to as "declaring" variables.
You declare JavaScript variables with the var keyword:
Syntax:
After the declaration shown above, the variables are empty (they have no values yet).var x;
var carname;
However, you can also assign values to the variables when you declare them:
After the execution of the statements above, the variable x will hold the value 5, and carname will hold the value Volvo.var x=5;
var carname="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.
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:
Syntax:
will declare the variables x and carname as global variables (if they don't already exist).x=5;
carname="Volvo";
JavaScript Arithmetic
As with algebra, you can do arithmetic operations with JavaScript variables:
Syntax:
y=x-5;
z=y+5;
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