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

[chkn]

Description

When a static field has the [<FSharp.Core.DefaultValue>] attribute, Fable leaves its value as undefined, which can have different behavior than the .NET default value.

Repro code

type Foo() =
    [<FSharp.Core.DefaultValue>]
    static val mutable private count : int
    static member IncrCount() =
        Foo.count <- Foo.count + 1
        Foo.count

printfn "%d" (Foo.IncrCount())

Expected and actual results

dotnet fsi prints: 1
Fable REPL prints: 0

Related information

• Fable version: 3.7.0-beta-011
• Operating system: macOS 10.15.7

[chkn]

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

[github-actions]

🤖 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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