106Implement a circuit breaker with CLOSED, OPEN, and HALF_OPEN states

When a dependency is down, retrying every request makes things worse: users wait for timeouts and the struggling service gets hammered. A circuit breaker fails fast instead.

Implement createCircuitBreaker(fn, { failureThreshold = 5, resetTimeout = 10000, halfOpenMaxCalls = 1, now = Date.now, onStateChange = () => {} } = {}) returning { call(...args), state }:

• CLOSED (normal): calls go through. Count consecutive failures; a success resets the count. Reaching failureThreshold opens the circuit.
• OPEN: every call rejects immediately with an error whose name is "CircuitOpenError", without calling fn. Once resetTimeout ms have passed since opening, the next call moves to HALF_OPEN.
• HALF_OPEN: allow up to halfOpenMaxCalls trial calls at the same time; further calls fail fast. A successful trial closes the circuit (and resets the count); a failed trial opens it again (restarting the timeout).
• onStateChange(from, to) is called on every transition.

const getPrices = createCircuitBreaker((ids) => api.prices(ids), { failureThreshold: 3, resetTimeout: 30000 });
try {
  render(await getPrices.call(ids));
} catch (error) {
  if (error.name === "CircuitOpenError") renderCachedPrices();
  else throw error;
}