Replace function argument in body with undefined if the argument is unit

src/Fable.Transforms/FSharp2Fable.Util.fs

@@ -1825,8 +1825,7 @@ module Util =
     open TypeHelpers
     open Identifiers
 
-    let isUnitArg (ident: Fable.Ident) =
-        ident.IsCompilerGenerated && ident.Type = Fable.Unit
+    let isUnitArg (ident: Fable.Ident) = ident.Type = Fable.Unit
     // && (ident.DisplayName.StartsWith("unitVar") || ident.DisplayName.Contains("@"))
 
     let discardUnitArg (args: Fable.Ident list) =

src/Fable.Transforms/Fable2Babel.fs

@@ -2986,13 +2986,17 @@ but thanks to the optimisation done below we get
 
         | _ -> transformDecisionTreeWithExtraSwitch com ctx returnStrategy targets treeExpr
 
-    let transformIdent (com: IBabelCompiler) ctx id =
-        let e = identAsExpr id
+    let transformIdent (com: IBabelCompiler) ctx (id: Fable.Ident) =
+        match id.Type with
+        // Remove unit idents, because the declaration may have been erased #4041
+        | Fable.Unit -> undefined id.Range None
+        | _ ->
+            let e = identAsExpr id
 
-        if com.IsTypeScript && ctx.ForcedIdents.Contains id.Name then
-            Expression.unaryExpression (UnaryNot, e, isSuffix = true)
-        else
-            e
+            if com.IsTypeScript && ctx.ForcedIdents.Contains id.Name then
+                Expression.unaryExpression (UnaryNot, e, isSuffix = true)
+            else
+                e
 
     let rec transformAsExpr (com: IBabelCompiler) ctx (expr: Fable.Expr) : Expression =
         match expr with

tests/Integration/Integration/data/transforms/RemoveUnitIdents.fs

@@ -0,0 +1,8 @@
+module RemoveUnitIdents
+
+type Model = unit
+
+let update (model: Model) =
+    model, ()
+
+update () |> ignore

tests/Integration/Integration/data/transforms/RemoveUnitIdents.jsx.expected

@@ -0,0 +1,7 @@
+
+export function update() {
+    return [undefined, undefined];
+}
+
+update();
+

tests/Integration/Integration/data/transforms/transforms.fsproj

@@ -0,0 +1,17 @@
+<Project Sdk="Microsoft.NET.Sdk">
+
+  <PropertyGroup>
+    <TargetFramework>net9.0</TargetFramework>
+    <RollForward>Major</RollForward>
+    <GenerateDocumentationFile>true</GenerateDocumentationFile>
+  </PropertyGroup>
+
+  <ItemGroup>
+    <Compile Include="RemoveUnitIdents.fs" />
+  </ItemGroup>
+
+  <ItemGroup>
+    <PackageReference Include="Fable.Core" Version="4.2.0" />
+  </ItemGroup>
+
+</Project>