I am messing around with some "classical" inheritance and I'm running into an issue. I am using Object.defineProperty() to add properties to my LivingThing "class". I want to have a default value, along with a property getter/setter.

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I am running into the following error:

Uncaught TypeError: Invalid property.  A property cannot both have accessors and be writable or have a value, #<Object>

Why am I getting this error and what would be the best approach to have a default value and a getter/setter for a property, using Object.defineProperty()?

I am messing around with some "classical" inheritance and I'm running into an issue. I am using Object.defineProperty() to add properties to my LivingThing "class". I want to have a default value, along with a property getter/setter.

http://jsfiddle/fmpeyton/329ntgcL/

I am running into the following error:

Uncaught TypeError: Invalid property.  A property cannot both have accessors and be writable or have a value, #<Object>

Why am I getting this error and what would be the best approach to have a default value and a getter/setter for a property, using Object.defineProperty()?

• javascript

• Depends. Are you using the setter to do anything special, or is the default value all the behavior you want? – Touffy Commented Sep 25, 2015 at 20:12
• The setter is just used to replace the old property value with a new value. I believe this is the default behavior, though I'm not sure. – Fillip Peyton Commented Sep 25, 2015 at 20:15
• OK so you don't actually need a setter and getter ;) – Touffy Commented Sep 25, 2015 at 20:16

3 Answers 3

Use a function scoped variable to back the defined property and set that variable's initial value to the default:

function LivingThing(){
    self = this;
    var isAlive = true;

    Object.defineProperty(self, 'isAlive', {
        get: function(){
            return isAlive;
        },
        set: function(newValue){
            isAlive = newValue;
        },
        configurable: true
    });

    self.kill = function(){
        self.isAlive = false;
    };
}

http://jsfiddle/329ntgcL/5/

writable isn't necessary because you have a setter. That's what's causing your error. You can either have value/writable (data descriptor) OR get/set (accessor descriptor).

As a result, when you call var l = new LivingThing, l.isAlive == true and after you call l.kill(), l.isAlive == false

Your getter/setter pair presents a puted property, you can (and should) back that up with a real property if you need storage:

self._isAlive = true;
Object.defineProperty(self, 'isAlive', {
    get: function(){
        return this._isAlive;
    },
    set: function(newValue){
        this._isAlive = newValue;
    },
    writable: true,
    configurable: true
});

Presumably, put some logic there to justify a setter/getter vs a regular property. Setters are for wrapping property access.

Without a setter and getter, you can define a default value by just assigning it to the prototype:

LivingThing.prototype.isAlive = true

var o = new LivingThing()

console.log(o.isAlive) // true

Just to be clear, changing that property on an instance of LivingThing will create a property for that instance, not change the value in its __proto__.

o.isAlive = false

console.log(new LivingThing().isAlive) // still true