Test Data Guide

Defining Type-Safe Test Fixtures with as const satisfies

Define dummy data for tests with as const satisfies Type. This preserves discriminant literal types and prevents widening.

const waitingRequest = {
  kind: "Waiting",
  passengerId: "passenger-1" as PassengerId,
} as const satisfies Waiting;

// waitingRequest.kind is the "Waiting" literal type (not string)

Why as const Alone Is Not Enough

as const preserves literal types, but does not verify that the object is compatible with the expected type. Adding satisfies Type guarantees type compatibility at compile time while still keeping the literal types.

// ❌ No type checking — typos go undetected
const bad = {
  kind: "Waitng", // typo is silently ignored
  passengerId: "passenger-1" as PassengerId,
} as const;

// ✅ Type checked + literal types preserved
const good = {
  kind: "Waiting",
  passengerId: "passenger-1" as PassengerId,
} as const satisfies Waiting;