Add days to a Date in vanilla JavaScript

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

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.

Add days to a new instance of JavaScript Date

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);

Update a JavaScript Date with added days

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

Add a day to a JavaScript Date

const date = new Date();
date.setDate(date.getDate() + 1);

Add a week to a JavaScript Date

const date = new Date();
date.setDate(date.getDate() + 7);

Gotchas and examples: months in JavaScript dates roll over

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

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

const monthRollsOver = addDays(myDate, 31);
console.log(monthsRollOver)
// 2019-05-15

Find the live examples at: observablehq.com/@hugodf/add-days-to-a-javascript-date

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

Wim van 't Einde