Thanks for the reply.
Domenic Denicola wrote:
In JavaScript, our data structures are specified to use SameValue (or
SameValueZero, or Strict Equality Comparison); none of them use a
user-overridable hook. Furthermore, years of best-practice advice have made a
lot of programmers prefer using `===` exclusively, ignoring `==` altogether. I
for one would find it uncomfortable to start switching to `==` on the hope that
it does something useful, in case the left-hand side is a value object with a
proper overload implementation. (I would of course want to*avoid* it for the
primitives, since its coercion behavior there is rarely desired.)
Indeed, writing something like
if (urlObj == "http://foo.com";) {...}
today could take advantage of toString or valueOf to "overload".
So ... I guess I am saying, I am not sure that making `==` work for mutable
objects is salvagable. It can be made to work for immutable objects, maybe,
definitely
assuming that their immutability is also used to modify SameValue et al. and
thus allow a consistent behavior between `==` and e.g. `map.has`.
No, that's ===. For value objects, == is completely overloadable.
Of course, a buggy or perverse implementation could give inconsistent
results, so my argument against mutability is weaker and relies on best
effort/testing. So I'm not totally, unalterably opposed, but I am opposed.
Your point about it being too late to salvage == (vs. ===) is good, but
perhaps with value objects plus further work to disable implicit
conversions, == will make a come-back -- but that's far down the road.
(I suppose I am asking for invariants `x == y&& Type(x) is Object&& Type(y)
is Object` implies `map.has(x) === map.has(y)`.)
That follows from the x == y && typeof x == typeof y <=> x === y
relation already in the language, except for NaNs (which you ignored, so
I will too!).
But unless we are prepared to allow the overloaded `==` to propagate
throughout the language, which seems unlikely since the language rarely uses
Abstract Equality Comparison, then I can't really see a place for `==`
overloading on mutable objects.
Good argument, thanks for making it.
/be
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