Add support for Temporal types

[SamPearlma]

Adds serialization support for all eight Temporal types: Instant,
Duration, PlainDate, PlainDateTime, PlainMonthDay, PlainTime,
PlainYearMonth, ZonedDateTime.

Values are encoded as their ISO / RFC 9557 format string form and restored via each
constructor's .from(). Temporal globals are referenced lazily, so
runtimes without Temporal are unaffected.

Also generalizes the test skipOnNode10 flag into a reusable skip
predicate to gate Temporal cases on runtime support.

Test plan

• npm test passes
• npm run test:temporal passes on a Temporal-capable Node
• npm run build succeeds

Greptile Summary

Adds first-class serialization support for all eight TC39 Temporal types by introducing a lazy-initialized temporalRule in transformer.ts and corresponding isTemporal type guards in is.ts. Temporal globals are only accessed at call time, so runtimes without Temporal are unaffected. The test suite is well-structured with gated cases for all eight types, and the skip generalization cleanly replaces the old skipOnNode10 flag.

Confidence Score: 5/5

Safe to merge — all remaining findings are P2 style suggestions.

The core Temporal serialization/deserialization logic is correct: lazy init guard is safe for non-Temporal runtimes, toString()/from() round-trips are correct for all 8 types, annotation lookup via constructor name is consistent, and test gating with skip: !hasTemporal works correctly. The only open item is a P2 suggestion to use NODE_OPTIONS instead of a hardcoded internal vitest path.

package.json — test:temporal script uses a vitest-internal file path that could break on upgrade.

Important Files Changed

Filename	Overview
src/transformer.ts	Adds temporalRule using compositeTransformation with lazy-initialized constructor map; serializes via .toString(), deserializes via ctor.from(); ordering in compositeRules correctly places it after custom/class rules.
src/is.ts	Adds isTemporal and per-type guards using Object.prototype.toString tag matching; safely returns false on non-Temporal runtimes since no Temporal global is referenced at runtime.
src/index.test.ts	Generalises skipOnNode10 to skip predicate; adds 10 Temporal test cases all correctly guarded with skip: !hasTemporal; output expectations and custom assertions look correct for all 8 Temporal types.
src/is.test.ts	Imports and tests isTemporal; false-path tests run unconditionally, true-path tests correctly gated with test.skipIf(typeof Temporal === 'undefined').
package.json	Adds test:temporal script using a direct path to vitest internals (vitest.mjs) which is fragile across upgrades; TypeScript bumped to 6.0.2.
tsconfig.json	Adds ignoreDeprecations: '6.0' for TypeScript 6 upgrade and types: ['node'] to restrict global type auto-inclusion; intentional to enable Temporal type support.
package-lock.json	Lockfile updated to reflect TypeScript 6.0.2 and removal of some optional peer dependencies; no anomalies.

Flowchart

%%{init: {'theme': 'neutral'}}%%
flowchart TD
    A[Input value] --> B{isTemporal?}
    B -- Yes --> C[annotation: temporal + constructor.name]
    C --> D[transform: t.toString()]
    D --> E[JSON string e.g. 2024-01-01T10:00:00Z]
    E --> F{untransformValue type === temporal?}
    F -- Yes --> G[getTemporalConstructors]
    G --> H{Temporal defined?}
    H -- No --> I[throw: Temporal not available]
    H -- Yes --> J[Lazy-init name to ctor map]
    J --> K[lookup ctor by annotation name]
    K --> L[ctor.from serializedString]
    L --> M[Restored Temporal instance]
    B -- No --> N[other rules / pass through]

Loading

Prompt To Fix All With AI

This is a comment left during a code review.
Path: package.json
Line: 19

Comment:
**`test:temporal` hardcodes vitest internals**

`./node_modules/vitest/vitest.mjs` couples the script to vitest's internal package layout. When vitest is upgraded this path could change silently. The standard pattern for passing Node.js flags through an npm script is `NODE_OPTIONS`:

```suggestion
    "test:temporal": "NODE_OPTIONS='--harmony-temporal' vitest run",
```

This also ensures the flag propagates to any child/worker processes that vitest may spawn, whereas the current approach relies on V8 flags being shared with worker-threads (which they are in the default `--threads` mode, but wouldn't be if `--pool=forks` is ever used).

How can I resolve this? If you propose a fix, please make it concise.

_{Reviews (2): Last reviewed commit: "fix: address scary Greptile style concer..." | Re-trigger Greptile}

[greptile-apps]

Inverted variable name and misleading comment

temporalConstructorToName maps name → constructor (a Record<string, TemporalConstructor>), which is the opposite of what the name implies. The comment above it also says "lazily initialize array" but this is a plain object/Record. A future reader looking for how a constructor maps to its name will be confused. Consider renaming to nameToTemporalConstructor (or just temporalConstructors) and updating the comment.

Suggested change

	// Can not define temporalConstructorToName within global scope
	// as Temporal may be undefined (don't introduce breaking changes).
	// So lazily initialize array
	let temporalConstructorToName: Record<string, TemporalConstructor> | undefined;
	// Lazily initialized to avoid referencing Temporal at module load time on
	// runtimes that don't support it.
	let nameToTemporalConstructor: Record<string, TemporalConstructor> | undefined;

Prompt To Fix With AI

This is a comment left during a code review.
Path: src/transformer.ts
Line: 260-263

Comment:
**Inverted variable name and misleading comment**

`temporalConstructorToName` maps *name → constructor* (a `Record<string, TemporalConstructor>`), which is the opposite of what the name implies. The comment above it also says "lazily initialize array" but this is a plain object/Record. A future reader looking for how a constructor maps to its name will be confused. Consider renaming to `nameToTemporalConstructor` (or just `temporalConstructors`) and updating the comment.

```suggestion
// Lazily initialized to avoid referencing Temporal at module load time on
// runtimes that don't support it.
let nameToTemporalConstructor: Record<string, TemporalConstructor> | undefined;
```

How can I resolve this? If you propose a fix, please make it concise.

Note: If this suggestion doesn't match your team's coding style, reply to this and let me know. I'll remember it for next time!

[SamPearlma]

Mirrors earlier constructorToName

[SamPearlma]

In case anyone comes across this without it being implemented, you can achieve the same functionality by doing

const temporalTypes = [
	Temporal.Instant,
	Temporal.ZonedDateTime,
	Temporal.PlainDateTime,
	Temporal.PlainDate,
	Temporal.PlainTime,
	Temporal.PlainYearMonth,
	Temporal.PlainMonthDay,
	Temporal.Duration,
] as const;

for (const Type of temporalTypes) {
	superjson.registerCustom<InstanceType<typeof Type>, string>(
		{
			isApplicable: (v): v is InstanceType<typeof Type> => v instanceof Type,
			serialize: (v) => v.toString(),
			deserialize: (v) => Type.from(v as never) as InstanceType<typeof Type>,
		},
		`Temporal.${Type.name}`,
	);
}

If you are sending between separate servers and clients, make sure you register the custom encoders on both