JavaScript Date Comparisons Don't Equal [duplicate]

Question

I've tried searching for people with similar questions, but haven't found anything.

I have two dates in JavaScript, both set to the same value... Equality Testing fails on ==, but >= and <= evaluate true.

Below is the code I have in play:

var startDate = new Date( 2011, 7, 30, 0, 0, 0, 0 ); var dt = new Date( 2011, 7, 30, 0, 0, 0, 0 ); if( startDate == dt ) document.write('They Equal<br />'); if( startDate > dt ) document.write('Start Date is > dt<br />'); if( startDate >= dt ) document.write('Start Date is >= dt<br />'); if( startDate < dt ) document.write('Start Date is < dt<br />'); if( startDate <= dt ) document.write('Start Date is <= dt<br />'); if( dt == startDate ) document.write('They Equal<br />'); if( dt > startDate ) document.write('dt > startDate<br />'); if( dt >= startDate ) document.write('dt >= Start Date <br />'); if( dt < startDate ) document.write('dt < Start Date <br />'); if( dt <= startDate ) document.write('dt <= Start Date <br />'); document.write( dt ); document.write( '<br />'); document.write( startDate );

Has anyone encountered anything like this, or am I doing something fundamentally wrong?

I tested this is Internet Explorer (9), Firefox 5+, and Chrome.

Update:

So two people posted great answers to my problem, and I thank both of you: xdazz and DaveRandom. I had read an earlier post on stackoverflow.com on a similar question and a guy said that date objects could be compared like any others, and any other example I found always did a < or > type of compare, never a full equality so I wasn't able to make the connection as to why I was doing it wrong.

Thanks to you two, and the others that posted similar answers.

solo · Accepted Answer · 2015-12-16 14:50:03Z

When you use <= or >= to compare two date objects, they are compared via valueOf, which is the same as getTime for Date.

But when you use ==, they are two different objects of the same type, so it returns false.

Added some examples:

> new Date(2011, 7, 30, 0, 0, 0, 0) == new Date( 2011, 7, 30, 0, 0, 0, 0 ) false > new Date(2011, 7, 30, 0, 0, 0, 0).getTime() == new Date( 2011, 7, 30, 0, 0, 0, 0).getTime() true > new Date(2011, 7, 30, 0, 0, 0, 0).valueOf() == new Date( 2011, 7, 30, 0, 0, 0, 0).valueOf() true > new Date(2011, 7, 30, 0, 0, 0, 0).valueOf() == new Date( 2011, 7, 30, 0, 0, 0, 0).getTime() true

DaveRandom · Accepted Answer · 2012-08-01 11:52:48Z

I think if you do

var startDate = (new Date( 2011, 7, 30, 0, 0, 0, 0 )).getTime(); var dt = (new Date( 2011, 7, 30, 0, 0, 0, 0 )).getTime();

At the top of the script you will find it works.

The getTime() method returns the date as an integer, what you are doing there is comparing objects, rather than specific values.

EDIT Fixed above code

@Shef Only by about 50 seconds, and I'll give you a +1 for your troubles ;-)

6502 · Accepted Answer · 2018-10-31 17:26:30Z

This is one of the many illogical parts of Javascript. You can work around this problem by converting the date to a number using .getTime() that will return how many milliseconds passed from that date and 00:00:00 of Jan 1, 1970.

if (a.getTime() === b.getTime()) ...

The === operator on the objects themselves will return always false because those two objects are indeed distinct mutable javascript objects.

PS: Don't use == operator with Javascript. Ever. What it does takes the term "crazy" to a whole new level. Seriously. The problem is that applies all sort of type conversions that simply make no sense and for example you end up with things like "1"==1, x==false && (x?1:2)==1 or even a==b && b==c && a!=c being true (e.g. with x=[] or with a=[1], b=1, c=[[1]]). Just don't use ==. Ever. It's "FUBAR". Fullstop.

I mostly agree with the postscript, but it should be supported with something far more meaningful, like this: 2ality.com/2011/12/strict-equality-exemptions.html
@MichaelScheper: I agree that that article is a nice analysis of many allegedly valid use cases, but I think it's important to also point out why it can be very dangerous (i.e. that == is not transitive, or that you can have x == false but x being truthy).

Shef · Accepted Answer · 2011-08-30 14:09:50Z

var startDate = new Date( 2011, 7, 30, 0, 0, 0, 0 ); var dt = new Date( 2011, 7, 30, 0, 0, 0, 0 ); if( startDate.getTime() == dt.getTime() ) console.log('They Equal<br />');

user3202283 · Accepted Answer · 2014-01-16 11:26:12Z

<Script> function checkval() { var strfromdate=document.frmadmin.txtfromdate.value; strfromdate=strfromdate.split("/"); //alert(strfromdate[2]+" "+strfromdate[0]+" "+strfromdate[1]); var strtodate=document.frmadmin.txttodate.value; strtodate=strtodate.split("/"); var fromDate = (new Date( strfromdate[2], strfromdate[0], strfromdate[1], 0, 0, 0, 0 )).getTime(); var toDate = (new Date( strtodate[2], strtodate[0], strtodate[1], 0, 0, 0, 0 )).getTime(); //alert(fromDate+" "+toDate); if(toDate<fromDate) { alert("To date should be greater than from date."); document.frmadmin.txttodate.focus(); return false; } return true; } </Script>

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