Make sure you are on the 3-debugging branch in the "exercise" folder to follow along with this section.

Deno apps can be debugged either from the browser, or with VS Code. Much like Node, Deno has --inspect and --inspect-brk flags. The --inspect flag starts the program and listens for the debugger. The --inspect-brk does the same thing except that it automatically breaks on the first line of the program.

When a process is in debug mode, you can attach to it with browser dev tools to provide a robust debugging experience.

Using the browser dev tools

This should work in any Chromium based browser - including Chrome itself and Microsoft Edge.

First, make sure the app.ts file contains the following code as we left it in the last module.

const message: String = Deno.args[0];
console.log(`Hello ${message}`.toUpperCase());

Run the program as you normally would, but this time, pass in the --inspect flag.

deno run --inspect app.ts World

Passing the --inspect flag opens the debugger port. However, the program runs to completion and you don't have a chance to debug it. This is because you haven't set any breakpoints and your code has no errors. The solution to this is to use the --inspect-brk flag instead.

Run the program again, this time passing in the --inspect-brk flag.

deno run --inspect-brk app.ts World

This time, the program runs, and then pauses. Now you can attach your browser developer tools to the running program.

Attach with browser dev tools

Note that this requires a Chromium-based browser such as Chrome or Edge

All you need here is an open browser tab. Navigate to chrome://inspect or edge://inspect. Both will work. In Edge, typing chrome://inspect will take you to edge://inspect.

The browser should automatically see your running program and list it as a "target" as shown below. Click the "inspect" link.

The chrome inspect page

This will open the browser dev tools and you will see the your program is broken on the first line.

The chrome dev tools

Step over to the next line and hover over the Deno.args object. You can see the "World" argument that was passed in.

Hovering over the args argument in Deno

Click the continue button to finish execution and then close the Dev Tools.

Note that at the time of this writing, this does not work with WSL2 as Chrome/Edge looks for Deno source code in //home/, which is not a valid path on Windows.

Debugging with VS Code

The Deno extension for VS Code includes a debugger. It also knows how to automatically configure a launch.config with the correct settings to launch the program with --inspect and attach the VS Code debugger.

  • Press F1 to open the Command Palette in VS Code
  • Type "launch" and select "Open launch.json"
  • Select "Deno"

VS Code creates a launch configuration and adds a file called launch.json and a folder called ".vscode". By default, the configuration looks for a file called main.ts. But in the exercise, we have called the file app.ts. Modify the launch.config file so that the program is app.ts.

"program": "app.ts",
  • Open the app.ts file.
  • Click in the gutter next to line 1 to add a breakpoint. A red circle should appear next to the line.

Breakpoint in VS Code

  • Press F5 to start the application.
  • The application breaks on the line where you breakpoint is.
  • Mouse over the Deno.args object to see the arguments that were passed in. There are none! This is because the debugger is launching the program and you need to pass the argument in there.
  • Press the stop button in the debugger to stop the process.
  • Open the launch.json file and add an "args" property underneath the "runtimeArgs" section.

    "args": [

Note that if you added "World" to runtime args, it would be passed BEFORE the name of the file to execute (app.ts) and the command would fail.

  • Press F5 to run the application again.
  • Notice that Deno.args now contains "World".
  • Press the continue button to finish execution and close the debugger

Notice that you can't see any output. Why is that? Where is the "Hello World" being logged out to?

By default, VS Code runs terminal apps on in internal console that you cannot see. You can alter this by specifying the "console" property in the launch.config file. Set it to "integrated" to see the output in the VS Code terminal.

"args": [
"console": "integratedTerminal",
  • Press F5 to run the application again.
  • Press the "step over" button in the debug bar.
  • "Hello World" is written to the Integrated Terminal.

Integrated terminal