Question

I noticed this situation in my code (unfortunately), and was able to duplicate it in my own JS file. So I have this code:

var date1 = new Date();  // today
var date2 = date1;

date2 = date2.setDate(date2.getDate() + 1);
// what is date1?

After this code executes, date1 is today’s date + 1! This harkens back to my undergrad days when I learned about pointers, and I guess I’m a little rusty. Is that what’s happening here? Obviously I’ve moved the assignment away from date1, and am only modifying date2, but date1 is being changed. Why is this the case?

Incidentally, after this code executes date2 is a long number like 1272123603911. I assume this is the number of seconds in the date, but shouldn’t date2 still be a Date object? setDate() should return a Date object.

Answer 1

Classic case of Rerence Types vs. Value Types.

When the assignment operator works on
primitive values (numbers, strings,
Boolean, null, and undefined), a copy
of the value is made. When the
assignment operator works on
JavaScript objects, references to the
objects are copied.

Assignment by Value Versus Assignment by Reference

Therefore:

// creates a new Date object in memory with date1 as a Reference to its location
var date1 = new Date();

// date2 will now point to the same Object in memory as date1
var date2 = date1;

// Since both date1 and date2 point to the same object,
// modifying one changes the other
date2 = date2.setDate(date2.getDate() + 1);

As for the resulting value, you’re correct. It’s getting converted inside the expression to the number of seconds since Epoch.