Private Values on Angular Factory - angularjs

Question

I have private values on my Factory service, but when I update a private value in one instance, all instances are updated as well.
Whats the best way to accomplish that without Updating all instances:
angular.module('app').factory('instanceItem', function () {
var _valid = true;
var item = function (id,data){
this.id = id;
this.data = data;
};
item.prototype.setValid = function(bool) {
_valid = bool;
}
item.prototype.getValid = function() {
return _valid
}
return { item:item }
});
What I am getting:
var itemA = new instanceItem.item(1,{});
var itemB = new instanceItem.item(2,{});
itemA.setValid(false);
console.log(itemA.getValid());
//False
console.log(itemB.getValid());
//False
What I am looking for
var itemA = new instanceItem.item(1,{});
var itemB = new instanceItem.item(2,{});
itemA.setValid(false);
console.log(itemA.getValid());
//False
console.log(itemB.getValid());
//True

Answer 1

A service is a singleton. So there is a unique instance of _valid. _valid should be a field of item:
var item = function (id,data){
this.id = id;
this.data = data;
this._valid = true;
};
item.prototype.setValid = function(bool) {
this._valid = bool;
}
item.prototype.getValid = function() {
return this._valid;
}
Or, if you really want to make it private (and also make your service more natural to use):
function createItem(id, data) {
var _valid = true;
return {
id: id,
data: data,
setValid: function(bool) {
_valid = bool;
},
getValid: function() {
return _valid;
}
};
}
return { createItem: createItem }