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With "useDefineForClassFields": true and an es2015 target, tsc turns every class field into an Object.defineProperty(this, ...) call in the constructor, 110 of them in the bundle. BaseNode and ObjectNode are constructed for every model instance and every scalar property value, so these calls dominate node creation. The option was enabled with MobX 6 support (mobxjs#1569), when the node and type classes called makeObservable(this). They no longer do, and no class in src relies on define semantics: no field name collides with an accessor, nothing enumerates or `in`-checks node or type instances, and the type declarations are unchanged. With plain assignments, tsc emits nothing for a field without an initializer, so a field that is set lazily (pathAtom, _internalEvents, middlewares, identifierCache, ...) is created on its first write. Nodes then get one V8 hidden class for each order of those writes. Give these fields an `= undefined` initializer in BaseNode, ObjectNode and StoredReference, so each class keeps one hidden class. Bun ignores this option when it runs src, so add a test that compiles the node classes with the project's compiler options and checks that construction assigns every declared field.
Author
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hey @coolsoftwaretyler, sorry for spamming with these PRs, but they will help us a lot. Please let me know if you need any more context or work from our side. |
Collaborator
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Thanks @lavrton - happy to get all the PRs. I'll get around to them as quickly as I can. Once they all merge, I'll cut a new version you can use. |
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What does this PR do and why?
MST is compiled with
useDefineForClassFields: true, so every class field becomes anObject.definePropertycall in the constructor. MST creates a lot of nodes (one per model instance and per property value), and these calls make that slow.This PR switches the option to
false, so fields are plain assignments. I also added= undefinedto a few node fields that are set later, so V8 keeps one hidden class per node type.Results on my machine (Node, production build):
One thing gets a bit slower: reading node paths (getPath, getParent) is about 15% slower, because node objects are bigger now. Everything else I measured is the same or faster.
Bun ignores this compiler option when running tests, so I added a small test that compiles the node classes and checks the fields are assigned in the constructor.