Compare JavaScript Date objects

Question

When comparing date objects in JavaScript, I found that it doesn't return true even when comparing identical dates.

var startDate1 = new Date("02/10/2012");
 var startDate2 = new Date("01/10/2012");
 var startDate3 = new Date("01/10/2012");
 alert(startDate1>startDate2); // true
 alert(startDate2==startDate3); //false

How do I compare these dates for equality? I want to use a native JavaScript Date object instead of any third-party library because it is not appropriate to use a third-party JavaScript library just to compare dates.

Answer 1

Use the getTime() method to compare dates, which returns the number of milliseconds since the epoch (i.e. a number) for comparison:

var startDate1 = new Date("02/10/2012");
var startDate2 = new Date("01/10/2012");
var startDate3 = new Date("01/10/2012");
alert(startDate1.getTime() > startDate2.getTime()); // true
alert(startDate2.getTime() == startDate3.getTime()); //true

Also, consider constructing Date objects using explicit year/month/day numbers rather than relying on string representations (see: Date.parse()). And remember, dates in JavaScript are always represented using the client's (browser's) time zone.

Answer 2

This is because in the second case, the actual date objects are compared and the two objects are never equal. Cast them to numbers:

alert( +startDate2 == +startDate3 ); // true

If you want to convert it to a number more explicitly, you can use one of the following methods:

alert( startDate2.getTime() == startDate3.getTime() ); // true

oor

alert( Number(startDate2) == Number(startDate3) ); // true

is a reference to the specification §11.9.3 Abstract Equality Comparison Algorithm, basically it says that when comparing objects, it is true only if obj1 == obj2 refers to the same object, otherwise the result is false.