Vielzeug

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/herald/testing';

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() uses the same synchronous delivery as createBus() and records dispatch before listeners run, including dispatches whose listeners throw.
  • emitted(event) returns every recorded payload in order and returns a copied array.
  • reset() clears only recording history; existing listeners remain active.
  • allEmitted() omits __proto__, constructor, and prototype keys to keep its plain result object safe. Use emitted(event) for those names.