Question

With a single property this is fairly easy:

var jsonobj = {
    "test": "ok"
}
var propname = "test";
// Will alert "ok"
alert(jsonobj[propname]);

But what I want to do is use a nested property:

var jsonobj = {
    "test": {
        "test2": "ok"
    }
}
var propname = "test.test2";
// Alerts undefined
alert(jsonobj[propname]);

Is there any way of selecting a nested “dynamic” property?
I know I can do jsonobj.test.test2, but the problem is that propname can change to a property that goes 1,2 or 3 levels deep. (e.g test, test.test2, …)

Answer 1

function resolve(cur, ns) {

    var undef;

    ns = ns.split('.');

    while (cur && ns[0])
        cur = cur[ns.shift()] || undef;

    return cur;

}

E.g.

// 1:
resolve({
    foo: { bar: 123 }
}, 'foo.bar'); // => 123


// 2:
var complex = {
    a: {
        b: [
            document.createElement('div')
        ]
    }
};

resolve(complex, 'a.b.0.nodeName'); // => DIV

The benefit in using this is that it won’t throw an error if you try accessing something that doesn’t exist — it’ll gracefully return undefined.

---

EDIT:

In the comment, Andy mentioned that this doesn’t throw errors where one might expect it to. I agree that getting undefined is a little bit generic and there is no way to tell whether your value was really resolved. So, to remedy that, try this:

var resolve = (function(){

    var UNRESOLVED = resolve.UNRESOLVED = {};
    return resolve;

    function resolve(cur, ns) {

        var undef;

        ns = ns.split('.');

        while (cur && ns[0])
            cur = cur[ns.shift()] || undef;

        if (cur === undef || ns[0]) {
            return UNRESOLVED;
        }

        return cur;

    }

}());

It’ll return an UNRESOLVED object that can be checked like so:

var result = resolve(someObject, 'a.b.c');

if (result === resolve.UNRESOLVED) {...}

It’s not perfect, but it is (IMO) the best way to determine an unresolved namespace without having to throw errors. If you want errors, then just go ahead with:

someObject.a.b.c; //...