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Only keys

onlyKeys validates that a plain object contains only the specified allowed keys.

It performs a strict structural check ensuring the value is a plain object, then validates that all own keys are present in the allowed list. If the value is not a plain object or contains any disallowed keys, the rule emits a single validation event. Otherwise, it produces no validation output.

Signature

Through the API:

.onlyKeys(allowedKeys: readonly string[])

And internally:

export const onlyKeys = (allowedKeys: readonly string[]): ValidationRule
(value: unknown, path: FieldPath) => Promise<ReadonlyArray<JaneEvent>>

Events

Event code Description
object.not.plain-object Value is not a JSON-compatible plain object
object.has.extra-key Object contains keys not in the allowed list

Design rationale

  • Provides a strict, predictable allowed keys validation.
  • First ensures the value is a plain object, then checks key membership.
  • Accepts an array of allowed keys and reports all extra keys at once.
  • Useful for enforcing strict schemas where only specific keys are permitted.
  • 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

onlyKeys 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 a plain object, emits object.not.plain-object.
  • If the value is a plain object but contains any keys not in the allowed list, emits object.has.extra-key.
  • If the value is a plain object containing only allowed keys → returns an empty result set.

Examples

Valid object with only allowed keys

await onlyKeys(["name", "age"])({ name: "John", age: 30 }, "$");
// → []

Object with extra keys

await onlyKeys(["name", "age"])({ name: "John", age: 30, city: "NYC" }, "$");
// → [
//     JaneEvent{
//       kind: "error",
//       code: "object.has.extra-key",
//       path: "$",
//       ...
//     }
//   ]

Array (invalid)

await onlyKeys(["name", "age"])([1, 2, 3], "$");
// → [
//     JaneEvent{
//       kind: "error",
//       code: "object.not.plain-object",
//       path: "$",
//       ...
//     }
//   ]