Utilizing String and Date Functions

We can find the day difference between two dates in JavaScript through built-in string and date functions. For example, consider the following code:

const parseDate = (str) => {

const [month, day, year] = str.split('/');

return new Date(year, month - 1, day);

}

const datediff = (first, second) => {

return Math.round((second - first) / (1000 * 60 * 60 * 24));

}

const diff = datediff(parseDate("1/1/2000"), parseDate("1/1/2001"));

console.log(diff);

The parseDate function accepts a date string formatted as MM/DD/YYYY. It splits the string by '/' and uses destructuring to extract the year, month, and day. This information is then passed to the Date constructor, creating a date object.

Note that the month is decremented by 1 because JavaScript counts months starting from zero. To find the difference, we use the datediff function to subtract the first date from the second, converting both dates to timestamps automatically. The result is then divided by the number of milliseconds in one day, and we round it using Math.round.

Upon running this code, we find that diff equals 366.

Using moment.js for Simplicity

Alternatively, the moment.js library simplifies the process of calculating the difference between two dates. For example:

const start = moment("2000-11-03");

const end = moment("2001-11-04");

const diff = end.diff(start, "days");

console.log(diff);

By passing date strings into the moment function, we can easily calculate the difference. The diff method compares the current moment date with the passed moment date, and the second argument specifies the unit of measurement for the difference. In this case, diff also equals 366, confirming that the two dates are 366 days apart.