count(callback) {
		this.find(emptyItemQuery, data => {
			const total = data.length;

			let i = total;
			let completed = 0;

			while (i--) {
				completed += data[i].completed;
			}
			callback(total, total - completed, completed);
		});
	}
Count total, active, and completed todos.

@param {function(number, number, number)} callback Called when the count is completed