Cron-Expression Scheduler with Field Parser and a Tick Loop in TypeScript

import { CronFields, parseExpression, matches } from "./cron-parser";

interface ScheduledTask {
  id: string;
  fields: CronFields;
  handler: () => void | Promise<void>;
}

export class CronScheduler {
  private tasks = new Map<string, ScheduledTask>();
  private running = new Set<string>();
  private timer: ReturnType<typeof setInterval> | null = null;
  private lastRun: string | null = null;

  constructor(private onError: (id: string, err: unknown) => void = () => {}) {}

  register(id: string, expression: string, handler: () => void | Promise<void>): void {
    if (this.tasks.has(id)) throw new Error(`Task "${id}" already registered`);
    this.tasks.set(id, { id, fields: parseExpression(expression), handler });
  }

  unregister(id: string): void {
    this.tasks.delete(id);
  }

  start(): void {
    if (this.timer) return;
    this.timer = setInterval(() => this.tick(new Date()), 15_000);
  }

  stop(): void {
    if (this.timer) clearInterval(this.timer);
    this.timer = null;
  }

  private tick(now: Date): void {
    const minuteKey = `${now.getFullYear()}-${now.getMonth()}-${now.getDate()}-${now.getHours()}-${now.getMinutes()}`;
    if (minuteKey === this.lastRun) return;
    this.lastRun = minuteKey;

    for (const task of this.tasks.values()) {
      if (!matches(task.fields, now)) continue;
      if (this.running.has(task.id)) continue;
      this.run(task);
    }
  }

  private async run(task: ScheduledTask): Promise<void> {
    this.running.add(task.id);
    try {
      await task.handler();
    } catch (err) {
      this.onError(task.id, err);
    } finally {
      this.running.delete(task.id);
    }
  }
}

import { CronScheduler } from "./CronScheduler";

const scheduler = new CronScheduler((id, err) => {
  console.error(`Task "${id}" failed:`, err);
});

scheduler.register("cleanup-temp", "*/5 * * * *", async () => {
  console.log("Purging temp files...");
  // await purgeTempFiles();
});

scheduler.register("daily-report", "0 6 * * 1-5", async () => {
  console.log("Generating weekday morning report...");
  // await sendDailyReport();
});

scheduler.register("hourly-sync", "0 * * * *", () => {
  console.log("Syncing upstream inventory...");
});

scheduler.start();

process.on("SIGINT", () => {
  console.log("Shutting down scheduler...");
  scheduler.stop();
  process.exit(0);
});