## Debugger

Deno supports [V8 Inspector Protocol](https://v8.dev/docs/inspector).

It is possible to debug Deno programs using Chrome Devtools or other clients
that support the protocol (eg. VSCode).

To activate debugging capabilities run Deno with `--inspect` or `--inspect-brk`
flag.

`--inspect` flag allows to attach debugger at any point in time, while
`--inspect-brk` will wait for debugger being attached and pause execution on the
first line of code.

### Chrome Devtools

Let's try debugging simple program using Chrome Devtools; for this purpose we'll
use [file_server.ts](https://deno.land/std@v0.50.0/http/file_server.ts) from
`std`; a simple static file server.

Use `--inspect-brk` flag to break execution on the first line.

```shell
$ deno run --inspect-brk --allow-read --allow-net https://deno.land/std@v0.50.0/http/file_server.ts
Debugger listening on ws://127.0.0.1:9229/ws/1e82c406-85a9-44ab-86b6-7341583480b1
Download https://deno.land/std@v0.50.0/http/file_server.ts
Compile https://deno.land/std@v0.50.0/http/file_server.ts
...
```

Open `chrome://inspect` and click `Inspect` next to target:

![chrome://inspect](../images/debugger1.jpg)

It might take a few seconds after opening the devtools to load all modules.

![Devtools opened](../images/debugger2.jpg)

You might notice that Devtools paused execution on the first line of
`_constants.ts` instead of `file_server.ts`. This is an expected behavior and is
caused by the way ES modules are evaluated by V8 (`_constants.ts` is left-most,
bottom-most dependency of `file_server.ts` so it is evaluated first).

At this point all source code is available in the Devtools, so let's open up
`file_server.ts` and add a breakpoint there; go to "Sources" pane and expand the
tree:

![Open file_server.ts](../images/debugger3.jpg)

_Looking closely you'll find duplicate entries for each file; one written
regularly and one in italics. The former is compiled source file (so in case of
`.ts` files it will be emitted JavaScript source), while the latter is a source
map for the file._

Add a breakpoint in `listenAndServe` method:

![Break in file_server.ts](../images/debugger4.jpg)

As soon as we've added the breakpoint Devtools automatically opened up source
map file, which allows us step through the actual source code that includes
types.

Let's send a request and inspect it in Devtools:

```
$ curl http://0.0.0.0:4500/
```

![Break in request handling](../images/debugger5.jpg)

At this point we can introspect contents of the request and go step-by-step to
debug the code.

### VSCode

Deno can be debugged using VSCode.

Official support in plugin is being worked on -
https://github.com/denoland/vscode_deno/issues/12

We can still attach debugger by manually providing simple `launch.json` config:

```json
{
  "version": "0.2.0",
  "configurations": [
    {
      "name": "Deno",
      "type": "node",
      "request": "launch",
      "cwd": "${workspaceFolder}",
      "runtimeExecutable": "deno",
      "runtimeArgs": ["run", "--inspect-brk", "-A", "<entry_point>"],
      "port": 9229
    }
  ]
}
```

**NOTE**: Replace `<entry_point>` with actual script name.

This time let's try with local source file, create `server.ts`:

```ts
import { serve } from "https://deno.land/std@v0.50.0/http/server.ts";
const s = serve({ port: 8000 });
console.log("http://localhost:8000/");

for await (const req of s) {
  req.respond({ body: "Hello World\n" });
}
```

Change `<entry_point>` to `server.ts` and run created configuration:

![VSCode debugger](../images/debugger6.jpg)

![VSCode debugger](../images/debugger7.jpg)

### Other

Any client that implements Devtools protocol should be able to connect to Deno
process.

### Limitations

Devtools support is still immature, there are some functionalities that are
known to be missing/buggy:

- autocomplete in Devtools' Console causes Deno process to exit
- profiling and memory dumps might not work correctly