The State Machine

// The smallest useful circuit breaker — three states, one rule.
type State = "CLOSED" | "OPEN" | "HALF_OPEN";

class CircuitBreaker {
  private state: State = "CLOSED";
  private failures = 0;          // current window
  private trials: boolean[] = [];
  private openUntil = 0;

  constructor(
    private failThreshold = 5,   // how many fails in CLOSED trip it
    private cooldownMs = 4000,   // how long OPEN waits before probing
    private trialCount = 3,      // how many HALF_OPEN trials to decide
  ) {}

  async call<T>(work: () => Promise<T>): Promise<T> {
    if (this.state === "OPEN") {
      if (Date.now() >= this.openUntil) this.state = "HALF_OPEN";
      else throw new Error("circuit open — fail fast");
    }

    try {
      const result = await work();
      this.onSuccess();
      return result;
    } catch (err) {
      this.onFailure();
      throw err;
    }
  }

  private onSuccess() {
    if (this.state === "HALF_OPEN") {
      this.trials.push(true);
      if (this.trials.length >= this.trialCount) this.close();
    } else {
      this.failures = 0;          // reset on a healthy run
    }
  }

  private onFailure() {
    if (this.state === "HALF_OPEN") {
      this.trip();                // one bad trial re-opens
      return;
    }
    if (++this.failures >= this.failThreshold) this.trip();
  }

  private trip()  { this.state = "OPEN";    this.openUntil = Date.now() + this.cooldownMs; this.trials = []; }
  private close() { this.state = "CLOSED";  this.failures = 0; this.trials = []; }
}