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?

EDIT - thanks for the responses!

• 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.

EDIT 2

• 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 (section 15.9.5.9) and this behavior is in the standard, which is exactly what I'm looking for.

My final function, based on Borgar's solution:

function isValidDate(d) {
  if ( Object.prototype.toString.call(d) !== "[object Date]" )
    return false;
  return !isNaN(d.getTime());
}

Answer 1

Instead of using "new Date()" you should use:

var timestamp=Date.parse('foo')

if (isNaN(timestamp)==false)
{
    var d=new Date(timestamp);

}

Date.parse() returns a timestamp, an integer representing the number of milliseconds since 01/Jan/1970. It will return NaN if it cannot parse the supplied date string.

Answer 2

Here's how I would do it:

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

Answer 3

You can check the validity of a Date object d via

d instanceof Date && isFinite(d)

To avoid cross-frame issues, one could replace the instanceof check with

Object.prototype.toString.call(d) === '[object Date]'

A call to getTime() as in Borgar's answer is unnecessary as isNaN() and isFinite() both implicitly convert to number.