Javascript - How to pass a nested dictionary key through a function?

Question

If I have a dictionary that looks like this:

const test = {
    "auth": {
        "username": "UserABC",
        "password": "abc123!"
    }
}

to access the username I would simply use test.auth.username to get "UserABC".

My question is how can I, if it's at all possible, pass 'auth.username' as a function to a variable. For example, if the variable test is part of a class and I only want to use getters and setters to alter any key/value pairs, how can I write a getter and setter to reference a nested dictionary object?

If I do something like

function getValue(key) {
    return test[key]
}

then I can get the username by calling getValue('auth')['username'], but this is a bit ugly for dictionaries that have more that just two levels. Also, this wouldn't solve my setter function problem.

How could I do something along the lines of getValue('auth.username');?