javascript - Getter/setter in constructor

Question

I recently read about the fact that there is a possibility of defining getters/setters in JavaScript. It seems extremely helpful - the setter is a kind of 'helper' which can parse the value to be set first, before actually setting it.

For example, I currently have this code:

var obj = function(value) {
    var test = !!value; // 'test' has to be a boolean
    return {
        get test() { return test },
        set test(value) { test = !!value }
    };
};

var instance = new obj(true);

This code always converts value to a boolean. So if you code instance.test = 0, then instance.test === false.

However, for this to work you have to actually return an object, which means that the new instance is not of type obj but just is a plain object. This means that changing the prototype of obj has no effect on instances. For example, this does not work - instance.func is undefined:

obj.prototype.func = function() { console.log(this.value); };

because instance is not of type obj. To get the prototype functions work, I guess I should not return a plain object, but rather not return anything so that instance would just be of type obj, like a regular constructor works.

The problem then is how to implement getters/setters? I can only find articles describing how to add these to an object, not as being part of the constructor of a custom type.

So how do I implement getters/setters in the constructor so as to be able to both use getters/setters and extending the prototype?

question from:https://stackoverflow.com/questions/5222209/getter-setter-in-constructor

Answer 1

You can't do that.

You can set setter/getters for properties of objects though. I advice you use ES5 Object.defineProperties though. of course this only works in modern browsers.

var obj = function() {
    ...
    Object.defineProperties(this, {
        "test": {
             "get": function() { ... },
             "set": function() { ... }
        }
    });
}

obj.prototype.func = function() { ... }

var o = new obj;
o.test;
o.func();