Bun Hot Reloading & HMR FAQs

Question 1

How do I prevent state from being lost during a hot reload?

Accepted Answer

To preserve state across reloads, you can attach data to globalThis (e.g., globalThis.cache ??= new Map()) or use the import.meta.hot API to manage state transitions between module versions.

Question 2

Does this skill help with running Bun tests automatically?

Accepted Answer

Yes, it provides configurations for using bun --watch test, which automatically reruns your test suite or specific test files whenever you save changes.

Question 3

What is the difference between --watch and --hot in Bun?

Accepted Answer

The --watch flag restarts the entire Bun process when files change, which is reliable but clears memory state. The --hot flag reloads modules in-place without restarting the process, allowing you to preserve variables and active connections.

Question 4

Can I use Bun's hot reloading with an Express-like app?

Accepted Answer

Absolutely. The skill includes patterns for wrapping server logic in functions that can be re-invoked by the Bun.serve reload handler when your source code changes.

Question 5

How do I handle cleanup, like closing database connections, during a reload?

Accepted Answer

You can use the import.meta.hot.dispose() callback to define cleanup logic, such as clearing intervals or closing server instances, before the module reloads.

Bun Hot Reloading & HMR

Key Features

Use Cases

Bun Hot Reloading & HMR

Key Features

Use Cases