Accessing get/set methods for JavaScript property created using Object.create

Question

What I'd like is the ability to wrap a JavaScript property to modify behavior on get/set.

For properties that are values, I can do the following:

var obj = {
    myProperty : 0
};

function notifyOfChange(obj, propertyName) {
    var propertyValue = obj[propertyName];
    Object.defineProperty(obj, propertyName, {
        get : function() { return propertyValue; },
        set : function(newValue) {
            var propertyValue = newValue;
            console.log("Message from notifyOfChange."); 
        }
    });
};

obj.myProperty = 10; // outputs "Message from notifyOfChange."

However, what if myProperty already has a getter/setter?

var obj = Object.create({}, {
    myProperty : {
        get : function() { return this._myProperty; },
        set : function(value) {
            console.log("Message from obj itself.");
            this._myProperty = value;
        },
        configurable : true
    }
});

obj.myProperty = 10; // outputs "Message from obj itself";

notifyOfChange(obj, "myProperty");

obj.myProperty = 10; // outputs "Message from notifyOfChange."

Is there a way to detect the myProperty anonymous setter so that I can call it in notifyOfChange?

Note: I'd like to make notifyOfChange work with any object, so just using a named function for the myProperty setter doesn't work.

kpozin · Accepted Answer · 2012-04-10T21:02:12.207

function notifyOfPropertyChange(obj, propertyName) {

    var desc = Object.getOwnPropertyDescriptor(obj, propertyName);

    if ("value" in desc && desc.writable) {
        // Value property

        var propertyValue = obj[propertyName];

        Object.defineProperty(obj, propertyName, {
            get: function () { return propertyValue; },
            set: function (newValue) {
                propertyValue = newValue;
                console.log("Message from notifyOfChange.");
            },
            configurable: true,
            enumerable: desc.enumerable
        });

    } else if (desc.set) {
        // Getter/setter property

        // Redefine the setter, keep the getter if it exists
        var originalSet = desc.set.bind(obj);
        desc.set = function (newValue) {
            originalSet(newValue);
            console.log("Message from notifyOfChange.");
        };
        Object.defineProperty(obj, propertyName, desc);
    }
}

You can use Object.getOwnPropertyNames to get the full list of property names to modify.

See MDN for an explanation of Object.getOwnPropertyDescriptor.

Nice, but why do you use `desc.writable` in the if-condition? — Bergi, Apr 10 '12 at 20:51
If it's not a writable property to begin with, we probably don't want to make it writable by giving it a setter. — kpozin, Apr 10 '12 at 20:59
OK, I would have logged the "notify" message before throwing the error :-) — Bergi, Apr 10 '12 at 21:27

Accessing get/set methods for JavaScript property created using Object.create

1 Answers1