JavaScript date comparison

Question

Why does the equality operator return false in the first case?

var a = new Date(2010, 10, 10);
var b = new Date(2010, 10, 10);
alert(a == b); // <- returns false
alert(a.getTime() == b.getTime()); // returns true

Why?

Answer 1

Since dates are built-in objects, and not primitives, an equality check is done using the objects references.

In this case, objects a and b are not the same object, and so the test fails.
You can see the same using

var a = new String("a");
var b = new String("a");
alert(a == b); //false

By using .getTime or .valueOf you are converting the objects value into a primitive, and these are always compared by value rather than by reference.

If you want to do a comparison by value of two dates there is also a more obscure way to do this

var a = new Date(2010, 10, 10);
var b = new Date(2010, 10, 10);

alert(+a == +b); //true

In this case the unary + operator forces the javascript engine to call the objects valueOf method - and so it is two primitives that are being compared.