javascript setMonth() - setting a month to date2 from date1. Why also date1 change?

Question

I know, title isn't probably clear, but I have this code:

var date1=new Date();
alert(date1);
var date2=date1;
alert(date2);
date2.setMonth(date1.getMonth() + 6);
alert(date1+" - "+date2);

Why date1 change? I think date1 should remain the current date and date2 six months later ...

Thanks

http://stackoverflow.com/questions/6605640/javascript-by-reference-vs-by-value — alebianco, Mar 28 '17 at 08:34
You are modifying same object from different reference. Is any language does what you expect ? — Panther, Mar 28 '17 at 08:35

score 5 · Answer 1 · answered Mar 28 '17 at 08:33

5

Dates are objects in JavaScript. When you set date2=date1, both date1 and date2 will reference the same object. Since they are both references to the same date object, the object may be updated or inspected using either.

answered Mar 28 '17 at 08:33

qubyte

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...to create a copy: `date2 = new Date(date1.getTime());` – T.J. Crowder Mar 28 '17 at 08:34
1

Thanks so much... and also to you T.J. Crowder – Francesco G. Mar 28 '17 at 08:39

score 1 · Accepted Answer · answered Mar 28 '17 at 08:40

1

Both the variables are pointing to same date object. You can do this instead

var date1=new Date();
alert(date1);
var date2=new Date(date1.valueOf());
alert(date2);
date2.setMonth(date1.getMonth() + 6);
alert(date1+" - "+date2);

In this approach, I am creating new object using values of existing one. Now there are two objects being point by different variables.

While you had assigned same object to both variables.

answered Mar 28 '17 at 08:40

Panther

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The Date can be copied as `var date2=new Date(date1)`. To accommodate ancient IE and dates from year 0 to 99, use `var date2=new Date(+date1)` (which still works everywhere, even in not old IE). ;-) – RobG Mar 28 '17 at 12:26

SaXXuM · Answer 3 · 2017-03-28T08:43:39.027

0

Because date2 gets the reference to date1, not value.

edited Mar 28 '17 at 08:43

answered Mar 28 '17 at 08:37

SaXXuM

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javascript setMonth() - setting a month to date2 from date1. Why also date1 change?

3 Answers3