When initializing a new Date object with a number versus a string, the resulting values will vary

As I delve deeper into JavaScript, I stumbled upon an interesting quirk in its behavior.

When creating date objects like this:

var stack = new Date(1404187200000) // 07-01-2014
var overflow = new Date('07-01-2014')

I noticed that when comparing these two date objects:

stack == overflow // returns false
stack.getTime() == overflow.getTime() // returns true

I speculated that this discrepancy is due to them not being the same object. However, I am aware that '==' checks for equality while '===' compares identity, as shown in this example:

var stack = 1;
var overflow = '1';
stack == overflow // returns true
stack === overflow // returns false

So why then, does comparing new Date([NUMBER]) and new Date([STRING]) yield different results even though they represent the same date?

Your insights on this matter would be greatly appreciated!

Answer №1

You might be misunderstanding the distinction between == and ===. It's not simply a matter of one conducting equality checks while the other handles reference checking.

With ===, both operands must share the same type. However, with ==, type coercion can occur before the comparison for equality.

In this scenario, since the two objects are of identical types, there isn't really a discrepancy between using == or ===; the comparison is more about reference equality than value equality. The correct approach to assessing value equality with dates is by doing something like:

stack.getTime() == overflow.getTime()
.

Another option would be to use +stack == +overflow, which will first convert them both, leading to a value equality examination.

Answer №2

When using the new Date method, it generates a new object each time it is called. As a result, these objects are not identical. However, the getTime method retrieves the same value from these separate objects.

Answer №3

This concept is a bit more intricate. The triple equals sign `===` involves checking the type, whereas the double equals sign `==` attempts to convert to a common type. This is why `1 == '1'` evaluates to true but `1 === '1'` evaluates to false - in the former case, '1' is transformed into a number (if I remember correctly).

You can refer to the specific specification on how this process is handled here: http://www.ecma-international.org/ecma-262/5.1/#sec-11.9.3 - the section that may interest you most in this situation is 1.f.

If x and y point to the same object, return true. Otherwise, return false.

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