103 lines
3.7 KiB
Plaintext
103 lines
3.7 KiB
Plaintext
---
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title: Debugging Bun with the web debugger
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sidebarTitle: Web debugger
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mode: center
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---
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Bun speaks the [WebKit Inspector Protocol](https://github.com/oven-sh/bun/blob/main/packages/bun-inspector-protocol/src/protocol/jsc/index.d.ts). To enable debugging when running code with Bun, use the `--inspect` flag. Consider the following web server.
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```ts server.ts icon="/icons/typescript.svg"
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Bun.serve({
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fetch(req) {
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console.log(req.url);
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return new Response("Hello, world!");
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},
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});
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```
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---
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Run the file with the `--inspect` flag.
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This starts a WebSocket server on port `6499` (pass `--inspect=<port>` to use a different port). Debugging tools connect to this server to introspect the running Bun process.
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Bun hosts a web-based debugger at [debug.bun.sh](https://debug.bun.sh). It is a modified version of WebKit's [Web Inspector Interface](https://webkit.org/web-inspector/web-inspector-interface/), which looks familiar to Safari users.
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```sh terminal icon="terminal"
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bun --inspect server.ts
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```
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```txt
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--------------------- Bun Inspector ---------------------
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Listening:
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ws://localhost:6499/204e4940-4a8b-4ebc-8fee-1c66926bbc21
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Inspect in browser:
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https://debug.bun.sh/#localhost:6499/204e4940-4a8b-4ebc-8fee-1c66926bbc21
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--------------------- Bun Inspector ---------------------
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```
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---
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Open the provided `debug.bun.sh` URL in your browser to start a debugging session. From this interface, you can view the source code of the running file, view and set breakpoints, and execute code with the built-in console.
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<Frame>
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</Frame>
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---
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Next, set a breakpoint. Open the Sources tab, which shows the code from earlier, and click line number `3` to set a breakpoint on the `console.log(req.url)` statement.
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<Frame>
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</Frame>
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---
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Then visit [`http://localhost:3000`](http://localhost:3000) in your browser to send an HTTP request to the server. The page hangs because the program is paused at the breakpoint you set.
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Note how the UI has changed.
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<Frame>
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</Frame>
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---
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Use the console at the bottom to run arbitrary code in the context of the program, with full access to the variables in scope at the breakpoint.
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<Frame>
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</Frame>
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---
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The right side of the Sources pane lists the local variables in scope; drill down to see their properties and methods. The screenshot shows the `req` variable.
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<Frame>
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</Frame>
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---
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The buttons in the upper left of the Sources pane control the program's execution.
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<Frame>
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</Frame>
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---
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Here's what each control flow button does.
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- _Continue script execution_ — runs the program until the next breakpoint or exception.
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- _Step over_ — continues to the next line.
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- _Step into_ — if the current statement contains a function call, steps into the called function.
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- _Step out_ — if the current statement is a function call, finishes executing it, then steps out of the function to the location where it was called.
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<Frame>
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</Frame>
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