Debugging your code
Deno supports the V8 Inspector Protocol used by Chrome, Edge and Node.js. This makes it possible to debug Deno programs using Chrome DevTools or other clients that support the protocol (for example VSCode).
To activate debugging capabilities run Deno with the
--inspect flag allows attaching the debugger at any point in time, while
--inspect-brk will wait for the debugger to attach and will pause execution on
the first line of code.
⚠️ If you use
--inspectflag, the code will start executing immediately. If your program is short, you might not have enough time to connect the debugger before the program finishes execution. In such cases, try running with
--inspect-brkflag instead, or add a timeout at the end of your code.
Let’s try debugging a program using Chrome Devtools. For this, we’ll use
std, a static file server.
--inspect-brk flag to break execution on the first line:
$ deno run --inspect-brk --allow-read --allow-net https://firstname.lastname@example.org/http/file_server.ts Debugger listening on ws://127.0.0.1:9229/ws/1e82c406-85a9-44ab-86b6-7341583480b1 Download https://email@example.com/http/file_server.ts Compile https://firstname.lastname@example.org/http/file_server.ts ...
In a Chromium derived browser such as Google Chrome or Microsoft Edge, open
chrome://inspect and click
Inspect next to target:
It might take a few seconds after opening the DevTools to load all modules.
You might notice that DevTools pauses execution on the first line of
_constants.ts instead of
file_server.ts. This is expected behavior caused by
_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
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 the case
source map for the file.
Next, add a breakpoint in the
As soon as we’ve added the breakpoint, DevTools automatically opens up the source map file, which allows us step through the actual source code that includes types.
Now that we have our breakpoints set, we can resume the execution of our script so that we can inspect an incoming request. Hit the “Resume script execution” button to do so. You might even need to hit it twice!
Once our script is running, try send a request and inspect it in Devtools:
$ curl http://0.0.0.0:4507/
At this point we can introspect the contents of the request and go step-by-step to debug the code.
Deno can be debugged using VSCode. This is best done with help from the official
vscode_deno extension. Documentation for this can be found
Note: make sure you have this Deno plugin installed and enabled in Preferences / Settings | Plugins. For more information, see this blog post.
You can debug Deno using your JetBrains IDE by right-clicking the file you want
to debug and selecting the
Debug 'Deno: <file name>' option.
This will create a run/debug configuration with no permission flags set. If you
want to configure them, open your run/debug configuration and add the required
flags to the
Any client that implements the DevTools protocol should be able to connect to a Deno process.