•
The value can be something that has to be calculated,
or something that will be provided later, like user input.
Variable declared without a value will have the value
undefined
.
•
The variable
carname
will have the
value
undefined
after the execution of the following
statement:
•
var carname;
14
Re-Declaring JavaScript Variables
•
If you re-declare a JavaScript variable, it will not lose its
value:.
•
The value of the variable
carname
will still have the
value "
Volvo
" after the execution of the following two
statements:
•
var carname="Volvo";
var carname;
15
Variable Scope
•
A variable declared (using var) within a JavaScript
function becomes
LOCAL
and can only be accessed
from 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 deleted as soon as the function is
completed.
•
Variables declared outside a function, become
GLOBAL
,
and all scripts and functions on the web page can
access it.
16
The Lifetime of JavaScript Variables
•
The lifetime of JavaScript variables starts when they are
declared.
•
Local variables are deleted when the function is
completed.
•
Global variables are deleted when you close the page
•
If you assign a value to a variable that has not yet been
declared, the variable will automatically be declared as
a
GLOBAL
variable.
•
This statement:
•
carname="Volvo“; will declare the variable
carname
as
a global variable , even if it is executed inside a
function.
17
JavaScript Arithmetic
•
Given that
y = 5
, the table below explains the
arithmetic operators:
•
Arithmetic operators are used to perform arithmetic
between variables and/or values
18
JavaScript Data Types
•
String, Number, Boolean, Array, Object, Null,
Undefined.
•
JavaScript has dynamic types.
•
This means that the same variable can be used as
different types:
•
Example
•
var x;
// Now x is undefined
var x = 5;
// Now x is a Number
var x = "John";
// Now x is a String
19
Addition
1.
<!DOCTYPE html>
2.
<html>
3.
<body>
4.
<p>y = 5, calculate x = y + 2, and display x:</p>
5.
<button onclick="myFunction()">Click Me</button>
6.
<p id="demo"></p>
7.
<script>
8.
function myFunction() {
9.
var y = 5;
10.
var x = y + 2;
11.
document.getElementById("demo").innerHTML = x;
12. }
13. </script>
14. </body>
15. </html>
20
Subtraction
•
<!DOCTYPE html>
•
<html>
•
<body>
•
<p>y = 5, calculate x = y - 2, and display x:</p>
•
<button onclick="myFunction()">Click Me</button>
•
<p id="demo"></p>
•
<script>
•
function myFunction() {
•
var y = 5;
•
var x = y - 2;
•
document.getElementById("demo").innerHTML = x;
•
}
•
</script>
•
</body>
•
</html>
21
JavaScript Strings
•
A string is a variable which stores a series of characters
like "John Doe".
•
A string can be any text inside quotes.
