Fix util.inspect negative fractional number formatting with numericSeparator

Fixes #23098

When numericSeparator: true is set, negative fractional numbers between -1 and 0
(like -0.123) were incorrectly formatted as "0..123" instead of "-0.123".

The issue was that Math.trunc(-0.123) returns -0, but String(-0) becomes "0",
losing the negative sign. Fixed by checking for -0 with Object.is() and
explicitly using "-0" as the string representation.

Also fixed scientific notation handling to check the original number string
instead of the truncated integer string.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

src/js/internal/util/inspect.js

@@ -1967,16 +1967,22 @@ function formatNumber(fn, number, numericSeparator) {
     return fn(`${number}`, "number");
   }
   const integer = MathTrunc(number);
-  const string = String(integer);
+  // Handle -0 specially to preserve the negative sign
+  const string = ObjectIs(integer, -0) ? "-0" : String(integer);
   if (integer === number) {
-    if (!NumberIsFinite(number) || StringPrototypeIncludes(string, "e")) {
-      return fn(string, "number");
+    // Check the original number string for scientific notation, not the integer string
+    if (!NumberIsFinite(number) || StringPrototypeIncludes(String(number), "e")) {
+      return fn(`${number}`, "number");
     }
     return fn(`${addNumericSeparator(string)}`, "number");
   }
   if (NumberIsNaN(number)) {
     return fn(string, "number");
   }
+  // Check the original number string for scientific notation before applying separators
+  if (StringPrototypeIncludes(String(number), "e")) {
+    return fn(`${number}`, "number");
+  }
   return fn(
     `${addNumericSeparator(string)}.${addNumericSeparatorEnd(StringPrototypeSlice(String(number), string.length + 1))}`,
     "number",

test/regression/issue/23098.test.ts

@@ -0,0 +1,28 @@
+import { test, expect } from "bun:test";
+import { inspect } from "node:util";
+
+test("util.inspect formats negative fractional numbers correctly with numericSeparator: true (#23098)", () => {
+  // Test the specific cases from the issue
+  const values = [0.1234, -0.12, -0.123, -0.1234, -1.234];
+  const text = inspect(values, { numericSeparator: true });
+  expect(text).toBe("[ 0.123_4, -0.12, -0.123, -0.123_4, -1.234 ]");
+
+  // Test individual values
+  expect(inspect(-0.12, { numericSeparator: true })).toBe("-0.12");
+  expect(inspect(-0.123, { numericSeparator: true })).toBe("-0.123");
+  expect(inspect(-0.1234, { numericSeparator: true })).toBe("-0.123_4");
+  expect(inspect(-0.123456789, { numericSeparator: true })).toBe("-0.123_456_789");
+
+  // Test edge cases
+  expect(inspect(-0, { numericSeparator: true })).toBe("-0");
+  expect(inspect(0, { numericSeparator: true })).toBe("0");
+
+  // Test scientific notation doesn't get separators
+  expect(inspect(1.23e-10, { numericSeparator: true })).toBe("1.23e-10");
+  expect(inspect(-1.23e-10, { numericSeparator: true })).toBe("-1.23e-10");
+  expect(inspect(1.23e10, { numericSeparator: true })).toBe("12_300_000_000");
+  expect(inspect(-1.23e10, { numericSeparator: true })).toBe("-12_300_000_000");
+
+  // Test large negative numbers still work
+  expect(inspect(-123456789.123456789, { numericSeparator: true })).toBe("-123_456_789.123_456_79");
+});