Javascript Setter Being Bypassed

Question

Say I have this code:

function test() {
    this._units = {};
}
test.prototype = {
    get units() { return this._units; },
    set units(val) {
        this._units = val;

        for (var unit in this._units) {
            if (this._units[unit] === 0)
                delete this._units[unit];
        }
    }
};

Now, I can assign a unit via the following:

x = new test();
x.units['foo'] = 1;

This all works. However, when I do this,

x.units['foo'] = 0;
// x.units = { 'foo' : 0 }

It doesn't remove foo from the units as it should. How can I change this?

Thanks!

The first issue is that the setter won't be called (you're modifying a property on the object returned by the getter, not actually calling the setter). — Qantas 94 Heavy, Jun 04 '14 at 10:27
Put the delete in the getter, then when you try to see `x.units` next, you'll see the emptied version. The other way would be to do `x.units = x.units` after the `x.units['foo'] = 1;` — Paul S., Jun 04 '14 at 10:34
Putting the delete part in the getter did not work. I tend to do stuff like `x.units['foo']++` — Jacob, Jun 04 '14 at 10:37

score 1 · Answer 1 · edited May 23 '17 at 11:49

You cannot intercept when a property is created, you'd need a Proxy for that. Unfortunately, it is only a harmony draft and currently only supported in Firefox' Javascript 1.8.5.

Your setter only detects an assignment to x.units = {foo:1}, but not to x.units.foo. You could create setters for every property on the units objects that you know of, but you need an extra method to make them known in the first place (assuming you're not only assigning objects to x.units).

However, you might better not do this at all. A property that deletes itself when being set to 0 is very counterintuitive. Think of

 x.units.foo = -1; x.units.foo += 2;

Would you expect this to do the same as

 x.units.foo = -1; x.units.foo++; x.units.foo++;

? The second would yield NaN instead of 1.

Javascript Setter Being Bypassed

1 Answers1