How to calculate the number of days between 2 dates in JavaScript

I had this problem: how do I calculate the number of days between 2 dates?

In particular, I wanted to count the number of nights that a person had to pay to rent a house and sleep in it, depending on the checkin date, and the checkout date.

I looked at different solutions, and the one that gave me the least problems, considering all the issues with dates (including DST), was this: starting from the starting date, we add one day until the date represents a date after the end date.

Here’s the code:

const numberOfNightsBetweenDates = (startDate, endDate) => {
  const start = new Date(startDate) //clone
  const end = new Date(endDate) //clone
  let dayCount = 0

  while (end > start) {
    dayCount++
    start.setDate(start.getDate() + 1)
  }

  return dayCount
}

I first clone the dates we are given, because dates are objects, and we get a reference to that object. This means that using setDate() in the function would also affect the variable outside of this function - not something we look forward to!

That’s it.

If instead you want to get the number of days between 2 dates (say, today to tomorrow is 2 days), just change while (end > start) to while (end >= start). That would work. Or increase the dayCount starting point to 1.