// Real world r15

Testing transitions

Walk every legal path with a tiny generator. Assert that illegal actions are refused. Your test suite gets graph coverage for free.

Code

traffic.test.ts

test('cycle ends where it began', () => {
  const m = traffic();
  ['next', 'next', 'next'].forEach(a => m.go(a));
  expect(m.state()).toBe('red');
});

test('cannot stop on red', () => {
  const m = traffic();
  expect(m.valid_action('stop')).toBe(false);
});

Notes

The graph itself is the contract. Tests stop being about implementation details and start being about which paths through the graph matter to your users.