How to add methods to a (JSON) object’s prototype?

Assuming your Object comes from some JSON library that parses the server output to generate an Object, it will not in general have anything particular in its prototype ; and two objects generated for different server responses will not share a prototype chain (besides Object.prototype, of course 😉 )

If you control all the places where a “Person” is created from JSON, you could do things the other way round : create an “empty” Person object (with a method like fullName in its prototype), and extend it with the object generated from the JSON (using $.extend, _.extend, or something similar).

var p = { first : "John", last : "Doe"};

function Person(data) {
   _.extend(this, data);
}

Person.prototype.fullName = function() {
   return this.first + " " + this.last;   
}

console.debug(new Person(p).fullName());