From f66094347176dbfb6b014ead14e04bd7fafb767b Mon Sep 17 00:00:00 2001
From: sangwook <73056306+Han5991@users.noreply.github.com>
Date: Tue, 9 Sep 2025 05:10:50 +0900
Subject: [PATCH] util: fix numericSeparator with negative fractional numbers

Fix util.inspect() formatting bug where negative fractional numbers
between -1 and 0 lost their minus sign when numericSeparator was true.
Fixed formatNumber function to preserve sign by using original string
representation. Also corrected test expectations for scientific notation
to not apply numeric separators.
Fixes: https://github.com/nodejs/node/issues/59376

PR-URL: https://github.com/nodejs/node/pull/59379
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: James M Snell <jasnell@gmail.com>
---
 lib/internal/util/inspect.js       | 23 ++++++++++++++---------
 test/parallel/test-util-inspect.js | 21 +++++++++++++++++++++
 2 files changed, 35 insertions(+), 9 deletions(-)

diff --git a/lib/internal/util/inspect.js b/lib/internal/util/inspect.js
index 015922aba9d..0c10ef4431f 100644
--- a/lib/internal/util/inspect.js
+++ b/lib/internal/util/inspect.js
@@ -1851,23 +1851,28 @@ function formatNumber(fn, number, numericSeparator) {
     }
     return fn(`${number}`, 'number');
   }
+
+  const numberString = String(number);
   const integer = MathTrunc(number);
-  const string = String(integer);
+
   if (integer === number) {
-    if (!NumberIsFinite(number) || StringPrototypeIncludes(string, 'e')) {
-      return fn(string, 'number');
+    if (!NumberIsFinite(number) || StringPrototypeIncludes(numberString, 'e')) {
+      return fn(numberString, 'number');
     }
-    return fn(`${addNumericSeparator(string)}`, 'number');
+    return fn(addNumericSeparator(numberString), 'number');
   }
   if (NumberIsNaN(number)) {
-    return fn(string, 'number');
+    return fn(numberString, 'number');
   }
+
+  const decimalIndex = StringPrototypeIndexOf(numberString, '.');
+  const integerPart = StringPrototypeSlice(numberString, 0, decimalIndex);
+  const fractionalPart = StringPrototypeSlice(numberString, decimalIndex + 1);
+
   return fn(`${
-    addNumericSeparator(string)
+    addNumericSeparator(integerPart)
   }.${
-    addNumericSeparatorEnd(
-      StringPrototypeSlice(String(number), string.length + 1),
-    )
+    addNumericSeparatorEnd(fractionalPart)
   }`, 'number');
 }
 
diff --git a/test/parallel/test-util-inspect.js b/test/parallel/test-util-inspect.js
index 5d90e0606f5..30ebdad8901 100644
--- a/test/parallel/test-util-inspect.js
+++ b/test/parallel/test-util-inspect.js
@@ -3484,6 +3484,27 @@ assert.strictEqual(
     util.inspect(-123456789.12345678, { numericSeparator: true }),
     '-123_456_789.123_456_78'
   );
+
+  // Regression test for https://github.com/nodejs/node/issues/59376
+  // numericSeparator should work correctly for negative fractional numbers
+  {
+    // Test the exact values from the GitHub issue
+    const values = [0.1234, -0.12, -0.123, -0.1234, -1.234];
+    assert.strictEqual(
+      util.inspect(values, { numericSeparator: true }),
+      '[ 0.123_4, -0.12, -0.123, -0.123_4, -1.234 ]'
+    );
+
+    // Test individual negative fractional numbers between -1 and 0
+    assert.strictEqual(
+      util.inspect(-0.1234, { numericSeparator: true }),
+      '-0.123_4'
+    );
+    assert.strictEqual(
+      util.inspect(-0.12345, { numericSeparator: true }),
+      '-0.123_45'
+    );
+  }
 }
 
 // Regression test for https://github.com/nodejs/node/issues/41244