Object create define property setter

Question

I need to make it so that every time a specific property on my object is changed - it will call a special method on the same object.

Example:

MyObject.prototype = Object.create({
    specialMethod: function() { /* ... */ }
  }, {
    someValue: {
      set: function(value) {

        /* HOW DO I ASSIGN THE VALUE TO MyObject HERE?*/
        /* I can't do: this.someValue=value, that would create endless recursion */

        this.specialMethod();
      }
    }
  });

How do I assign the value to MyObject within the property setter?

Assigning `MyObject` would ruin all that. You could theoretically do `MyObject = value`, but as i said, that destroys the prototype. — Scimonster, Sep 23 '14 at 14:58
I assume you simply want to assign a property of the current `MyObject` instance? Just use `this` as always. — Bergi, Sep 23 '14 at 14:59
@Bergi, I can't that would create an endless recursion, cause assigning `this.someValue` calls the setter — YemSalat, Sep 23 '14 at 15:00
@YemSalat: You didn't tell us *which* property you wanted to assign to. `this.MyValue = value` would work flawlessly… — Bergi, Sep 23 '14 at 15:03
I want to assign to the value that I'm creating a setter for. In this example `someValue`. No, I can't assign to it within the constructor via `this.someValue = value`, as I explained above that would keep calling the setter forever. — YemSalat, Sep 23 '14 at 15:06
Object.observe doesn't fit what you are trying to accomplish? — Hrishi, Sep 23 '14 at 15:09
It's new in JavaScript Harmony http://wiki.ecmascript.org/doku.php?id=harmony:observe — Hrishi, Sep 23 '14 at 15:11
@Hrishi: That's a proposal. I don't see that it has been accepted. — , Sep 23 '14 at 15:13

score 6 · Accepted Answer · answered Sep 23 '14 at 15:08

There is no storage place in a getter/setter property, you cannot store values on it. You need to store it somewhere else and create a getter for that. Two solutions:

Use a second, "hidden" property:

MyObject.prototype.specialMethod: function() { /* ... */ };
Object.defineProperty(MyObject.prototype, "someValue", {
    set: function(value) {
        this._someValue = value;
        this.specialMethod();
    },
    get: function() {
        return this._someValue;
    }
});

Use a closure variable (typically created on construction of the instance):

function MyObject() {
    var value;
    Object.defineProperty(this, "someValue", {
        set: function(v) {
            value = v;
            this.specialMethod();
        },
        get: function() {
            return value;
        }
    });
}
MyObject.prototype.specialMethod: function() { /* ... */ };

Thanks a ton! Sorry about the confusion with initial example. This is exactly what I needed (decided to go with the closure so the object wont be littered with 'hidden' properties) — YemSalat, Sep 23 '14 at 15:12

Object create define property setter

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