getUserMedia - what if user makes no choice?

I've adapted a library to record MP3 audio via the user's microphone. If the user allows or rejects the microphone access request, I'm fine, but I note that MDN says this:

Note that it is possible for the returned promise to neither resolve nor reject, as the user is not required to make a choice.

But it doesn't seem to say what, if anything, I can do catch that "no choice made" action. If the user merely quits the dialog, or blurs out of it without making a choice, can I catch that and adjust my UI accordingly?

Here's my current code:

navigator.mediaDevices.getUserMedia({audio: true}).then(function(stream) {
    build_ui();
    startUserMedia(stream);
}).catch(function(e) { //<-- doesn't fire if no choice made
    cfg.no_device_callback && cfg.no_device_callback(e);
});

You could implement a timeout on the promise you have.

For instance, you could extend the Promise object and prototype as follows:

Promise.wait = function (ms) {
    return new Promise(function (resolve) {
        setTimeout(resolve, ms);
    });
};

Promise.prototype.timeout = function(ms) {
    return Promise.race([
        this, 
        Promise.wait(ms).then(function () {
            throw new Error("time out");
        })
    ])
};

Once you have that, you can just chain in a .timeout(10000):

navigator.mediaDevices.getUserMedia({audio: true})
         .timeout(10000).then(function(stream) {
//       ^^^^^^^^^^^^^^^
    build_ui();
    startUserMedia(stream);
}).catch(function(e) { //<-- now also fires if no choice made within 10 secs
    cfg.no_device_callback && cfg.no_device_callback(e);
});