util: fix numericSeparator handling for scientific notation

lib/internal/util/inspect.js

@@ -2206,11 +2206,14 @@ function formatNumber(fn, number, numericSeparator) {
     }
     return fn(addNumericSeparator(numberString), 'number');
   }
-  if (NumberIsNaN(number)) {
+  if (NumberIsNaN(number) || StringPrototypeIncludes(numberString, 'e')) {
     return fn(numberString, 'number');
   }
 
   const decimalIndex = StringPrototypeIndexOf(numberString, '.');
+  if (decimalIndex === -1) {
+    return fn(addNumericSeparator(numberString), 'number');
+  }
   const integerPart = StringPrototypeSlice(numberString, 0, decimalIndex);
   const fractionalPart = StringPrototypeSlice(numberString, decimalIndex + 1);
 

test/parallel/test-util-inspect-scientific.js

@@ -0,0 +1,29 @@
+'use strict';
+
+require('../common');
+const assert = require('assert');
+const util = require('util');
+
+// Ensure scientific notation is not broken when numericSeparator is enabled
+{
+  const value = 1e-7;
+  const result = util.inspect(value, { numericSeparator: true });
+
+  assert.strictEqual(result, '1e-7');
+}
+
+// Another scientific notation case (positive exponent)
+{
+  const value = 1e+21;
+  const result = util.inspect(value, { numericSeparator: true });
+
+  assert.strictEqual(result, '1e+21');
+}
+
+// Control case: regular number should still apply numeric separators
+{
+  const value = 1000000;
+  const result = util.inspect(value, { numericSeparator: true });
+
+  assert.strictEqual(result, '1_000_000');
+}