Symbols as WeakMap keys

For Stage 2

Daniel Ehrenberg

Igalia in partnership with Bloomberg

July 2020 TC39 meeting

Why?

• I'm championing this in order to:�enable references from Records and Tuples to Objects via WeakMaps
• Some TC39 delegates reported other use cases

Details

• Any Symbol may be used as
• A WeakMap key
• A WeakSet entry
• The target of a WeakRef
• Registered in a FinalizationRegistry

Spec text

• PR: https://github.com/tc39/ecma262/pull/2038

Why not add a Box type?

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Boxmaker, Boxmaker....

Intuition: Box objects in a primitive

const obj = { hello: "world" };

const box = Box(obj);

assert(typeof box === "box", "boxes are a new primitive type");

assert(obj !== box, "boxes are not their boxed object");

assert(obj.hello === box.deref().hello, "boxes can deref props");

assert(obj === box.deref(), "boxes can deref the full object");

const server = #{

port: 8080,

handler: Box(function handler(req) { /* ... */ }),

};

server.handler.deref()({ /* ... */ });

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Box semantics in more detail

• Each Box has an associated [[Id]]
• Each Realm has an associated weak mapping [[Id]] -> Boxed object�
• Box(obj)
• Create a new [[Id]], and write [[Id]] -> obj into the Realm's mapping
• Return a Box value with an associated [[Id]]
• Box.prototype.deref
• If this Realm's mapping contains an entry for this.[[Id]]
• Return the associated object
• Else, return undefined

Sharing and isolating with Boxes

• Box.prototype.deref is the only way to get at that Realm's mapping
• Two different Realms cannot see into each other's Boxes
• Unless one Realm gives the other Realm its Box.prototype.deref
• A Realm can pull off its deref, delete it, etc, to control access to the mapping

Problem: isolating within a Realm

• If using membrane-based isolation within a Realm,�Box() + Box.prototype.deref bypasses the membrane
• Only option to preserve membrane is to disable the feature (early)

delete Box.prototype.deref

• Some committee members want all TC39 features to be available within a single frozen Realm, membrane-isolated world
• ⇒ no Box

Symbols as WeakMap keys omits Realm-wide mapping.�Manage your own mapping!

Stage 2?