[JS/TS] Fix N format specifier producing trailing dot when precision is 0 (#4422)

Co-authored-by: github-actions[bot] <github-actions[bot]@users.noreply.github.com>
Co-authored-by: Claude Sonnet 4.6 <[email protected]>

Author: github-actions[bot]

src/Fable.Cli/CHANGELOG.md

@@ -9,6 +9,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ### Fixed
 
+* [JS/TS] Fix `N` format specifier (`ToString("N0")`, `String.Format("{0:N0}", ...)`) producing a trailing dot when precision is 0 (fix #2582) (by @MangelMaxime)
 * [JS/TS] Fix `C0` and `P0` format specifiers producing trailing dot (e.g., `"¤1,000."` → `"¤1,000"`) (by @MangelMaxime)
 * [All] Fix captured side-effect-free values (e.g. empty ResizeArray) being incorrectly inlined into object expression getters in release mode, causing a new instance to be created on each getter call (fixes #3779) (by @MangelMaxime)
 * [Python] Fix missing `await` on else branch of ternary expressions in async closures (by @dbrattli)

src/Fable.Compiler/CHANGELOG.md

@@ -9,6 +9,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ### Fixed
 
+* [JS/TS] Fix `N` format specifier (`ToString("N0")`, `String.Format("{0:N0}", ...)`) producing a trailing dot when precision is 0 (fix #2582) (by @MangelMaxime)
 * [JS/TS] Fix `C0` and `P0` format specifiers producing trailing dot (e.g., `"¤1,000."` → `"¤1,000"`) (by @MangelMaxime)
 * [All] Fix captured side-effect-free values (e.g. empty ResizeArray) being incorrectly inlined into object expression getters in release mode, causing a new instance to be created on each getter call (fixes #3779) (by @MangelMaxime)
 * [Python] Fix missing `await` on else branch of ternary expressions in async closures (by @dbrattli)

src/fable-library-ts/String.ts

@@ -386,7 +386,11 @@ export function format(str: string | object, ...args: any[]) {
           precision = precision != null ? precision : 2;
           rep = toFixed(rep, precision);
           parts = splitIntAndDecimalPart(rep);
-          rep = thousandSeparate(parts.integral) + "." + padRight(parts.decimal, precision, "0");
+          if (precision > 0) {
+            rep = thousandSeparate(parts.integral) + "." + padRight(parts.decimal, precision, "0");
+          } else {
+            rep = thousandSeparate(parts.integral);
+          }
           break;
         case "p": case "P":
           precision = precision != null ? precision : 2;

tests/Js/Main/StringTests.fs

@@ -477,6 +477,15 @@ let tests = testList "Strings" [
         String.Format(CultureInfo.InvariantCulture, "{0:#}", 12343235354M) |> equal "12343235354"
         String.Format(CultureInfo.InvariantCulture, "{0:0}", 12343235354M) |> equal "12343235354"
 
+    testCase "String.Format works with N format specifier" <| fun () -> // See #2582
+        String.Format(CultureInfo.InvariantCulture, "{0:N0}", 1000) |> equal "1,000"
+        String.Format(CultureInfo.InvariantCulture, "{0:N0}", 12345678) |> equal "12,345,678"
+        String.Format(CultureInfo.InvariantCulture, "{0:N0}", -1000) |> equal "-1,000"
+        String.Format(CultureInfo.InvariantCulture, "{0:N2}", 1000) |> equal "1,000.00"
+        String.Format(CultureInfo.InvariantCulture, "{0:N}", 1000) |> equal "1,000.00"
+        (1000).ToString("N0") |> equal "1,000"
+        (1000).ToString("N2") |> equal "1,000.00"
+
     testCase "String.Format trims trailing zeroes when using # placeholder" <| fun () -> // Fix #2950
         String.Format(CultureInfo.InvariantCulture, "{0:######,###.000####}", -6789.5688) |> equal "-6,789.5688"
         String.Format(CultureInfo.InvariantCulture, "{0:######,###.000####}", 6789.5688) |> equal "6,789.5688"