关于JavaScript中的日期函数的问题

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

Answer 1

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.

Answer 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);

Answer 3

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.