Problem with classes in javascript

I wrote the following class in javascript

function main() 
{
  this.area ;
  this.$= function(target)
  {
    if(target == undefined)
    {
      this.area = document;
    }
    else
    {  
      this.area = document.getElementById(target);
      if( this.area == null )
      {
        this.area = document.getElementsByTagName(target);
        if( this.area[0] == null )
        alert("Error value "+target+" passed to $() is no an id or name");//code to create variable
      }
    }
  }
  this.display = function(value)
    {
      if(this.area == undefined)
      alert("Error in display() method...target not selectedddd");
      开发者_StackOverflow社区else area.innerHTML = value;
    }
}
    and i always get the error message I given as alert. Why area is always null? 

If I refer to area without using the this keyword will it work. Is there any way to nest classes in javascript like

function()
{
  this.a;
  function()
  {
  }
}

The reason why it does not work is that this refers to the inner function scope, not the outer function scope you call a "class".

Try something like this instead:

function MyClass() {
    // This is the constructor.
}

MyClass.prototype = {
    method: function() {
    },

    foo: function() {
    }
};

var instance = new MyClasss();
instance.method();

I tried the class, and I don't get the error message.

Usage:

var m = new main();
m.$('x');
m.display('Replaced');

Demo: http://jsfiddle.net/ksNSG/

There is an error in the display method, however. You are using area instead of this.area, but that doesn't cause the error message to be displayed, it just keeps the method from setting the content of the element.

Edit:

Regarding your comment to the question; the $ method doesn't return the object, so you can't chain it after the constructor like var m = new main().$('x');, you have to call the $ method after creating the object.

You would have to add a return this; last in the $ method in order to chain the calls that way.