Javascript object descriptors

Question

Is it possible to use getters/setters on Javascript objects like the following basic functionality mockup?

function Descriptor(x) {
     this._value = x;

     this.setter = function(x) {
          // Set internal value after some action or modification
          this._value = x + 1;
     }

     this.getter = function() {
          // Return the internal value
          return this._value;
     }
}

var obj = {};

obj.a = new Descriptor();
obj.a = 5;  // Would run the setter defined in the Descriptor object
obj.a == 6; // Should evaluate to true in this trivial example

// Same as above, just an example of being able to easily reuse the getter/setter
obj.b = new Descriptor();
obj.b = 10;  
obj.b == 11;

Ultimately, it should operate similarly to a Python descriptor set in a class definition. The only things I can find that accomplish something like this requires that the getter/setter be hooked up during the above obj creation, and cannot be easily reused on multiple attributes or objects.

Answer 1

You can try ES5 Object.defineProperty :

function addDescriptor(obj, prop) {
    var value = 0;
    Object.defineProperty(obj, prop, {
        get: function(x) {
            return value;
        },
        set: function(x) {
            value = x + 1;
        }
    });
}

var obj = {};

addDescriptor(obj, 'a');
obj.a = 5;
obj.a == 6; // true

addDescriptor(obj, 'b');
obj.b = 10;
obj.b == 11; // true

Answer 2

I don't know what your environment is (it's only tagged javascript), but ES6 Proxies provide this flexibility of get/set

copied from MDN :

var handler = {
    get: function(target, name){
        return name in target?
            target[name] :
            37;
    }
};

var p = new Proxy({}, handler);
p.a = 1;
p.b = undefined;

console.log(p.a, p.b); // 1, undefined
console.log('c' in p, p.c); // false, 37

Answer 3

You can either use Object.defineProperty to define getter and setter, or simply do it in an object initialiser :

var obj = {
    value: 0,
    get a(){
        return this.value;
    },
    set a(x){
        this.value = x + 1;
    }
};

---

I believe you are trying to do something like this: (Another way of doing what Oriol has done in his answer.)

Object.prototype.Descriptor = function(name){
    var value = 0;
    Object.defineProperty(this, name, {
        get: function(){ return value; },
        set: function(x){ value = x + 1; }
    });
};

var obj = {};
obj.Descriptor("a");
obj.Descriptor("b");

obj.a = 5;
obj.b = 10;

obj.a             //6
obj.b             //11

obj.value         //undefined

Everything is separated, such that the function can be reused meanwhile values are kept separated from all other object's value.