JavaScript String subclassing example. Based on http://webreflection.blogspot.com/2008/10/subclassing-javascript-native-string.html

string_subclassing.js

function Markdown(value) {
  this.value = value || '';
  this.length = this.value.length;
};
Markdown.prototype = new String;

// Instance methods
Markdown.prototype.toString = Markdown.prototype.valueOf = function(){return this.value};
Markdown.prototype.toHTML = function(){return Markdown.decode(this)};

// Static methods
Markdown.decode = function(){};

// Example:
m = new Markdown('*italic*');
m.toHTML();  // '<i>italic</i>'
Markdown.decode('*italic*');   // '<i>italic</i>'

If you have access to ES5, this would be better (replacing line 3):

Object.defineProperty(this, 'length', {get: function () { return this.value.length; }});

You then get:

m.length;  // » 8
m.value = '**bolder**';
m.length;  // » 10

I'm doubly confused by: "Markdown.decode = function(){};" --a function without arguments.
In the first place it is called with the parameter "this". In the second place decode() is not a native String method.

Presumably the reader is expected to write "decode()" him/herself and in the body convert "arguments[0].value" to HTML?