JavaScript日期对象-奇怪的行为

Question

I am new to JavaScript and attempting to convert some server-side code to it using NodeJS. A snippet is below; in general it takes a date and subtracts 2 weeks from it.

Can anyone explain why the below happens?

var currentDate = new Date('05/12/2014 14:32');
currentDate.setDate(currentDate.getDate() - 14);
console.log(typeof currentDate);
console.log(currentDate);

Output:

object
Mon Apr 28 2014 14:32:00 GMT-0500 (CDT)

Now, all is well. I've managed to subtract 14 days off of my date. However, it really isn't the 'currentDate' anymore so I want to assign it to say, 'futureDate'

var currentDate = new Date('05/12/2014 14:32');
var futureDate = currentDate.setDate(currentDate.getDate() - 14);
console.log(typeof futureDate);
console.log(futureDate);

Output:

number
1398713520000

Ick. That's no good. it looks like it returned milliseconds since epoch or something as a number. Bad times. But how come if I do this, it does work...

var currentDate = new Date('05/12/2014 14:32');
currentDate.setDate(currentDate.getDate() - 14);
var futureDate = currentDate;
console.log(typeof futureDate);
console.log(futureDate);

Output:

object
Mon Apr 28 2014 14:32:00 GMT-0500 (CDT)

It just makes no logical sense to me. JavaScript is hurting my head. I'd be grateful for any help in understanding what is happening here. I know I can proceed and my script will work, but I'd like to understand what is going on.

Thanks, Tom

Answer 1

What actually happens in your code now is this :

var futureDate = currentDate.setDate(currentDate.getDate() - 14);

You're setting the date of currentDate to currentDate.getDate() - 14 . currentDate.setDate returns the integer equivalent of that new date, and you save that to futureDate .

You should save it as a Date, and not as an integer as it currently is.

It's because futureDate is currently not a Date. currentDate.getDate() - 14 will return just a number, but for you to make it into a Date, you should initialize futureDate as a date first, then use futureDate.setDate to assign it a new date value.

var futureDate = new Date();
futureDate.setDate(currentDate.getDate() - 14);