FSharpValue.GetRecordFields skips None values with anonymous records #3477

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1. reinux

   changed the title [-]FSharpValue.GetRecordFields skips `None` values[/-] [+]FSharpValue.GetRecordFields skips `None` values with anonymous records[/+] on Jun 16, 2023
2. github-actions

   added

   bug

   javascript

   on Mar 16, 2026
3. fable-repo-assist commented on Apr 26, 2026

   

   fable-repo-assistBot

   on Apr 26, 2026

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   🤖 This is an automated response from Repo Assist.

   I've traced this to the BabelPrinter in src/Fable.Transforms/BabelPrinter.fs at around line 1211:

   properties
   |> Array.filter (function
       | ObjectProperty(_, UndefinedOrVoid, _, _) -> false  // ← filters out undefined values
       | _ -> true)

   When an anonymous record like {| b = (None: int option) |} is compiled, the None field becomes undefined (since mustWrapOption int = false). The BabelPrinter then filters out the property entirely from the object literal, so the emitted JS is { a: 3, c: 89 } without b at all.

   Since FSharpValue.GetRecordFields in Reflection.ts uses Object.keys(v).map(k => v[k]), it only sees keys that actually exist in the object — so the b field is simply absent from the result.

   This filter exists to avoid emitting noisy b: undefined in the output, which is a reasonable optimisation for TypeScript (where b?: int and b: int | undefined are interchangeable). However, it breaks reflection.

   Potential fixes:

   1. Pass type info to getRecordFields — augment the call in Replacements.fs to also pass the anonymous record's type info, then use it in Reflection.ts to enumerate fields in the correct order/count.
   2. Track omitted fields — instead of simply filtering, note which field indices are undefined so getRecordFields can reconstruct the full array.
   3. Don't filter in anonymous record construction — special-case anonymous records to always emit all keys (even if undefined), at the cost of slightly noisier JS output.

   Option 1 is the cleanest but requires threading type info through. Option 3 is the simplest change.

   Note: this likely affects regular records too when their fields are None, unless regular records are compiled as class instances rather than plain object literals (which would explain why the author found it only affects anonymous records).

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4. MangelMaxime

   closed this as completedin #4704on Jun 30, 2026