Tests

Keep logic that does not need the interface in plain modules, and test it with Vitest. Vitest reads the project's Vite config and needs no setup of its own:

npm install -D vitest

src/stock.ts:

/** How a stock count is shown. */
export function stockLabel(count: number): string {
  if (count <= 0) return 'Sold out';
  return count < 5 ? `Only ${count} left` : 'In stock';
}

src/stock.test.ts:

import { describe, expect, it } from 'vitest';
import { stockLabel } from './stock';

describe('stockLabel', () => {
  it('says when little is left', () => {
    expect(stockLabel(0)).toBe('Sold out');
    expect(stockLabel(3)).toBe('Only 3 left');
    expect(stockLabel(12)).toBe('In stock');
  });
});

npx vitest runs the tests on every save, and npx vitest run runs them once. Test files never end up in the plugin, since the build takes only what src/index.tsx imports. Try components in the CMS itself, with the dev server.