How to check if Date is valid and make it fast in JavaScript

February 01, 2024


This code sample and measuring of speed happened after one code review I was given and I wanted to share it since I got really surprised by coercion speed in JS and how some obvious choices happen to be slower than expected.

The problem

My problem was to take string with date and time in en-US format and check if this string is parsable to date object in JS.

Code I wrote first

isNaN(new Date('2022-01-01'));

was my obvious choice once I investigated some articles about how to check if string is valid Date.

However the code was written in TypeScript and only type that is expected in


is number so I was searching for a way how to (maybe) make one small step and please TypeScript and have everything IMO still readable.

I was reading through what happens when I call


and everywhere I looked this should happen “automagically”

isNaN(new Date('2022-01-01').valueOf());

so why not call it directly then. TypeScript is happy, it doesn’t hurt readability (that much), but let’s wait what tests will say.

Making it faster

Then I discussed with other colleagues and somehow (shame on me) I forgot about this guy


OK so now we have 3 ways of checking, all work, but which one is faster? I suppose no one expected the results but here they are.


Link to test in your browser

My results in latest browsers :

FireFox 122 results

Chrome 121 results

I surely would expect Date.parse to be fastest but I would really not guesses that there is some difference calling isNaN with just Date vs Date.valueOf(). Maybe someone can explain (drop me an email pls).

NodeJS results

Small snippet for NodeJS here :

const iterations = 1000000;

const date = '2022-01-01';

// Method 1: isNaN(Date.parse(date))
let start =;
for (let i = 0; i < iterations; i++) {
let end =;
console.log(`Method 1 took ${end - start} milliseconds.`);

// Method 2: isNaN(new Date(date).valueOf())
start =;
for (let i = 0; i < iterations; i++) {
  isNaN(new Date(date).valueOf());
end =;
console.log(`Method 2 took ${end - start} milliseconds.`);

// Method 3: isNaN(new Date(date))
start =;
for (let i = 0; i < iterations; i++) {
  isNaN(new Date(date));
end =;
console.log(`Method 3 took ${end - start} milliseconds.`);

with NodeJS 20.10.0 I see results like this :

NodeJS 20 results

And the bonus from Copilot

What Copilot has to say

Hope this helps.

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Written by Dušan Roštár - the "mr edge case" guy
my twitter : rostacik, my linkedin : rostar, drop me an email : here