might be for historic reasons. I do remember some browsers (at least IE) not doing too well with extending native types

@lilactown I think the idea is that you have to control the protocol or the type

if you use a protocol and a type you don’t control then you will have conflicts

> if you use a protocol and a type you don’t control then you will have conflicts I recall the case of MooTools monkeypatching core JS objects causing trouble for newer ECMAscripts, with conflicting methods changing underlying semantics. Are there are other good examples where this wasn't just a theoretical problem?

it happened all the time - in the early days it prevented using different libs together on the same webpage

definitely not theoretical issue - really a nightmare

does the introduction of Symbol and extending built ins using them eliminate most of the risk of conflicts?

@lilactown If you own the protocol, what is exactly the risk?

that's what I'm wondering

You either own the protocol OR the type, you don't have to own them both

Symbol is a means to an end, a unique string property would have the same effect I think?

(although as you pointed out, this stuff is done using Symbols now in JS)

yeah, there's certain other nice properties of using symbols that don't have to do with avoiding conflicts

you don't have to use Symbol.for. Symbol("foo") creates a completely unique symbol each time, so as long as you can reference the value returned by it, using Symbol.for is not needed and you can avoid conflicts compeletely

but also, Symbols as properties on objects don't show up in the enumerable properties of objects, so you can avoid problems with serialization too

so if have an object {} and do {}[Symbol("foo")] = 1 twice, it has two new properties?

right

verified:

> x = {}
{}
> x[Symbol("foo")] = 1
1
> x[Symbol("foo")] = 1
1
> Object.getOwnPropertySymbols(x);
[ Symbol(foo), Symbol(foo) ]

I guess if it's just MooTools-esque conflicts or old versions of IE to worry about (which I don't care), then a new language could use Symbols for protocols and extend built ins directly