Erkin Ünlü

Software Engineer

Sorting and validating string dates in Javascript

Posted on April 17, 2022


We have to handle dates as strings every day. The HTTP protocol doesn’t have any idea of a type, so we generally convey data in a json format and use strings for dates (it’s also recommended that we keep dates in ISO 8601 format:

This creates two problems, picking out the valid ones from an array of strings and sorting them. I’ll start with the validation of string dates.

Validating dates

So, one of the easiest way to validate string dates is to convert them to Date objects and check their return value from getTime() function call. An invalid date object will return the NaN object for getTime. And the NaN object is not equal to itself for some of the reasons outlined here:

So this information above could be utilised as follows:

const dates = ['foo', '2022/04/17'] const validDates = dates.filter((d) => { const rd = new Date(d) return rd.getTime() === rd.getTime() }) console.log(validDates) // will print [ '2022/04/17' ]

Tada 🎉!

Sorting dates

A very basic and nice way of sorting dates is literally to turn your strings into dates, and then subtract them to get a value that is either negative, positive, or zero.

const dates = ['2022/04/17', '1986/03/30'] const sorted = dates.sort((d1, d2) => { return new Date(d1) - new Date(d2) }) console.log(sorted) // will print [ '1986/03/30', '2022/04/17' ]

Hopefully this post would help anyone who is looking for a clean solution to these two generic and frequent problems we face every other day!

PS: It’s amazing how the sort function interface is this lean!