Properties and Methods

JavaScript is a dynamic language. We do not need to declare properties or methods on an object prior to using them, for example the following will automatically add a new property and method to an object

var o = new Object();
o.Message = "Clicked";
o.Clicked = function() {
   alert(this.Message);
}

So in the above code we create an instance of an Object and then by calling o.Message we basically add a new property to the object o. Next we create a method on the object. Note: We need to use “this” to access the property from within the method otherwise the code tries to access an undefined variable.

We can actually add properties and methods as well as access them using indexer notation, for example

var o = new Object();
o["Message"] = "Hello";

and this allows us to even call methods dynamically such as

var clicked = "Clicked";

var o = new Object();
   o["Message"] = "Clicked";
   o["Clicked"] = function() {
      alert(this["Message"]);
   }


   o[clicked]();

So in the above we can dynamically change the method called (assuming we had more than one method).

We can also iterate over the members of an object using the following

var o = new Object();
   o["Message"] = "Clicked";
   o["Clicked"] = function() {
      var s = "";
      // iterate over the object
      for(var p in o)
      {
         s = s + p + ";"
      }
      alert(s);
   }

Using the for..in loop we simply iterate over and object, the string “s” will be “Message;Clicked”.

Finally, we can add properties and methods, but we can also delete them using

delete o.Message;

This deletes the Message property from the object o.

Types

JavaScript is loosely types. We declare variables using the var keyword and a variable can change type at any time, for example

var tmp = 3.1415926;
tmp = "Hello";

The runtime will not complain about the type conversion from a floating point number to a string.

JavaScript has a small number of types which include, string, number, boolean, array and object.

• A string may be declared using either double quotes or single quotes, obviously this helps when the string is declare inline.
• A number can be declared with or without decimal places (in other words it can be floating point or an integer).
• A boolean may be true or false (as one would expect)
• An array can be declared in various ways (described later) and can store heterogeneous data types
• An object can be declared and properties and methods assigned to it (see above)

If a var is not assigned a value it is “undefined” (which is itself a type), basically this is a variable that has no value, but we can also use null to denote an “empty” value.

Declaring Arrays

Arrays can be declared in various ways, for the first example we declare an array and then we dynamically add items by creating an index to them

var vowels = new Array();
vowels[0] = "a";
vowels[1] = "e";
vowels[2] = "i";
vowels[3] = "o";
vowels[4] = "u";

A couple of alternatives are

var vowels1 = new Array("a", "e", "i", "o", "u");
var vowels2 = ["a", "e", "i", "o", "u"];