Question

Is this the correct way to make a javascript class with default properties (even though some here are null), yet also having the option to add a creation argument that can contain specific values for some properties listed in object format/json?

Also should i put any of this into Person’s prototype to save memory if I am creating many Person objects?

this is working for me but I wonder if it is a good way to do this ?

// PERSON Class -----------------------------------------------
 QM.Person=function(data)
 {
    /* Closure for this */
    var my = this;

    this.PersonID=null;
    this.Name_Last="";
    this.Name_First="";
    this.Date_Birth=null;
    this.Biography="";

    this.loadData=function(obj) {
        for (var i in obj) {
            if (my.hasOwnProperty(i)) {
                my[i] = obj[i];
            }
        }
    }

    this.loadData(data);
}

example of creating using this class:

jondoe = new Person();

bob = new Person({Name_First:"Bob",Name_Last:"Bar"});

jondoe.Name_First   /*  ""     */
bob.Name_First     /*   "Bob"  */

Answer 1

To avoid repeating yourself you could build a list of keys you expect and also the default values to insert if they aren’t specified:

MyType = function(data){
    var keys = this.keys,
        data = data || {},
        key

    for (key in keys) {
        if (keys.hasOwnProperty(key)) {
            if (data.hasOwnProperty(key)) {
                this[key] = data[key];
            } else {
                this[key] = keys[key];
            }
        }
    }      
};

MyType.prototype.keys = {
    personId: null,
    firstName: 'A default first name',
    lastName: ''
};

var o1 = new MyType();
var o2 = new MyType({
    personId: 1,
    firstName: 'Override'
});
console.log(o1, o2);