Question regarding date function in javascript

Question

I'm making a function for recalculating a time based on the current time. timeShowToday is a specific time everyday (set at 8:00 pm) to reveal some blinded answer.

_checkTime = () => {
    let timeNow = new Date();
    let timeShowToday = new Date(   //set at 8:00pm everyday
      timeNow.getFullYear(),
      timeNow.getMonth(),
      timeNow.getDate(),
      20,
      0
    );

    let timeShowYesterday = timeShowToday.setDate(timeShowToday.getDate() - 1);
    let timeDiff = timeShowToday.getTime() - timeNow.getTime();

    if (timeDiff < 0) {  //recalculate if current Time is past specific time(8:00pm) 
      let temp = new Date(
        timeNow.getFullYear(),
        timeNow.getMonth(),
        timeNow.getDate(),
        20,
        0
      );
      console.log(temp);
      timeShowYesterday = temp;
      timeShowToday = timeShowYesterday.setDate(
        timeShowYesterday.getDate() + 1
      );
      console.log(timeShowYesterday);
    }

The problem here is that variable temp and timeShowYesterday has different datetime eventhough I've just assigned temp to timeShowYesterday. This is the log I get when I console log it:

05-03 00:26:59.623 ReactNativeJS: temp: Fri May 03 2019 20:00:00 GMT+0900
05-03 00:26:59.623 ReactNativeJS: timeShowYesterday: Sat May 04 2019 20:00:00 GMT+0900

As you can see, temp logs the current time correctly but timeShowYesterday has +1 day. I have no idea why this is the case, because all I did was just assign temp to timeShowYesterday.

am I missing something? Thank you

The calls to `.setDate()` *modify* the date you're operating on — they do not create a new Date instance. — Pointy, May 02 '19 at 15:38
I see you are adding +1 `timeShowToday = timeShowYesterday.setDate( timeShowYesterday.getDate() + 1 );` — F.bernal, May 02 '19 at 15:38
you are logging `temp` here `console.log(temp);` before you add one day to `timeShowYesterday`, so it shows previous value (because the log converts the object to a string instead of showing the actual object's reference) — Kaddath, May 02 '19 at 15:38
@Pointy right, I thought it creates a new date instance. That is the mistake! Thank you very much guys for the quick help!!! — Eric Ahn, May 02 '19 at 15:41
@JonasWilms i created it just to show that there is the difference between the two. I thought timeShowYesterday.setDate would create new instance but it modified it — Eric Ahn, May 02 '19 at 15:42

F.bernal · Answer 1 · 2019-05-02T15:50:15.413

Be aware about setDate modifies your reference no creates a new one.

    let timeNow = new Date();
    let timeShowToday = new Date(   //set at 8:00pm everyday
      timeNow.getFullYear(),
      timeNow.getMonth(),
      timeNow.getDate(),
      20,
      0
    );        
      
    let timeShowYesterday = new Date(timeShowToday);
    timeShowYesterday.setDate(timeShowYesterday.getDate() - 1);
    
    
    console.log('Yesterday', timeShowYesterday);
    console.log('Today', timeShowToday);
    
    let timeDiff = timeShowToday.getTime() - timeNow.getTime();

    if (timeDiff < 0) {  //recalculate if current Time is past specific time(8:00pm) 
      const timeShowYesterday = new Date(
        timeNow.getFullYear(),
        timeNow.getMonth(),
        timeNow.getDate(),
        20,
        0
      );
      
      timeShowToday = new Date(timeShowYesterday);
      timeShowToday.setDate(timeShowToday.getDate() + 1);
      
       console.log('Yesterday', timeShowYesterday);
       console.log('Today', timeShowToday);
    }

I updated

let timeShowYesterday = new Date(timeShowToday); timeShowYesterday.setDate(timeShowYesterday.getDate() - 1);

and

timeShowToday = new Date(timeShowYesterday); timeShowToday.setDate(timeShowToday.getDate() + 1);

To avoid overwriting your refence, just create a new date and update it.

Downvotes can be removed. I don't have the time to recheck wether a bad answer was edited, thats why I comment, move on and check back if I get notified. — Jonas Wilms, May 02 '19 at 15:56

score 2 · Accepted Answer · answered May 02 '19 at 15:54

2

You seem to assume that setDate creates a new date without changing the original date. It does not. Instead it just modifies the date object and returns a self reference.

The usage of that is that this:

  date.setDate(10);
  date.setMinutes(10);

can be written more elegantly as:

  date.setDate(10).setMinutes(10);

If you want to create two independent dates, you have to copy the date:

  const copy = new Date(oldDate);

answered May 02 '19 at 15:54

Jonas Wilms

132,000
20
149
151

Thanks! I've fixed by code, but I've got another problem. Could you have a look at the update please? – Eric Ahn May 02 '19 at 16:28
Don't edit the question into another question. You should copy the date before you change anything. – Jonas Wilms May 02 '19 at 16:32
Oh okay, though I did copy the date and make another variable. I'll try again. Thanks ! – Eric Ahn May 02 '19 at 16:34

Eric Ahn · Answer 3 · 2019-05-02T15:52:40.297

0

Found the problem!

I thought the setDate

 timeShowToday = timeShowYesterday.setDate(
        timeShowYesterday.getDate() + 1
      );

would create a new instance from timeShowYesterday without affecting it and assign it to timeShowToday but actually modified it.

So +1 was the problem.

edited May 02 '19 at 15:52

answered May 02 '19 at 15:44

Eric Ahn

391
2
7
18

You can answer your own questions with the solution you found somewhere else. – Sebastian Speitel May 02 '19 at 15:48
@SebastianSpeitel Right. Thought I could only upvote someone else's answer. Edited it! – Eric Ahn May 02 '19 at 15:53

Question regarding date function in javascript

3 Answers3