util: fix numericSeparator with negative fractional numbers

Han5991 · richardlau · commit f66094347176 · 2025-09-20T06:17:30.000Z
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: #59376 PR-URL: #59379 Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de> Reviewed-By: James M Snell <jasnell@gmail.com>
diff --git 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
@@ -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
