Watch Mode - BunDocumentation Index
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Skip to main contentBun home pageAutomatic reloading in Bun with —watch and —hot modes
bun --watch index.tsxTo run your tests in --watch mode: terminal
bun --watch testIn --watch mode, Bun keeps track of all imported files and watches them for changes. When a change is detected, Bun restarts the process, preserving the same set of CLI arguments and environment variables used in the initial run. If Bun crashes, --watch will attempt to automatically restart the process.
bun run --watch watchy.tsxwatchy.tsx
import { serve } from "bun";
console.log("I restarted at:", Date.now());
serve({
port: 4003,
fetch(request) {
return new Response("Sup");
},
});
In this example, Bun is
Running bun test in watch mode and save-on-keypress enabled:
terminalbun --watch test
bun --hot server.tsStarting from the entrypoint (server.ts in the example above), Bun builds a registry of all imported source files (excluding those in node_modules) and watches them for changes. When a change is detected, Bun performs a “soft reload”. All files are re-evaluated, but all global state (notably, the globalThis object) is persisted. server.ts
// make TypeScript happy
declare global {
var count: number;
}
globalThis.count ??= 0;
console.log(`Reloaded ${globalThis.count} times`);
globalThis.count++;
// prevent `bun run` from exiting
setInterval(function () {}, 1000000);
If you run this file with bun --hot server.ts, you’ll see the reload count increment every time you save the file.
terminalbun --hot index.ts
Reloaded 1 times Reloaded 2 times Reloaded 3 timesTraditional file watchers like nodemon restart the entire process, so HTTP servers and other stateful objects are lost. By contrast, bun --hot is able to reflect the updated code without restarting the process. HTTP servers This makes it possible, for instance, to update your HTTP request handler without shutting down the server itself. When you save the file, your HTTP server will be reloaded with the updated code without the process being restarted. This results in seriously fast refresh speeds. server.ts
globalThis.count ??= 0;
globalThis.count++;
Bun.serve({
fetch(req: Request) {
return new Response(`Reloaded ${globalThis.count} times`);
},
port: 3000,
});
Implementation details
On hot reload, Bun: Resets the internal require cache and ES module registry (Loader.registry) Runs the garbage collector synchronously (to minimize memory leaks, at the cost of runtime performance) Re-transpiles all of your code from scratch (including sourcemaps) Re-evaluates the code with JavaScriptCore This implementation isn’t particularly optimized. It re-transpiles files that haven’t changed. It makes no attempt at incremental compilation. It’s a starting point.Was this page helpful?
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- Automatic reloading in Bun with --watch and --hot modes