How does JS date comparison work?

Question

Let\'s assume I have a proper Date object constructed from the string: \"Tue Jan 12 21:33:28 +0000 2010\".

var dateString = \"Tue          


        

Answer 1

I think yes. Using if (now < twitterDate), it evaluates to if (now.valueOf()<twitterDate.valueOf()). valueOf() delivers the number of milliseconds passed since 01/01/1970 00:00:00, so the comparison of those 2 numbers is valid.

check it like this

var then = new Date("Tue Jan 12 21:33:28 +0000 2010")
   ,now  = new Date;

console.log(then.valueOf(),'::',now.valueOf(),'::',now<then);
  //=> 1263332008000 :: 1352365105901 :: false

Answer 2

Relational operations on objects in ECMAScript rely on the internal ToPrimitive function (with hint number) that you can access, when it is defined, using valueOf.

Try

var val = new Date().valueOf();

You'll get the internal value of the date which is, as in many languages, the number of milliseconds since midnight Jan 1, 1970 UTC (the same that you would get using getTime()).

This means that you're, by design, ensured to always have the date comparison correctly working.

This article will give you more details about toPrimitive (but nothing relative to comparison).

Answer 3

Date values in Javascript are numbers, as stated in the ECMA Script specification. So the Date values are compared as numbers.

This is a demo of your code (I set twitterDate in the future).

(function(){
    var dateString = "Tue Jan 12 21:33:28 +0000 2014";
    var twitterDate = new Date(dateString);

    var now = new Date();
    if (now < twitterDate) {
         document.write('twitterDate is in the future');
    }
    else
    {
        document.write('twitterDate is NOT in the future');
    }

})()​