VSCode intellisense not working for namepsaced JavaScript

Question

I open VSCode and create test.js and write something like:

var test = (function () {
    return {
        foo: function () {
            //foo
        },
        bar: function () {
            //foo
        }
    };
})();

Start typing test and I can see suggested for me via intellisense foo and bar, and I F12 on either it takes me to the definition.

Now if I introduce a namespace, like so:

var test = test || {};
test.rob = (function () {
    return {
        foo: function () {
            //foo
        },
        bar: function () {
            //foo
        }
    };
})();

Intellisense 'loses' the definition, doesn't highlight foo and bar and won't go to definition when using F12.

Any idea why? Have I defined my JavaScript incorrectly perhaps?