Detecting an "invalid date" Date instance in JavaScript

Question

I'd like to tell the difference between valid and invalid date objects in JS, but couldn't figure out how:

var d = new Date("foo"); console.log(d.toString()); // shows 'Invalid Date' console.log(typeof d); // shows 'object' console.log(d instanceof Date); // shows 'true' 

Any ideas for writing an isValidDate function?

• Ash recommended Date.parse for parsing date strings, which gives an authoritative way to check if the date string is valid.
• What I would prefer, if possible, is have my API accept a Date instance and to be able to check/assert whether it's valid or not. Borgar's solution does that, but I need to test it across browsers. I also wonder whether there's a more elegant way.
• Ash made me consider not having my API accept Date instances at all, this would be easiest to validate.
• Borgar suggested testing for a Date instance, and then testing for the Date's time value. If the date is invalid, the time value is NaN. I checked with ECMA-262 and this behavior is in the standard, which is exactly what I'm looking for.

Answer 1

Here's how I would do it:

if (Object.prototype.toString.call(d) === "[object Date]") {   // it is a date   if (isNaN(d)) { // d.getTime() or d.valueOf() will also work     // date object is not valid   } else {     // date object is valid   } } else {   // not a date object } 

Update [2018-05-31]: If you are not concerned with Date objects from other JS contexts (external windows, frames, or iframes), this simpler form may be preferred:

function isValidDate(d) {   return d instanceof Date && !isNaN(d); } 

Update [2021-02-01]: Please note that there is a fundamental difference between "invalid dates" (2013-13-32) and "invalid date objects" (new Date('foo')). This answer does not deal with validating date input, only if a Date instance is valid.