You don't have to immediately chain a promise, so just use the if/else to set a promise variable, and then set up a chain on that.

gistfile1.txt

Setlist.count({}, (err, count) => {
	let promise;
	// if db is empty, create a setlist
	if (count === 0) {
		promise = Setlist.create({
			tracks: [req.body.track]
		});
	} else {
		// add track to setlist
		promise = Setlist.findOneAndUpdate({}, 
			{$push: {tracks: req.body.track}},
			{new: true}
		);
	}
	promise.then(setlist => {
		res.status(201).json(setlist);
	}).catch(err => {
		console.error(err);
		res.status(500).json({message: 'Internal server error'});
	});
});