sse-session/tests/utils/event-emitter.test.ts

import { expect, test, vi } from 'vitest';
import { EventEmitter } from '../../src/utils/event-emitter.js';

/** Simple event map used across these tests. */
type TestEvents = {
    message: string;
    count: number;
};

/**
 * Tests that EventEmitter invokes listeners when an event is emitted.
 */
const testEventEmitterCallsListeners = (): void => {
    const emitter = new EventEmitter<TestEvents>();
    const listener = vi.fn();

    // Register the listener and emit an event.
    emitter.on('message', listener);
    emitter.emit('message', 'hello');

    // Expect the listener to have been called with the emitted payload.
    expect(listener).toHaveBeenCalledOnce();
    expect(listener).toHaveBeenCalledWith('hello');
};

/**
 * Tests that EventEmitter invokes all listeners registered for the same event.
 */
const testEventEmitterCallsMultipleListeners = (): void => {
    const emitter = new EventEmitter<TestEvents>();
    const firstListener = vi.fn();
    const secondListener = vi.fn();

    // Register two listeners for the same event type.
    emitter.on('count', firstListener);
    emitter.on('count', secondListener);
    emitter.emit('count', 42);

    // Expect both listeners to receive the same payload.
    expect(firstListener).toHaveBeenCalledOnce();
    expect(firstListener).toHaveBeenCalledWith(42);
    expect(secondListener).toHaveBeenCalledOnce();
    expect(secondListener).toHaveBeenCalledWith(42);
};

/**
 * Tests that EventEmitter only invokes listeners registered for the emitted event type.
 */
const testEventEmitterCallsOnlyMatchingListeners = (): void => {
    const emitter = new EventEmitter<TestEvents>();
    const messageListener = vi.fn();
    const countListener = vi.fn();

    // Register listeners on different event types.
    emitter.on('message', messageListener);
    emitter.on('count', countListener);

    // Emit only the message event.
    emitter.emit('message', 'hello');

    // Expect only the matching listener to have been called.
    expect(messageListener).toHaveBeenCalledOnce();
    expect(countListener).not.toHaveBeenCalled();
};

/**
 * Tests that EventEmitter.emit returns false when no listeners are registered.
 */
const testEventEmitterEmitReturnsFalseWithNoListeners = (): void => {
    const emitter = new EventEmitter<TestEvents>();

    const hasListeners = emitter.emit('message', 'hello');

    // Expect emit to report that nobody was listening.
    expect(hasListeners).toBe(false);
};

/**
 * Tests that EventEmitter.emit returns true when listeners are registered.
 */
const testEventEmitterEmitReturnsTrueWithListeners = (): void => {
    const emitter = new EventEmitter<TestEvents>();

    emitter.on('message', vi.fn());

    const hasListeners = emitter.emit('message', 'hello');

    // Expect emit to report that at least one listener was invoked.
    expect(hasListeners).toBe(true);
};

/**
 * Tests that the off callback returned by on() removes the listener.
 */
const testEventEmitterOffCallbackRemovesListener = (): void => {
    const emitter = new EventEmitter<TestEvents>();
    const listener = vi.fn();

    // on() returns an off callback that removes the listener.
    const off = emitter.on('message', listener);
    emitter.emit('message', 'first');

    // Unsubscribe before emitting again.
    off();
    emitter.emit('message', 'second');

    // Expect the listener to have only received the first event.
    expect(listener).toHaveBeenCalledOnce();
    expect(listener).toHaveBeenCalledWith('first');
};

/**
 * Tests that off() removes a listener when given the same function reference.
 */
const testEventEmitterOffRemovesListenerByReference = (): void => {
    const emitter = new EventEmitter<TestEvents>();
    const listener = vi.fn();

    emitter.on('message', listener);
    emitter.off('message', listener);
    emitter.emit('message', 'hello');

    // Expect the listener to have been removed before the emit.
    expect(listener).not.toHaveBeenCalled();
};

/**
 * Tests that off() does nothing when given an unknown listener reference.
 */
const testEventEmitterOffIgnoresUnknownListener = (): void => {
    const emitter = new EventEmitter<TestEvents>();
    const listener = vi.fn();

    emitter.on('message', listener);

    // Try to remove a different function reference.
    emitter.off('message', vi.fn());
    emitter.emit('message', 'hello');

    // Expect the original listener to still receive the event.
    expect(listener).toHaveBeenCalledOnce();
};

/**
 * Tests that off() does nothing when called for an event type with no listeners.
 */
const testEventEmitterOffIgnoresUnregisteredEventType = (): void => {
    const emitter = new EventEmitter<TestEvents>();
    const listener = vi.fn();

    // Call off without ever registering this listener.
    emitter.off('message', listener);

    expect(listener).not.toHaveBeenCalled();
};

/**
 * Tests that once() listeners are invoked only one time.
 */
const testEventEmitterOnceListenerFiresOnce = (): void => {
    const emitter = new EventEmitter<TestEvents>();
    const listener = vi.fn();

    emitter.once('message', listener);
    emitter.emit('message', 'first');
    emitter.emit('message', 'second');

    // Expect the listener to auto-unsubscribe after the first emit.
    expect(listener).toHaveBeenCalledOnce();
    expect(listener).toHaveBeenCalledWith('first');
};

/**
 * Tests that once() can be added when regular listeners already exist for the event type.
 */
const testEventEmitterOnceWorksWithExistingListeners = (): void => {
    const emitter = new EventEmitter<TestEvents>();
    const existingListener = vi.fn();
    const onceListener = vi.fn();

    // Register a regular listener first so the event type already exists in the map.
    emitter.on('message', existingListener);
    emitter.once('message', onceListener);
    emitter.emit('message', 'hello');

    expect(existingListener).toHaveBeenCalledOnce();
    expect(onceListener).toHaveBeenCalledOnce();
};

/**
 * Tests that the off callback returned by once() removes the listener before it fires.
 */
const testEventEmitterOnceOffCallbackRemovesListener = (): void => {
    const emitter = new EventEmitter<TestEvents>();
    const listener = vi.fn();

    const off = emitter.once('message', listener);

    // Unsubscribe before the event is ever emitted.
    off();
    emitter.emit('message', 'hello');

    expect(listener).not.toHaveBeenCalled();
};

/**
 * Tests that removeAllListeners() clears every registered listener.
 */
const testEventEmitterRemoveAllListeners = (): void => {
    const emitter = new EventEmitter<TestEvents>();
    const messageListener = vi.fn();
    const countListener = vi.fn();

    emitter.on('message', messageListener);
    emitter.on('count', countListener);
    emitter.removeAllListeners();

    // Emit on both event types after clearing all listeners.
    emitter.emit('message', 'hello');
    emitter.emit('count', 1);

    expect(messageListener).not.toHaveBeenCalled();
    expect(countListener).not.toHaveBeenCalled();
};

/**
 * Tests that waitFor() resolves when a matching event is emitted.
 */
const testEventEmitterWaitForResolvesOnMatch = async (): Promise<void> => {
    const emitter = new EventEmitter<TestEvents>();

    // Wait until an event matches the predicate.
    const waitPromise = emitter.waitFor('count', (payload) => payload === 42);

    // Emit a non-matching event first, then the matching one.
    emitter.emit('count', 41);
    emitter.emit('count', 42);

    await expect(waitPromise).resolves.toBe(42);
};

/**
 * Tests that waitFor() ignores non-matching events while other listeners still receive them.
 */
const testEventEmitterWaitForIgnoresNonMatchingEvents = async (): Promise<void> => {
    const emitter = new EventEmitter<TestEvents>();
    const listener = vi.fn();

    const waitPromise = emitter.waitFor('message', (payload) => payload === 'done');

    // A regular listener should still receive every emit while waitFor filters.
    emitter.on('message', listener);
    emitter.emit('message', 'pending');
    emitter.emit('message', 'done');

    await expect(waitPromise).resolves.toBe('done');
    expect(listener).toHaveBeenCalledTimes(2);
};

/**
 * Tests that waitFor() rejects when the timeout expires.
 */
const testEventEmitterWaitForRejectsOnTimeout = async (): Promise<void> => {
    vi.useFakeTimers();

    try {
        const emitter = new EventEmitter<TestEvents>();

        const waitPromise = emitter.waitFor('message', () => true, 100);

        // Attach the rejection handler before advancing timers so the rejection is handled.
        const assertion = expect(waitPromise).rejects.toThrow('Timeout waiting for event "message"');

        await vi.advanceTimersByTimeAsync(100);

        await assertion;
    } finally {
        vi.useRealTimers();
        vi.restoreAllMocks();
    }
};

/**
 * Tests that waitFor() clears its timeout when it resolves before expiry.
 */
const testEventEmitterWaitForClearsTimeoutOnResolve = async (): Promise<void> => {
    vi.useFakeTimers();

    try {
        const emitter = new EventEmitter<TestEvents>();

        // Register waitFor with a timeout, then resolve it before the timer fires.
        const waitPromise = emitter.waitFor('message', (payload) => payload === 'done', 100);

        emitter.emit('message', 'done');

        await expect(waitPromise).resolves.toBe('done');

        // If clearTimeout was not called, advancing past the timeout would reject the promise.
        await vi.advanceTimersByTimeAsync(100);
    } finally {
        vi.useRealTimers();
        vi.restoreAllMocks();
    }
};

/**
 * Tests that waitFor() removes its listener after resolving.
 */
const testEventEmitterWaitForRemovesListenerAfterResolve = async (): Promise<void> => {
    const emitter = new EventEmitter<TestEvents>();

    const waitPromise = emitter.waitFor('message', () => true);

    emitter.emit('message', 'first');
    await expect(waitPromise).resolves.toBe('first');

    // Register a second waitFor so we can verify the first listener was cleaned up.
    const secondWaitPromise = emitter.waitFor('message', (payload) => payload === 'second');

    // Emit a payload that only the second waitFor should accept.
    emitter.emit('message', 'ignored');

    // Track whether the second waitFor resolves too early.
    let resolvedEarly = false;
    /* eslint-disable-next-line */
    secondWaitPromise.then(() => {
        resolvedEarly = true;
    });

    // Yield so any premature resolution would have a chance to run.
    await Promise.resolve();
    expect(resolvedEarly).toBe(false);

    emitter.emit('message', 'second');
    await expect(secondWaitPromise).resolves.toBe('second');
};

/**
 * Tests that the first debounced emit does not call clearTimeout.
 */
const testEventEmitterDebouncedFirstEmitDoesNotClearTimeout = (): void => {
    vi.useFakeTimers();

    try {
        const emitter = new EventEmitter<TestEvents>();
        const clearTimeoutSpy = vi.spyOn(globalThis, 'clearTimeout');
        const listener = vi.fn();

        emitter.on('message', listener, 100);
        emitter.emit('message', 'first');

        // The first emit starts the debounce timer; there is nothing to clear yet.
        expect(clearTimeoutSpy).not.toHaveBeenCalled();
        expect(listener).not.toHaveBeenCalled();
    } finally {
        vi.useRealTimers();
        vi.restoreAllMocks();
    }
};

/**
 * Tests that debounced on() listeners receive only the last payload after the debounce window.
 */
const testEventEmitterDebouncedOnListener = async (): Promise<void> => {
    vi.useFakeTimers();

    try {
        const emitter = new EventEmitter<TestEvents>();
        const listener = vi.fn();

        emitter.on('message', listener, 100);

        // Emit several events in quick succession.
        emitter.emit('message', 'first');
        emitter.emit('message', 'second');
        emitter.emit('message', 'third');

        // Expect the listener to not have fired yet.
        expect(listener).not.toHaveBeenCalled();

        // Advance past the debounce window.
        await vi.advanceTimersByTimeAsync(100);

        // Expect only the last payload to have been delivered.
        expect(listener).toHaveBeenCalledOnce();
        expect(listener).toHaveBeenCalledWith('third');
    } finally {
        vi.useRealTimers();
        vi.restoreAllMocks();
    }
};

/**
 * Tests that repeated debounced emits reset the debounce timer.
 */
const testEventEmitterDebouncedTimerResetsOnRepeatedEmits = async (): Promise<void> => {
    vi.useFakeTimers();

    try {
        const emitter = new EventEmitter<TestEvents>();
        const listener = vi.fn();

        emitter.on('count', listener, 100);
        emitter.emit('count', 1);

        // Advance halfway through the debounce window and emit again.
        await vi.advanceTimersByTimeAsync(50);
        emitter.emit('count', 2);
        await vi.advanceTimersByTimeAsync(50);

        // The timer was reset, so the listener should not have fired yet.
        expect(listener).not.toHaveBeenCalled();

        // Advance the remaining time for the reset timer to expire.
        await vi.advanceTimersByTimeAsync(50);

        expect(listener).toHaveBeenCalledOnce();
        expect(listener).toHaveBeenCalledWith(2);
    } finally {
        vi.useRealTimers();
        vi.restoreAllMocks();
    }
};

/**
 * Tests that a debounce time of zero behaves like a normal listener.
 */
const testEventEmitterZeroDebounceDoesNotDebounce = async (): Promise<void> => {
    vi.useFakeTimers();

    try {
        const emitter = new EventEmitter<TestEvents>();
        const listener = vi.fn();

        // A debounce time of zero should behave like a normal listener.
        emitter.on('message', listener, 0);
        emitter.emit('message', 'first');
        emitter.emit('message', 'second');

        expect(listener).toHaveBeenCalledTimes(2);
    } finally {
        vi.useRealTimers();
        vi.restoreAllMocks();
    }
};

/**
 * Tests that debounced once() listeners fire once with the last payload.
 */
const testEventEmitterDebouncedOnceListener = async (): Promise<void> => {
    vi.useFakeTimers();

    try {
        const emitter = new EventEmitter<TestEvents>();
        const listener = vi.fn();

        emitter.once('message', listener, 100);
        emitter.emit('message', 'first');
        emitter.emit('message', 'second');

        await vi.advanceTimersByTimeAsync(100);

        // Expect the debounced once listener to fire once with the last payload.
        expect(listener).toHaveBeenCalledOnce();
        expect(listener).toHaveBeenCalledWith('second');

        // Emit again after the debounce window; the once listener should stay removed.
        emitter.emit('message', 'third');
        await vi.advanceTimersByTimeAsync(100);

        expect(listener).toHaveBeenCalledOnce();
    } finally {
        vi.useRealTimers();
        vi.restoreAllMocks();
    }
};

const runTests = async (): Promise<void> => {
    test('EventEmitter: calls listeners when an event is emitted', testEventEmitterCallsListeners);
    test('EventEmitter: calls multiple listeners for the same event', testEventEmitterCallsMultipleListeners);
    test('EventEmitter: only calls listeners for the emitted event type', testEventEmitterCallsOnlyMatchingListeners);
    test('EventEmitter: returns false when emitting with no listeners', testEventEmitterEmitReturnsFalseWithNoListeners);
    test('EventEmitter: returns true when emitting with listeners', testEventEmitterEmitReturnsTrueWithListeners);
    test('EventEmitter: stops calling a listener after its off callback is invoked', testEventEmitterOffCallbackRemovesListener);
    test('EventEmitter: removes a listener when off is called with the same reference', testEventEmitterOffRemovesListenerByReference);
    test('EventEmitter: ignores off when the listener reference is unknown', testEventEmitterOffIgnoresUnknownListener);
    test('EventEmitter: ignores off for an event type with no listeners', testEventEmitterOffIgnoresUnregisteredEventType);
    test('EventEmitter: calls a once listener only one time', testEventEmitterOnceListenerFiresOnce);
    test('EventEmitter: registers once when listeners already exist', testEventEmitterOnceWorksWithExistingListeners);
    test('EventEmitter: stops a once listener after its off callback is invoked', testEventEmitterOnceOffCallbackRemovesListener);
    test('EventEmitter: removes all listeners when removeAllListeners is called', testEventEmitterRemoveAllListeners);
    test('EventEmitter: resolves waitFor when a matching event is emitted', testEventEmitterWaitForResolvesOnMatch);
    test('EventEmitter: ignores non-matching events while waiting with waitFor', testEventEmitterWaitForIgnoresNonMatchingEvents);
    test('EventEmitter: rejects waitFor when the timeout is reached', testEventEmitterWaitForRejectsOnTimeout);
    test('EventEmitter: clears the timeout when waitFor resolves before expiry', testEventEmitterWaitForClearsTimeoutOnResolve);
    test('EventEmitter: removes the waitFor listener after it resolves', testEventEmitterWaitForRemovesListenerAfterResolve);
    test('EventEmitter: does not clear a timeout on the first debounced emit', testEventEmitterDebouncedFirstEmitDoesNotClearTimeout);
    test('EventEmitter: debounces on listeners', testEventEmitterDebouncedOnListener);
    test('EventEmitter: resets the debounce timer on repeated emits', testEventEmitterDebouncedTimerResetsOnRepeatedEmits);
    test('EventEmitter: does not debounce when debounceMilliseconds is zero', testEventEmitterZeroDebounceDoesNotDebounce);
    test('EventEmitter: debounces once listeners and invokes them only once', testEventEmitterDebouncedOnceListener);
};

await runTests();