[<FSharp.Core.DefaultValue>] not handled properly on static fields #2739

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1. chkn commented on Jan 17, 2022

   

   chkn

   on Jan 17, 2022

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   I found a workaround: save the static field to a local first. This works as expected:

       static member IncrCount() =
           let count = Foo.count
           Foo.count <- count + 1
           Foo.count
2. github-actions

   added

   bug

   on Mar 15, 2026
3. github-actions commented on Mar 17, 2026

   

   github-actionsBot

   on Mar 17, 2026 – with GitHub Actions

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   🤖 This is an automated response from Repo Assist.

   Root cause: Fable doesn't emit static field initializers in the generated JavaScript class body. In declareClassWithParams in src/Fable.Transforms/Fable2Babel.fs (~line 3467), the classFields array only adds TypeScript type-annotation declarations for fields; no default-value initialization is emitted for either JavaScript or TypeScript:

   let classFields =
       if com.IsTypeScript && not ent.IsFSharpUnion then
           ent.FSharpFields
           |> List.mapToArray (fun field ->
               // Comment in the source: "Static fields need to be initialized by static constructor"
               ClassMember.classProperty (prop, isComputed = isComputed, isStatic = field.IsStatic, ...)
           )
       else
           Array.empty  // <-- nothing for JS

   So a field like static val mutable private count : int with [(DefaultValue)] is never emitted as static count = 0; in the JavaScript output. At runtime Foo.count is undefined. When you compute undefined + 1, JavaScript produces NaN, and since integer arithmetic is coerced via | 0, NaN | 0 = 0, causing the method to return 0 instead of 1.

   The fix would need to:

   1. Add a makeDefaultValue function for the JS target (the Python target already has getDefaultValueForType in Fable2Python.Util.fs as a reference)
   2. In declareClassWithParams, emit static count = 0; (or the appropriate default for the field's type) for static FSharpFields — either only when [(DefaultValue)] is present, or unconditionally for all static fields of value types

   Note: PR #4405 (currently open) fixes a related but distinct issue — static backing fields for static member val being incorrectly included as constructor parameters. This [(DefaultValue)] initialization issue is a separate bug that will still need a fix after #4405 is merged.

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4. MangelMaxime

   added a commit that references this issue on Mar 18, 2026

   [JS/TS] Fix `static val mutable` fields declared with `[<DefaultValue…

   ...

   This issue will close once commit 28f34f0 is merged into the 'main' branch.

   28f34f0
5. MangelMaxime

   closed this as completedin #4418on Mar 19, 2026