accessing-the-current-time-with-clock

description: Use the Clock service to get the current time, enabling deterministic testing with TestClock. globs: "**/*.ts" alwaysApply: true

Accessing the Current Time with Clock

Rule: Use the Clock service to get the current time, enabling deterministic testing with TestClock.

Example

This example shows a function that checks if a token is expired. Its logic depends on Clock, making it fully testable.

import { Effect, Clock, Duration } from "effect";

interface Token {
  readonly value: string;
  readonly expiresAt: number; // UTC milliseconds
}

// This function is pure and testable because it depends on Clock
const isTokenExpired = (
  token: Token
): Effect.Effect<boolean, never, Clock.Clock> =>
  Clock.currentTimeMillis.pipe(
    Effect.map((now) => now > token.expiresAt),
    Effect.tap((expired) =>
      Clock.currentTimeMillis.pipe(
        Effect.flatMap((currentTime) =>
          Effect.log(
            `Token expired? ${expired} (current time: ${new Date(currentTime).toISOString()})`
          )
        )
      )
    )
  );

// Create a test clock service that advances time
const makeTestClock = (timeMs: number): Clock.Clock => ({
  currentTimeMillis: Effect.succeed(timeMs),
  currentTimeNanos: Effect.succeed(BigInt(timeMs * 1_000_000)),
  sleep: (duration: Duration.Duration) => Effect.succeed(void 0),
  unsafeCurrentTimeMillis: () => timeMs,
  unsafeCurrentTimeNanos: () => BigInt(timeMs * 1_000_000),
  [Clock.ClockTypeId]: Clock.ClockTypeId,
});

// Create a token that expires in 1 second
const token = { value: "abc", expiresAt: Date.now() + 1000 };

// Check token expiry with different clocks
const program = Effect.gen(function* () {
  // Check with current time
  yield* Effect.log("Checking with current time...");
  yield* isTokenExpired(token);

  // Check with past time
  yield* Effect.log("\nChecking with past time (1 minute ago)...");
  const pastClock = makeTestClock(Date.now() - 60_000);
  yield* isTokenExpired(token).pipe(
    Effect.provideService(Clock.Clock, pastClock)
  );

  // Check with future time
  yield* Effect.log("\nChecking with future time (1 hour ahead)...");
  const futureClock = makeTestClock(Date.now() + 3600_000);
  yield* isTokenExpired(token).pipe(
    Effect.provideService(Clock.Clock, futureClock)
  );
});

// Run the program with default clock
Effect.runPromise(
  program.pipe(Effect.provideService(Clock.Clock, makeTestClock(Date.now())))
);