Treat two Invalid Dates as equal in deepEquals and fix their failure-message rendering

src/jsc/bindings/bindings.cpp

@@ -1180,7 +1180,10 @@ std::optional<bool> specialObjectsDequal(JSC::JSGlobalObject* globalObject, Mark
         JSC::DateInstance* left = uncheckedDowncast<DateInstance>(c1);
         JSC::DateInstance* right = uncheckedDowncast<DateInstance>(c2);
 
-        return left->internalNumber() == right->internalNumber();
+        double leftTime = left->internalNumber();
+        double rightTime = right->internalNumber();
+        // Two Invalid Dates (NaN time values) are equal, matching Node.js.
+        return leftTime == rightTime || (std::isnan(leftTime) && std::isnan(rightTime));
     }
     case RegExpObjectType: {
         if (c2Type != RegExpObjectType) {

src/runtime/test_runner/pretty_format.rs

@@ -1937,7 +1937,10 @@ impl<'a> Formatter<'a> {
                                 Err(_) => b"",
                             }
                         };
-                        if out_buf.len() > 2 {
+                        if out_buf == b"null" {
+                            // JSON.stringify(new Date(NaN)) is `null` (unquoted)
+                            out_buf = b"Invalid Date";
+                        } else if out_buf.len() > 2 {
                             // trim the quotes
                             out_buf = &out_buf[1..out_buf.len() - 1];
                         }

test/js/bun/bun-object/deep-equals.spec.ts

@@ -14,6 +14,7 @@ describe.each([true, false])("Bun.deepEquals(a, b, strict: %p)", strict => {
     [new Set(), new Set()],
     [Symbol.for("foo"), Symbol.for("foo")],
     [NaN, NaN],
+    [new Date(NaN), new Date(NaN)],
   ])("Bun.deepEquals(%p, %p) === true, regardless of strict modee", (a, b) => {
     expect(Bun.deepEquals(a, b, true)).toBe(true);
     expect(Bun.deepEquals(a, b, false)).toBe(true);
@@ -24,6 +25,7 @@ describe.each([true, false])("Bun.deepEquals(a, b, strict: %p)", strict => {
     [-0, +0], //
     [{ a: 1 }, { a: 2 }],
     ["foo", "bar"],
+    [new Date(NaN), new Date(0)],
   ])("Bun.deepEquals(%p, %p) !== true, regardless of strict modee", (a, b) => {
     expect(Bun.deepEquals(a, b, true)).toBe(false);
     expect(Bun.deepEquals(a, b, false)).toBe(false);

test/js/bun/test/expect-invalid-date-message.test.ts

@@ -0,0 +1,29 @@
+// Failure messages for Invalid Date must print "Invalid Date".
+// JSON.stringify(new Date(NaN)) is `null` (unquoted), and the quote-trimming
+// pass used to slice it down to the garbage string "ul".
+
+import { expect, test } from "bun:test";
+import { bunEnv, bunExe, tempDir } from "harness";
+
+test("expect() failure messages render Invalid Date, not a sliced 'null'", async () => {
+  using dir = tempDir("expect-invalid-date", {
+    "invalid-date.test.ts": `
+      import { test, expect } from "bun:test";
+      test("invalid date vs valid date", () => {
+        expect(new Date(NaN)).toEqual(new Date(0));
+      });
+    `,
+  });
+  await using proc = Bun.spawn({
+    cmd: [bunExe(), "test", "invalid-date.test.ts"],
+    env: { ...bunEnv, NO_COLOR: "1", FORCE_COLOR: undefined },
+    cwd: String(dir),
+    stdout: "pipe",
+    stderr: "pipe",
+  });
+  const [, stderr, exitCode] = await Promise.all([proc.stdout.text(), proc.stderr.text(), proc.exited]);
+
+  expect(stderr).toContain("Expected: 1970-01-01T00:00:00.000Z");
+  expect(stderr).toContain("Received: Invalid Date");
+  expect(exitCode).toBe(1);
+});

test/js/bun/test/expect.test.js

@@ -722,6 +722,16 @@ describe("expect()", () => {
     expect(d).toEqual(e);
     expect(e).toEqual(d);
 
+    if (isBun) {
+      // Two Invalid Dates are equal, matching Node.js (jest disagrees).
+      expect(new Date(NaN)).toEqual(new Date(NaN));
+      expect(new Date(NaN)).toStrictEqual(new Date(NaN));
+      expect(new Date(NaN)).not.toEqual(new Date(0));
+      expect(new Date(NaN)).not.toStrictEqual(new Date(0));
+      expect(new Date(0)).not.toEqual(new Date(NaN));
+      expect(new Date(0)).not.toStrictEqual(new Date(NaN));
+    }
+
     class Date2 extends Date {
       constructor() {
         // @ts-ignore

test/js/node/assert/deep-equal.test.ts

@@ -287,6 +287,8 @@ const cases: Case[] = [
   // Date.
   { name: "two equal dates", a: () => new Date(0), b: () => new Date(0), strict: true, loose: true },
   { name: "two different dates", a: () => new Date(0), b: () => new Date(1), strict: false, loose: false },
+  { name: "two invalid dates", a: () => new Date(NaN), b: () => new Date(NaN), strict: true, loose: true },
+  { name: "an invalid and a valid date", a: () => new Date(NaN), b: () => new Date(0), strict: false, loose: false },
   {
     name: "a date with an extra own property",
     a: () => withExtraProperty(new Date(0)),