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Testing with createTestBus ​

Problem ​

You want to write unit tests for code that uses an event bus — asserting that specific events were emitted, in the right order, with the right payloads, without real side effects.

Solution ​

ts
import { describe, it, expect } from 'vitest';
import { createTestBus } from '@vielzeug/eventit/test';

type CartEvents = {
  'item:added': { id: string; qty: number };
  'cart:cleared': void;
};

describe('cart module', () => {
  it('emits item:added when item is added', () => {
    using bus = createTestBus<CartEvents>();

    addItemToCart(bus, { id: 'sku-1', qty: 2 });

    expect(bus.emitted('item:added')).toEqual([{ id: 'sku-1', qty: 2 }]);
  });

  it('emits cart:cleared on reset', () => {
    using bus = createTestBus<CartEvents>();

    clearCart(bus);

    expect(bus.emitted('cart:cleared')).toHaveLength(1);
  });

  it('reset() clears emission records without removing listeners', () => {
    using bus = createTestBus<CartEvents>();

    addItemToCart(bus, { id: 'sku-1', qty: 1 });
    bus.reset();

    expect(bus.emitted('item:added')).toHaveLength(0);
  });
});

Pitfalls ​

  • createTestBus delivers events synchronously. If your production code relies on microtask-level async delivery, the test may pass when the real code would fail. Verify the delivery timing matches production.
  • testBus.emitted(event) returns all payloads ever emitted under that name — not just the most recent. Assert on a specific index when emission order matters.
  • Calling testBus.reset() clears both emitted history and all active listeners. Re-register any listeners set up in beforeEach after calling reset().