[JS] Rework how we handle static field on class #4419

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1. MangelMaxime

   added

   enhancement

   TypeScript

   javascript

   on Mar 18, 2026
2. github-actions commented on Mar 28, 2026

   

   github-actionsbot

   on Mar 28, 2026 – with GitHub Actions

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   🤖 This is an automated response from Repo Assist.

   Great proposal! I had a look at the existing infrastructure — it's closer to ready than it might seem.

   Existing building blocks

   ClassProperty with isStatic: bool already exists in the Babel AST (src/Fable.Transforms/Global/Babel.fs), and ClassMember.classProperty is already used for TypeScript class property declarations (e.g. for [(DefaultValue)] fields in declareDefaultValueStaticFieldInits). So the AST and printer already support native static class fields.

   Where the IIFE currently comes from

   static let bindings in F# are compiled by FCS into a class static constructor (.cctor). Fable emits the .cctor body as an ActionDeclaration immediately after the class declaration, which then becomes an IIFE in the JS output. The class body itself doesn't see the static initializer at all — it arrives as a separate top-level declaration.

   Key technical consideration

   Native static field = value runs when the class expression is evaluated (i.e. when the module is first imported), not after the full module has been initialised. For simple literal values (static let count = 42) this is always safe. For initializers that reference other module-level bindings (static let items = ResizeArray(), static let logger = createLogger()) it may also be safe, but for ones that refer to values declared after the class in the source, the IIFE pattern provides a guarantee the native field initializer does not.

   Suggested implementation path

   1. In FSharp2Fable.fs, identify when a .cctor action contains only simple assignments to one class's static fields (i.e. no cross-class or cross-module dependencies).
   2. Tag those ActionDeclarations (or the corresponding MemberDecls) so Fable2Babel.fs can recognise them.
   3. In Fable2Babel.fs, instead of emitting those as an IIFE after the class, fold them into the class body as ClassMember.classProperty with isStatic = true.
   4. For initializers that can't be safely inlined (complex expressions with potential order-of-init dependencies), keep the IIFE as a fallback.

   A simpler first cut could handle only the [(DefaultValue)] path (which declareDefaultValueStaticFieldInits already handles) and the literal-initializer .cctor case.

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