Adding days to a date using Vanilla JS

This is a short post about how we can manage a common situation with our propper implementation.

Despite the JavaScript Date warts, it’s straightforward to add days to a date in JavaScript.

Today we have a ton of libraries and external modules to work with dates. It's overwhelming!

While it would be very easy to reach for moment.js or another date manipulation library (date-fns, luxon, dayjs) to do something as simple as adding days to a Date in JavaScript, writing a short helper function might just be easier.

A utility function that creates a Date copy

function addDays(date, days) {
  const copy = new Date(Number(date))
  copy.setDate(date.getDate() + days)
  return copy

const date = new Date();
const newDate = addDays(date, 10);

Mutating the Date directly

const date = new Date();
date.setDate(date.getDate() + 10);

Gotchas and examples

This actually works as expected, eg. the month rolls over.

const d = new Date('2019-04-14');

const monthRollsOver = addDays(myDate, 31);
// 2019-05-15

Of course, if you’re already using a data manipulation library elsewhere in your code, you should leverage that since you’re already paying the download/parse cost.