Objects don't have a .length property...not in the way you're thinking (it's undefined), it's Arrays that have that, to get a length, you need to count the keys, for example:

var length = 0;

for(var k in obj) if(obj.hasOwnProperty(k)) length++;

Or, alternatively, use the keys collection available on most browsers:

var length = obj.keys.length;

MDN provides an implementation for browsers that don't already have .keys:

Object.keys = Object.keys || function(o) {

    var result = [];

    for(var name in o) {

        if (o.hasOwnProperty(name))

          result.push(name);

    }

    return result;

};

Or, option #3, actually make your JSON an array, since those keys don't seem to mean much, like this:

[{"parent_id":1649,"id":"1803","last_update_on":"2010-12-24 07:01:49","message":"dhb;lxd","created_by_id":"21","created_by_name":"Amol Deshpande"},{"parent_id":1649,"id":"1804","last_update_on":"2010-12-24 07:02:49","message":"amol","created_by_id":"21","created_by_name":"Amol Deshpande"}]

Then you can use .length like you want, and still access the members by index.