fix: preserve invalid Date instead of dropping it to null

src/index.test.ts

@@ -197,6 +197,25 @@ describe('stringify & parse', () => {
       },
     },
 
+    'works for invalid dates': {
+      input: {
+        date: new Date('invalid'),
+      },
+      output: {
+        date: 'Invalid Date',
+      },
+      outputAnnotations: {
+        values: {
+          date: ['Date'],
+        },
+      },
+      dontExpectEquality: true,
+      customExpectations: value => {
+        expect(value.date).toBeInstanceOf(Date);
+        expect(Number.isNaN(value.date.getTime())).toBe(true);
+      },
+    },
+
     'works for Errors': {
       input: {
         e: new Error('epic fail'),
@@ -1431,3 +1450,23 @@ test('#310 fixes backwards compat', () => {
     },
   });
 });
+
+test('#355 invalid dates are backwards compatible', () => {
+  // payloads from previous versions (ISO strings) still parse unchanged
+  const date = new Date(2020, 1, 1);
+  expect(
+    SuperJSON.deserialize({
+      json: { date: date.toISOString() },
+      meta: { values: { date: ['Date'] } },
+    })
+  ).toEqual({ date });
+
+  // the "Invalid Date" sentinel goes through the same `new Date(string)`
+  // path older versions use, so they revive it to an invalid Date too
+  const result = SuperJSON.deserialize<{ date: Date }>({
+    json: { date: 'Invalid Date' },
+    meta: { values: { date: ['Date'] } },
+  });
+  expect(result.date).toBeInstanceOf(Date);
+  expect(Number.isNaN(result.date.getTime())).toBe(true);
+});

src/transformer.ts

@@ -1,6 +1,5 @@
 import {
   isBigint,
-  isDate,
   isInfinite,
   isMap,
   isNaNValue,
@@ -73,9 +72,13 @@ const simpleRules = [
     }
   ),
   simpleTransformation(
-    isDate,
+    (v): v is Date => v instanceof Date,
     'Date',
-    v => v.toISOString(),
+    // An invalid Date (`new Date(NaN)`) has no ISO representation and would
+    // otherwise be dropped to `null`, losing its type. Serialize it as
+    // "Invalid Date", which `new Date('Invalid Date')` revives back into an
+    // invalid Date - also on older versions that just call `new Date(v)`.
+    v => (isNaN(v.valueOf()) ? 'Invalid Date' : v.toISOString()),
     v => new Date(v)
   ),