sometimes $(document).ready() isn’t called after a postback is made. This happens thrice in 5 attempts or more.

I have the below code in the $(document).ready() which needs to be executed on every postback which is not executed sometimes.

$(document).ready(function () {

Sys.StringBuilder.prototype.appendFormat = function (mask) { this.append(String.format.apply(null, arguments)); };

});

As a result i get the below error

'''Object has no method 'appendFormat''''

Does anyone know why this is happening ?????