Numbers
In JavaScript all numbers are treated as floating point
numbers . JavaScript does support integers , octal numbers
, hexadecimal numbers etc but at the lowest level , JavaScript
sees all numbers as floating point numbers .
Integers
These are numbers with no fractional parts , they
can be positive or negative and they can be decimal , hexadecimal
or octal . The most common type of integers are decimal
integers (or base 10 ) . these are numbers ranging from
0 - 9 and cannot have a 0 in front of the numbers . Here
are some examples .
valid : 2 , 23 , 900 , 54
invalid : 04 , 0300 , 079
Octal integers (also referred to as base-8 ) must
begin with a leading zero and then each digit after the
leading zero can be in the range of 0 - 7.
hexadecimal integers (also known as base-16) must
begin with 0x or 0X . Each digit following this can be in
the range of 0 - 9 and from a - f where a - f is the equivalent
of 10 - 15 .
Example
A simple example here
<script language = "JavaScript">
<!--
document.write("45 is " + 45 + " decimal (base 10)");
document.write("<br>045 is " + 045 + " octal (base 8)");
document.write("<br>0x45 is " + 0x45 + " hexadecimal (base 16)");
//-->
</script>
now can you see the importance of not putting 0 or 0x in
front of your numbers , the results are different in octal
and hexadecimal.
Strings
A string is made up of a number of characters . Strings
are declared by placing the characters inside either double
quotes ( " " ) or inside single quotes( ' ' )
. When a backslash character(\) appears in a string literal
, it escapes the character that follows it this means you
can place special characters in the string . ( see our special
characters reference )
Boolean
A boolean data type can only have two values either true
or false . These are often represented by 1 for true and
0 for false in javaScript . It is sometimes better to think
of true or false as on or off or even as yes or no .
null
This is provided by the JavaScript keyword null which represents
a condition where no value exists .
Naming Variables
There are some guidelines to follow when naming variables
in JavaScript . These are as follows
- The first character of the name must be a letter or
an underscore ( _ )
- All characters following the first character can be
letters , underscore , or digits
- Letters can be either upper or lower case . Note that
JavaScript treats the two cases differently so for example
firstname is different from FirstName or FIRSTname .
Assigning Variables
To declare a variable in JavaScript we use the var keyword
followed by a variable name . You can put multiple variables
using the same var keyword if this is the case you use commas
to separate the variable names .
Now that you have defined a variable , you can assign a
value to it with the assignment operator (=) . Often the
declaration and assignment take place at the same time .
If a value is assigned to a variable that has not been declared
with the var keyword . JavaScript creates a global variable
.
Lets see an example of all this .
<script language = "JavaScript">
<!--
//variable declaration with no assignment
var name ;
//assignment with no use of var keyword
name = "iain";
//variable declaration and assignment combined
var age = 29 , height = 6;
//print details
document.write(name ," is ", age ," and " ,height, " foot " );
//-->
</script>
Which gives this result .
Variable Scope
Variables in JavaScript can be either local or global .
All variables are global unless they are declared in a function
in this case the variable is local to that function . You
can have two variables with the same name if one is global
and one is local to a function . When you access the variable
in the function you access the local variable , from outside
the function you are accessing the global variable .
You should always use the var keyword to declare a variable
inside a function if you wish it to be a local variable
. If you do not JavaScript creates a global variable.
