Skip to content

Is undefined

isUndefined validates that a value is exactly undefined.

It performs a strict undefined check and rejects any value that is not undefined. If the value is not undefined, the rule emits a single validation event. Otherwise, it produces no validation output.

Signature

Through the API:

.isUndefined()

And internally:

export const isUndefined: ValidationRule
(value: unknown, path: FieldPath) => Promise<ReadonlyArray<JaneEvent>>

Events

Event code Description
value.not.undefined Value is not exactly undefined

Design rationale

  • Provides a strict, predictable undefined value validation.
  • Accepts only the exact undefined value.
  • Rejects null, empty strings, and other falsy values.
  • Never coerces or normalizes — validation is explicit and opt-in.
  • Emits exactly one event per failure for clarity and composability.
  • Async-compatible and returns a readonly array of JaneEvent objects.

Invoke

isUndefined runs only when explicitly included in a boundary or pipeline. It does not run automatically.

The rule activates when:

  • The value is any JavaScript value.
  • If the value is not exactly undefined, emits value.not.undefined.
  • If the value is undefined → returns an empty result set.

Examples

Valid undefined value

await isUndefined(undefined, "$");
// → []

Null value

await isUndefined(null, "$");
// → [
//     JaneEvent{
//       kind: "error",
//       code: "value.not.undefined",
//       path: "$",
//       ...
//     }
//   ]

Other values

await isUndefined("", "$");
// → [
//     JaneEvent{
//       kind: "error",
//       code: "value.not.undefined",
//       path: "$",
//       ...
//     }
//   ]