Javascript: Date object being changed [duplicate]

Question

I have a situation when I set expire to a date object. After altering 'expire' the 'object.created' is changed when it should not be. Why is 'object.created' be changed? Thanks!

let expire = object.created; // object.created: Wed Mar 02 2016
expire.setDate(12);

console.log(expire); // Wed Mar 12 2016
console.log(object.created); //Wed Mar 12 2016 <-- WHY?!

Answer 1

Assigning a reference to an object from one place to another does not involve making a copy. If you want a complete copy of the date:

let expire = new Date(object.created);

If you don't do that, there's only one Date instance involved; both expire and object.created refer to the same object in your code.