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Bun supports the --hot flag to run a file with hot reloading enabled. When any module or file changes, Bun re-runs the file.

terminal

bun --hot run index.ts

Bun detects when you are running an HTTP server with Bun.serve(). It reloads your fetch handler when source files change, without restarting the bun process. This makes hot reloads nearly instantaneous.

Note that this doesn’t reload the page on your browser.

index.ts

Bun.serve({
  port: 3000,
  fetch(req) {
    return new Response("Hello world");
  },
});

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Send an HTTP request using fetch

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Start a cluster of HTTP servers

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