Why javascript date.setDate() change the value of other date variables

date, html, javascript, jquery, variable-assignment

Solution

You need to make a copy of your dates, like this:

date3 = new Date(date1.getTime());
date2 = new Date(date1.getTime());

or simply:

date3 = new Date(date1);
date2 = new Date(date1);

instead of

date3 = date1;
date2 = date1;

Otherwise, your variables all point to the same Date object (initially referenced by `date1`).

EDIT (about memory allocation)

- Example 1:

var date1 = new Date();           // Memory allocation for an object
var date2 = new Date(1991,4,11);  // Memory allocation n°2
var date3;                        // Obviously no memory allocation here

date3 = date1;                    // No memory allocation either, date2 and date3
date2 = date1;                    // become references of the object in date1

In this example, there are two memory allocations but only one of them is useful as the object in date2 is not used.

Note: The object that was originally in date2 still exists, but is not referenced anymore (it will be garbage-collected).

- Example 2:

var date1 = new Date();            // Memory allocation for an object
var date2 = new Date(date1);       // Memory allocation n°2
var date3 = new Date(date1);       // Memory allocation n°3

In this example, there are three memory allocations for 3 different objects. The 2nd and 3rd allocations consist in creating new Date objects, containing a copy of the object in date1.

I hope it is clearer with this little explanation. If you're interested in memory management in JavaScript, I suggest you have a look at this:

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Memory_Management

Problem

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