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Pascual Vila

Frontend Instructor // Code Syllabus

Common Pitfalls & Errors

The Error //

Using setInterval directly for a recurring task in an application deployed with multiple replicas

// Wrong: runs once PER REPLICA, not once total
setInterval(sendDailyDigest, 24 * 60 * 60 * 1000);

// Correct: Redis-coordinated, exactly one execution regardless of replica count
await queue.add("dailyDigest", {}, { repeat: { pattern: "0 9 * * *" } });

The Solution //

Each replica runs its own completely independent setInterval timer with no awareness of the others, meaning a task intended to run once actually executes once per replica simultaneously — a silent multiplication bug that can cause duplicate emails, duplicate charges, or other duplicated side effects. Use a distributed-safe scheduling mechanism instead, like BullMQ repeatable jobs or a Kubernetes CronJob.

The Error //

Scheduling a cron job without explicitly specifying its intended timezone

// Ambiguous: depends on the server's ambient local timezone
new CronJob("0 9 * * *", task);

// Correct: explicit, unambiguous
new CronJob("0 9 * * *", task, null, true, "America/New_York");

The Solution //

A cron schedule with no explicit timezone typically runs based on the server's local timezone (which may not be UTC, and may even differ between deployment environments) — this can cause a job intended to run at 9am in a specific business timezone to actually run at an unintended hour. Always specify the timezone explicitly.