The prototype keyword lets you add methods and properties to custom objects, even some predefined objects like the string object!
return('Quote: “' + this + '“');
String.prototype.Quoted = AddQuotes;
var msg = 'this sentence is not mine but a quote.';
Lousy example, but imagine what you can do with these possibilities...
I used this in a large webapplication to add things like Trim(), LTrim() en RTrim() to the string object.
(I know, I know...
Not cutting edge .NET technology. Just thought I 'dump' it on the blog in case someone might find it usefull.)