[JS/TS] Fixed quotation for union string cases (#4420)

Author: MangelMaxime

src/Fable.Cli/CHANGELOG.md

@@ -10,6 +10,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 ### Fixed
 
 * [JS/TS] Fix `Unchecked.defaultof<'T>` for struct types with fields returning `undefined` instead of zero-initialized values (by @MangelMaxime)
+* [JS/TS] Fixed quotation for union string cases (by @MangelMaxime)
 * [Python] Fix `Unchecked.defaultof<'T>` for struct types with fields returning incorrect values instead of zero-initialized instances (by @MangelMaxime)
 * [Python] Fix `Unchecked.defaultof<char>` returning `""` (empty string) instead of `"\u0000"` (null character) (by @MangelMaxime)
 * [Python] Improve `Unchecked.defaultof<_>` for declared entities (by @MangelMaxime)

src/Fable.Compiler/CHANGELOG.md

@@ -10,6 +10,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 ### Fixed
 
 * [JS/TS] Fix `Unchecked.defaultof<'T>` for struct types with fields returning `undefined` instead of zero-initialized values (by @MangelMaxime)
+* [JS/TS] Fixed quotation for union string cases (by @MangelMaxime)
 * [Python] Fix `Unchecked.defaultof<'T>` for struct types with fields returning incorrect values instead of zero-initialized instances (by @MangelMaxime)
 * [Python] Fix `Unchecked.defaultof<char>` returning `""` (empty string) instead of `"\u0000"` (null character) (by @MangelMaxime)
 * [Python] Improve `Unchecked.defaultof<_>` for declared entities (by @MangelMaxime)

src/fable-library-ts/CHANGELOG.md

@@ -7,6 +7,10 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## Unreleased
 
+### Fixed
+
+* [JS/TS] Fixed quotation for union string cases (by @MangelMaxime)
+
 ## 2.0.0-rc.4 - 2026-03-19
 
 ### Fixed

src/fable-library-ts/Types.ts

@@ -39,16 +39,23 @@ export function toString(x: any, callStack = 0): string {
 }
 
 export function unionToString(name: string, fields: any[]) {
+  function unionFieldToString(x: any): string {
+    if (typeof x === "string") {
+      return '"' + x + '"';
+    }
+    return toString(x);
+  }
+
   if (fields.length === 0) {
     return name;
   } else {
     let fieldStr;
     let withParens = true;
     if (fields.length === 1) {
-      fieldStr = toString(fields[0]);
+      fieldStr = unionFieldToString(fields[0]);
       withParens = fieldStr.indexOf(" ") >= 0;
     } else {
-      fieldStr = fields.map((x: any) => toString(x)).join(", ");
+      fieldStr = fields.map((x: any) => unionFieldToString(x)).join(", ");
     }
     return name + (withParens ? " (" : " ") + fieldStr + (withParens ? ")" : "");
   }

tests/Js/Main/UnionTypeTests.fs

@@ -97,6 +97,8 @@ type LangCode =
     | EN
     | ``pt-Br``
 
+type S = S of string
+
 let tests =
   testList "Unions" [
     testCase "Union cases matches with no arguments can be generated" <| fun () ->
@@ -272,4 +274,8 @@ let tests =
         let x = LangCode.``pt-Br``
 
         equal x LangCode.``pt-Br``
+
+    testCase "sprintf formats strings cases correctly" <| fun () ->
+        let s = sprintf "%A" (S "1")
+        equal s "S \"1\""
   ]