- Welcome
- Basics
- Advanced Topics
- Debugging
- Dynamic Web Client Programming (DHTML)
- Advanced Web Client Programming
- JavaScript Outside HTML
- Standards and Best Practices
- Appendices
JavaScript is a loosely typed language. What this means is that you
can use the same variable for different types of information, but you
may also have to check what type a variable is yourself if the
differences matter. For example, if you wanted to add two numbers, but
one variable turned out to be a string, the result wouldn't necessarily
be what you expected.
Contents[hide] |
[edit] Variable declaration
Variables are commonly explicitly declared by the
var
statement, as shown below:var c;
The above variable is created, but has the default value of
undefined
. To be of value, the variable needs to be initialized:var c = 0;
After being declared, a variable may be assigned a new value which will replace the old one:
c = 1;
But make sure to declare a variable with
var
before (or while) assigning to it; otherwise you will create a "scope bug."[edit] Naming variables
When naming variables there are some rules that must be obeyed:
- Upper case and lower case letters of the alphabet, underscores, and dollar signs can be used
- Numbers are allowed after the first character
- No other characters are allowed
- Variable names are case sensitive: different case implies a different name
- A variable may not be a reserved word
[edit] Primitive Types
Primitive types are types provided by the system, in this case by
javascript. Primitive type for javascript are booleans, numbers and
text. In addition to the primitive types, users may define their own
classes.
The primitive types are treated by Javascript as value types and when
you pass them around they go as values. Some types, such as string,
allow method calls.
[edit] Boolean Type
Boolean variables can only have 2 possible values, true or false.
var mayday = false; var birthday = true;
[edit] Numeric Types
You can use Integer and Float types on your variables, but they are treated as a numeric type.
var sal = 20; var pal = 12.1;
In ECMA Javascript your number literals can go from 0 to
-+1.79769e+308. And because 5e-324 is the smallest infinitesimal you can
get, anything smaller is rounded to 0.
[edit] String Types
The String and char types are all strings, so you can build any string literal that you wished for.
var myName = "Some Name"; var myChar = 'f';
[edit] Complex Types
A complex type is an object, be it either standard or custom made. Its home is the heap and goes everywhere by reference.
[edit] Array Type
In Javascript, all Arrays are untyped, so you can put everything you
want in an Array and worry about that later. Arrays are objects, they
have methods and properties you can invoke at will. (The ".length"
property indicates how many things are currently in the array. If you
add more things to the array, the value of the ".length" gets larger).
You can build yourself an array by using the statement
new
followed by Array
, as shown below.var myArray = new Array(0, 2, 4); var myOtherArray = new Array();
Arrays can also be created with the array notation, which uses square brackets:
var myArray = [0, 2, 4]; var myOtherArray = [];
Arrays are accessed using the square brackets:
myArray[2] = "Hello"; var text = myArray[2];
There is no limit to the number of items that can be stored in an array.
[edit] Object Types
An object within Javascript is created using the new operator:
var myObject = new Object();
Objects can also be created with the object notation, which uses curly braces:
var myObject = {};
JavaScript Objects can be built using inheritance and overriding, and
you can use polymorphism. There are no scope modifiers, with all
properties and methods having public access. More information on
creating objects can be found in Object Oriented Programming.
You can access browser built-in objects and objects provided through browser JavaScript extensions.
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