Croner

Croner lets JavaScript and TypeScript code trigger functions on a cron schedule, or simply evaluate cron expressions to compute upcoming/previous run times, without pulling in any runtime dependencies. It extends standard cron syntax with seconds and year fields plus L (last), W (nearest weekday), # (nth weekday occurrence), and + (AND logic) modifiers, and it runs unmodified across Node.js, Deno, Bun, and the browser (as a standalone script, UMD bundle, or ES module).

Beyond simple scheduling, Croner builds in the operational guardrails that hand-rolled setInterval-based schedulers usually lack: overrun protection so a slow-running job doesn’t overlap itself, structured error handling for both sync and async job functions, per-job pause/resume/stop control, and explicit time-zone targeting for schedules that must run relative to a specific locale rather than server local time. Because everything runs in-memory with no database or config files, it fits equally well as a lightweight in-process scheduler for a small service or as the timing engine embedded inside a larger job-processing system.

What You Get

• new Cron(pattern, fn) to schedule a function against a cron expression with extended syntax (seconds, years, L/W/#/+)
• .nextRun() / .nextRuns(n) / previous-run helpers to evaluate a cron expression without actually scheduling anything
• Built-in overrun protection so a long-running job invocation doesn’t overlap its next scheduled trigger
• Structured error handling for both synchronous and asynchronous job functions
• Per-job .pause() / .resume() / .stop() control after a job has been scheduled
• Explicit time-zone targeting so schedules run against a specific zone rather than server local time

Common Use Cases

• Running periodic maintenance jobs (cleanup, cache refresh, report generation) inside a Node.js/Bun backend service
• Scheduling in-browser reminders or periodic UI refreshes without a server-side cron daemon
• Computing ‘next N occurrences’ of a cron expression for a scheduling UI or admin dashboard
• Embedding cron-based timing as the scheduling engine inside a larger task/job-processing system

Under The Hood

Architecture - The library is authored Deno-first: deno.json defines the canonical ./src/croner.ts export, and a build/build.ts script (using esbuild plus esbuild-plugin-d.ts) compiles the Deno source down to the published npm package, so a single TypeScript codebase targets JSR, npm, and browser bundles from one source of truth. Core logic is split across src/croner.ts (the Cron class and scheduling loop), src/pattern.ts (cron-expression parsing including the extended L/W/#/+ modifiers), src/date.ts (date math and next/previous run computation), and src/helpers/timezone.ts (time-zone-aware evaluation). Tech Stack - Pure TypeScript with zero runtime dependencies; builds and tests run through Deno’s own toolchain (deno test, deno lint, deno fmt) rather than a Node-centric bundler, with esbuild used only for the npm/browser distribution step. Code Quality - The test/ directory contains a large, expression-focused Deno test suite (pattern.test.ts, range.test.ts, question-mark.test.ts, previous-runs.test.ts, several ocps-*.test.ts conformance suites), reflecting the project’s emphasis on correctly matching cron-expression edge cases across many real-world patterns. API Design - The primary API is a single Cron(pattern, fn, options) constructor plus a handful of instance methods (nextRun, nextRuns, pause, resume, stop), which keeps the surface area small; supporting both ‘trigger a function’ and ‘just evaluate an expression’ from the same constructor lets the same mental model cover both scheduling and pure cron-math use cases.