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145 lines
4.6 KiB
Markdown
145 lines
4.6 KiB
Markdown
## Debugger
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Deno supports the [V8 Inspector Protocol](https://v8.dev/docs/inspector).
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It's possible to debug Deno programs using Chrome Devtools or other clients that
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support the protocol (eg. VSCode).
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To activate debugging capabilities run Deno with the `--inspect` or
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`--inspect-brk` flags.
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The `--inspect` flag allows attaching the debugger at any point in time, while
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`--inspect-brk` will wait for the debugger to attach and will pause execution on
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the first line of code.
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### Chrome Devtools
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Let's try debugging a program using Chrome Devtools. For this, we'll use
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[file_server.ts](https://deno.land/std@$STD_VERSION/http/file_server.ts) from
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`std`, a static file server.
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Use the `--inspect-brk` flag to break execution on the first line:
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```shell
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$ deno run --inspect-brk --allow-read --allow-net https://deno.land/std@$STD_VERSION/http/file_server.ts
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Debugger listening on ws://127.0.0.1:9229/ws/1e82c406-85a9-44ab-86b6-7341583480b1
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Download https://deno.land/std@$STD_VERSION/http/file_server.ts
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Compile https://deno.land/std@$STD_VERSION/http/file_server.ts
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...
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```
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Open `chrome://inspect` and click `Inspect` next to target:
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It might take a few seconds after opening the Devtools to load all modules.
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You might notice that Devtools paused execution on the first line of
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`_constants.ts` instead of `file_server.ts`. This is expected behavior and is
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caused by the way ES modules are evaluated by V8 (`_constants.ts` is left-most,
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bottom-most dependency of `file_server.ts` so it is evaluated first).
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At this point all source code is available in the Devtools, so let's open up
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`file_server.ts` and add a breakpoint there; go to "Sources" pane and expand the
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tree:
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_Looking closely you'll find duplicate entries for each file; one written
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regularly and one in italics. The former is compiled source file (so in the case
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of `.ts` files it will be emitted JavaScript source), while the latter is a
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source map for the file._
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Next, add a breakpoint in the `listenAndServe` method:
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As soon as we've added the breakpoint Devtools automatically opened up the
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source map file, which allows us step through the actual source code that
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includes types.
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Now that we have our breakpoints set, we can resume the execution of our script
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so that we might inspect an incoming request. Hit the Resume script execution
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button to do so. You might even need to hit it twice!
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Once our script is running again, let's send a request and inspect it in
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Devtools:
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```
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$ curl http://0.0.0.0:4500/
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```
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At this point we can introspect the contents of the request and go step-by-step
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to debug the code.
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### VSCode
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Deno can be debugged using VSCode.
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Official support via the plugin is being worked on -
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https://github.com/denoland/vscode_deno/issues/12
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We can still attach the debugger by manually providing a
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[`launch.json`](https://code.visualstudio.com/docs/editor/debugging#_launch-configurations)
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config:
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```json
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{
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"version": "0.2.0",
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"configurations": [
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{
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"name": "Deno",
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"type": "pwa-node",
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"request": "launch",
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"cwd": "${workspaceFolder}",
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"runtimeExecutable": "deno",
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"runtimeArgs": ["run", "--inspect-brk", "-A", "${file}"],
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"attachSimplePort": 9229
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}
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]
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}
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```
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**NOTE**: This uses the file you have open as the entry point; replace `${file}`
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with a script name if you want a fixed entry point.
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Let's try out debugging a local source file. Create `server.ts`:
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```ts
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import { serve } from "https://deno.land/std@$STD_VERSION/http/server.ts";
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const server = serve({ port: 8000 });
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console.log("http://localhost:8000/");
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for await (const req of server) {
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req.respond({ body: "Hello World\n" });
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}
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```
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Then we can set a breakpoint, and run the created configuration:
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### JetBrains IDEs
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You can debug Deno using your JetBrains IDE by right-clicking the file you want
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to debug and selecting the `Debug 'Deno: <file name>'` option. This will create
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a run/debug configuration with no permission flags set. To configure these flags
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edit the run/debug configuration and modify the `Arguments` field with the
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required flags.
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### Other
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Any client that implements the Devtools protocol should be able to connect to a
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Deno process.
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### Limitations
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Devtools support is still immature. There is some functionality that is known to
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be missing or buggy:
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- autocomplete in Devtools' console causes the Deno process to exit
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- profiling and memory dumps might not work correctly
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