Javascript Prototype Shared Data

Consider the following:

var User = function(data){
    if(data) this.data = data;
};

User.prototype.data = {};

This seems to work well.

var admin = User({id:1});
console.log(admin.data.id) //1

The issue becomes apparent in the following example:

var guest = User();
var admin = User({id:1});
console.log(guest.data.id); //1