fix(js/ts): fix G/g format specifier corrupting exponential notation when trimming trailing zeros (#4587)

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
Co-authored-by: github-actions[bot] <github-actions[bot]@users.noreply.github.com>
Co-authored-by: Maxime Mangel <me@mangelmaxime.fr>

Author: 4 people

package-lock.json

@@ -390,11 +390,23 @@ export function format(str: string | object, ...args: any[]) {
             rep = parts.integral + "." + padRight(parts.decimal, precision, "0");
           }
           break;
-        case "g": case "G":
+        case "g": case "G": {
           rep = precision != null ? toPrecision(rep, precision) : toPrecision(rep);
-          // TODO: Check why some numbers are formatted with decimal part
-          rep = trimEnd(trimEnd(rep, "0"), ".");
+          // Handle exponential notation: only trim trailing zeros from mantissa, not from exponent.
+          // .NET G format guarantees an exponent of at least 2 digits with an explicit sign (e.g. "E-07").
+          const eIdx = rep.indexOf("e");
+          if (eIdx >= 0) {
+            const mantissa = trimEnd(trimEnd(rep.slice(0, eIdx), "0"), ".");
+            const expSign = rep[eIdx + 1]; // toPrecision always emits "+" or "-"
+            const expDigits = rep.slice(eIdx + 2);
+            const paddedExpDigits = expDigits.length < 2 ? "0" + expDigits : expDigits;
+            const eChar = format === "G" ? "E" : "e";
+            rep = mantissa + eChar + expSign + paddedExpDigits;
+          } else {
+            rep = trimEnd(trimEnd(rep, "0"), ".");
+          }
           break;
+        }
         case "n": case "N":
           precision = precision != null ? precision : 2;
           rep = toFixed(rep, precision);

src/fable-library-ts/String.ts

@@ -511,6 +511,21 @@ let tests = testList "Strings" [
         String.Format(CultureInfo.InvariantCulture, "{0:P2}", 0.1234) |> equal "12.34 %"
         String.Format(CultureInfo.InvariantCulture, "{0:C2}", 1000) |> equal "¤1,000.00"
 
+    testCase "G format specifier trims trailing zeros without corrupting exponential notation" <| fun () ->
+        (123.45).ToString("G17", CultureInfo.InvariantCulture) |> equal "123.45"
+        (10000.0).ToString("G17", CultureInfo.InvariantCulture) |> equal "10000"
+        (0.000222).ToString("G17", CultureInfo.InvariantCulture) |> equal "0.000222"
+        (1e-10).ToString("G17", CultureInfo.InvariantCulture) |> equal "1E-10"
+        (1e-10).ToString("g17", CultureInfo.InvariantCulture) |> equal "1e-10"
+        (1.5e10).ToString("G17", CultureInfo.InvariantCulture) |> equal "15000000000"
+
+    testCase "G format specifier pads exponent to at least 2 digits" <| fun () ->
+        (1e-7).ToString("G2", CultureInfo.InvariantCulture) |> equal "1E-07"
+        (1e7).ToString("G2", CultureInfo.InvariantCulture) |> equal "1E+07"
+        (1.5e-7).ToString("G2", CultureInfo.InvariantCulture) |> equal "1.5E-07"
+        (-1e-7).ToString("G2", CultureInfo.InvariantCulture) |> equal "-1E-07"
+        (1e-7).ToString("g2", CultureInfo.InvariantCulture) |> equal "1e-07"
+
     testCase "x and X format specifiers work with no defined precision" <| fun () ->
         String.Format(CultureInfo.InvariantCulture, "{0:X}", 0) |> equal "0"
         String.Format(CultureInfo.InvariantCulture, "{0:X}", 0u) |> equal "0"