add days to date in javascript

AddingDates.js

Date.prototype.addDays = function (days) {
    return new Date(this.valueOf() + days * 864e5);
}

var date = new Date("2019-10-01");
date.addDays(50);


// Or

var someDate = new Date("2019-10-01");
var numberOfDaysToAdd = 50;
someDate.setDate(someDate.getDate() + numberOfDaysToAdd);