Symbols, Protocols, Frames, and Versioning

Brendan Eich brendan at
Wed Oct 3 12:01:09 PDT 2012

Thanks for pointing this out. Python's dunder-prefixing or anything like 
it in JS has that advantage: you can spell the magic property name with 
a string that works in any frame or global object. Of course strings can 

Symbols are useful in spite of this, but it is telling that we want 
@iterator to be a singleton across all potentially connected frames.

So should there be a way in the language to create singleton symbols? If 
so, how?


Kevin Smith wrote:
> One of the main use cases for symbols is for defining object 
> "protocols" that don't suffer from property name conflicts.  The 
> recently discussed `iterator` and `toStringTag` method names fall into 
> this category.  The idea is that we can implement the protocol by 
> defining methods using symbols, and thus avoid namespacing considerations.
> Designing and maintaining a global namespace is, well, no fun.
> But consider the multiple-global case in which we have scripts running 
> in more than one frame.  It seems like protocols should be 
> transferrable across frames.  For built-in protocols like `iterator`, 
> this has to work:
>     function f(iterable) {
>       for (x of iterable) {
>         // This must work regardless of which frame `iterable` comes from
>       }
>     }
> But what about user-defined protocols?  Let's say we have a 
> "Persistable" protocol:
>     export var persistName = new Symbol; // unique, not "private"
> And a function which makes use of this protocol:
>     import persistName from "Persistable.js";
>     function usePersistable(obj) {
>       if (obj[persistName])
>         obj[persistName]();
>     }
> It seems like `usePersistable` should be able to work as expected even 
> if `obj` comes from a different frame (in which "Persistable.js" was 
> separately loaded).
> Another expression of the same problem occurs with versioning.
> Suppose that in a fairly complex module dependency graph, 
> "Persistable-0.1.js" and "Persistable-0.2.js" are simultaneously 
> loaded. ("Persistable" is on github and therefore in perpetual 
> version-zero purgatory.)  It seems reasonable to expect that objects 
> implementing the protocol defined by "Persistable-0.2.js" should be 
> able to work with functions consuming the "Persistable-0.1.js" 
> protocol.  But that is not possible with unique symbols.
> In summary, I don't think that we can really avoid global namespacing 
> issues using system-generated unique symbols as we currently conceive 
> of them.  Built-in protocols like `iterator` are a special "cheating" 
> case, but we need to have an equally consistent story for user-defined 
> protocols.
> Kevin
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