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Weekday

weekday validates that a Date instance falls on a specific weekday (0-6, where 0=Sunday, 6=Saturday).

It performs a strict type check ensuring the value is a valid Date instance, then validates that the UTC weekday matches the required value. If the value is not a Date, is invalid, or falls on a different weekday, the rule emits a single validation event. Otherwise, it produces no validation output.

Signature

Through the API:

.weekday(required: number)

And internally:

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

Events

Event code Description
date.not.date Value is not a Date instance
date.is.invalid Date instance represents an invalid date
date.not.weekday Date does not fall on the required weekday

Design rationale

  • Provides a strict, predictable weekday validation.
  • First ensures the value is a valid Date instance.
  • Then compares UTC weekday using getUTCDay() (0=Sunday, 6=Saturday).
  • Useful for business logic requiring specific days of the week.
  • 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

weekday 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 Date instance, emits date.not.date.
  • If the value is a Date but invalid, emits date.is.invalid.
  • If the value is a valid Date but wrong weekday, emits date.not.weekday.
  • If the value is a valid Date on the required weekday → returns an empty result set.

Examples

Correct weekday (Monday = 1)

await weekday(1)(new Date("2023-01-02"), "$"); // Monday
// → []

Wrong weekday

await weekday(1)(new Date("2023-01-01"), "$"); // Sunday
// → [
//     JaneEvent{
//       kind: "error",
//       code: "date.not.weekday",
//       path: "$",
//       ...
//     }
//   ]

Non-Date value

await weekday(1)("2023-01-02", "$");
// → [
//     JaneEvent{
//       kind: "error",
//       code: "date.not.date",
//       path: "$",
//       ...
//     }
//   ]