Layer 0: Replace importMeta object with importMetaHook function

[nicolo-ribaudo]

ECMA-262 exposes two host hooks to initialize import.meta: HostGetImportMetaProperties and HostFinalizeImportMeta.

• HostGetImportMetaProperties doesn't receives any parameter and returns a list of properties, that ECMA-262 will copy on the import.meta object
• HostFinalizeImportMeta receives the import.meta object, and can modify it as it wants

The second host hook is enough to cover all the use cases of the first hook, but not the other way around.

This proposal currently implements HostGetImportMetaProperties: the importMeta parameter of the Module constructor is equivalent to the list of properties returned by the hook. However, this is not enough to replicate the behavior of the various hosts:

• XS freezes the import.meta object (source: "XS is freezing import.meta today" in https://github.com/tc39/notes/blob/167155eeb708d84e1758d99c88b15670f9b81f75/meetings/2020-03/march-31.md#importmeta-for-stage-4)
• Bun installs a custom [[Prototype]] on import.meta (source: run console.log(Object.getPrototypeOf(import.meta)); in Bun)

With importMetaHook, both behaviors would be possible:

new Module(source, {
  importMetaHook(importMeta) {
    importMeta.url = someURL;
    Object.freeze(importMeta);
  }
});

new Module(source, {
  importMetaHook(importMeta) {
    importMeta.url = someURL;
    Object.setPrototypeOf(importMeta, ImportMetaPrototype);
  }
});

This has the same serializability implications of importHook: modules with a non-default importMetaHook would not be serializable.

[caridy]

Few clarifications needed:

1. is the argument passed into importMetaHook an ordinary object created by the engine, empty at first, and passed into the hook?
2. If the answer to 1 is "yes", then what's the __proto__ of the importMeta object argument?
3. how many times is this hook called? once and then cached as it is today for the host hooks?

[nicolo-ribaudo]

1. yes
2. I propose null, which is what is already happening with the host hook (so that by default import.meta objects have a null prototype): step 4.a of https://tc39.es/ecma262/#sec-meta-properties-runtime-semantics-evaluation
3. Yes, it's cached per-module and only called when import.meta is evaluated the first time.

[caridy]

I'm fine with this proposal.