Explicit base constructor call not properly called #2425

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1. alfonsogarciacaro

   closed this as completedin b31b6f3on Apr 5, 2021
2. alfonsogarciacaro

   reopened this on Apr 5, 2021
3. github-actions

   added

   bug

   on Mar 15, 2026
4. github-actions commented on Mar 19, 2026

   

   github-actionsbot

   on Mar 19, 2026 – with GitHub Actions

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   🤖 This is an automated response from Repo Assist.

   Thanks for filing this, @ncave! This is a known gap in Fable's class hierarchy support.

   Root cause: Fable has limited support for F#'s explicit val field syntax combined with the record-constructor syntax ({ inherit Node(x); Y = y }). In the generated JavaScript, the base constructor call (inherit Node(x)) doesn't result in super(x) being invoked properly, so Node's field initialisation never runs and X remains undefined.

   Workaround: Use the standard F# primary-constructor syntax, which Fable handles reliably:

   type Node(x: int) =
       member val X = x
   
   type Leaf(x: int, y: string) =
       inherit Node(x)
       member val Y = y
   
   [(EntryPoint)]
   let main _args =
       let node = Leaf(3, "4")
       let x = node.X  // correctly returns 3
       let y = node.Y
       System.Console.WriteLine($"x: {x}, y: {y}")
       0

   This pattern maps cleanly to ES6 classes with super(x) and getter properties.

   Where to fix: The issue lives in the FSharp2Fable or Fable2Babel stage — when a class uses val fields with new () = { inherit Base(args); ... }, the base constructor call needs to be translated into a super(...) call in the generated JavaScript class constructor.

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