Create property in OOP JavaScript

i found 2 define for property in OOP javascript ,which is correct?

1. define property inner current object with private field(with var)
2. define property in prototype object with public field(with this)

i think first is more like to c# property?

  function Pet() {

               this._age = 0;                       //second public feild

               _name = 'm2';                         //first private feild
              this.get_name= function () {          //first define
                    return _name;
                }

                set_name= function (value) {
                    this._name = value;
                }
            }


            Pet.prototype = {
                speak: function () {                        
                    throw Error.notImplemented();
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                get_age: function () {          //second define
                    return this._age;
                },
                set_age: function (value) {
                    if (isNaN(value) || value < 0) {
                        throw Error.argument('age');
                    }
                    this._age = 0;
                }
            }

            Pet.registerClass('Pet');

            var pet = new Pet();
            var u = d.get_name();

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first option with var as local vaariable and two getter and setter defines better as property because it give option to control input validation and do custom code when setting value like raiseing event.

With this keyword, it is again works property but is more like public field.