How do I get the number of days between two dates in JavaScript

let today = new Date().toISOString().slice(0, 10)

const startDate  = '2021-04-15';
const endDate    = today;

const diffInMs   = new Date(endDate) - new Date(startDate)
const diffInDays = diffInMs / (1000 * 60 * 60 * 24);


alert( diffInDays  );