Common Async Timing Mistakes in Node.js Code

Common Async Timing Mistakes in Node.js Code

Async timing mistakes cause subtle bugs that are hard to find. Here are the common ones.

Assuming setTimeout(0) Runs Immediately

It runs on the next tick, not immediately. Code after it may run first, and microtasks run before it. Assuming it is instant causes bugs when code relies on timing.

Forgetting That Microtasks Run Before Macrotasks

Promise callbacks run before the next setTimeout or I/O callback. If your code assumes the order is the order you wrote it, you get bugs. Understand the order: sync, nextTick, promises, then macrotasks.

Not Realizing process.nextTick Runs Before Promises

nextTick is Node.js-specific and runs before promise microtasks. If you use nextTick expecting it to run after promises, you get bugs. It runs first.

Race Conditions From Shared State

Multiple async operations modifying shared state can interleave in unexpected ways. Be careful with shared state across async operations.

Time-Based Logic Bugs

Using Date.now() in async code can give unexpected results if the operation takes longer than expected. Use Date.now() at the right point, and account for async delays in timing logic.

Unhandled Rejections From Async Paths

An error thrown in an async path that is not caught becomes an unhandled rejection. Always use try/catch in async functions, or .catch on promises.

The Takeaway

Common async timing mistakes include assuming setTimeout(0) is instant, forgetting microtasks run before macrotasks, not realizing nextTick runs before promises, race conditions, time-based bugs, and unhandled rejections from async paths. Understanding event loop order prevents most of these.