CLI Tour

Switch to the 3-cli-tour branch to follow along with this exercise.

For a full list of arguments you can pass to Deno, execute deno -h.

There are quite a few built-in features for Deno, including a formatter, a linter and a built-in test runner. For any of the built-in arguments, you can select the argument and the pass -h again after the argument to find out what you can do with a command, what options are available and what flags you can pass.

Inspect the fmt argument using the -h command.

deno fmt -h

Change the formatting in the app.ts file so that a default value is passed for the message if args doesn't exist. Make sure to use single quotes.

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

Run the formatter on the app.ts file

deno fmt app.ts

Notice that the formatter changes the single-quotes to double-quotes. Formatting in Deno is done by the dprint library with the default configuration. As of the time of this writing, there is no way to pass a configuration to dprint. My personal preference is to use Prettier and let the editor worry about formatting. The same goes for linting.

Passing in Arguments

Arguments can be passed to the Deno CLI after the path of the file to be executed.

deno run app.ts a b c

These arguments are referenced within the file by looking at the Deno.args object.

Modify the code in "app.ts" to log out all incoming arguments...


Run the code from the terminal. Pass in the values a, b and c as args

deno run app.ts a b c

Check file:///home/burkeholland/dev/deno-first-look-exercises/app.ts
[ "a", "b", "c" ]

Modify the code to echo out the value of the argument passed in and concatenate it with "Hello"

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

Run the program

deno run app.ts World

Check file:///home/burkeholland/dev/deno-first-look-exercises/app.ts
Hello World

Deno doesn't do named arguments by default, but these can be passed in as flags using the Standard Library, which we'll examine later on in this course.


Deno has built-in flags that can be passed to the runtime. You've already used many of them. We used "--no-check" to skip TypeScript checking and the "--allow-env" and "--allow-read" permissions flags.

Flags that are passed to Deno must be passed before the name of the file to execute. If they are passed after, they will be ignored. This is important as it can cause quite a bit of confusion if your flags are working the way you think they will.

There are several flags listed from the help, but let's take a look at a few of the ones you will use the most often.

--watch & --unstable

One of my favorite features of Deno is the built-in watcher. It's kind of like supervisor or nodemon if you've used Node before. It just watches for file changes and then re-builds your TypeScript and restarts the process.

  • Execute the app.ts file with the built-in watcher by passing the --watch flag.
deno run --watch app.ts World

error: The following required arguments were not provided:

    deno run <SCRIPT_ARG>... --unstable --watch

For more information try --help

Uh oh - what happened? The CLI is telling us that we are missing a required flag to use the --watch feature. That is the --unstable flag. The --unstable flag allows Deno to run with still unstable features. Watch is one of those features. You'll encounter quite a few features in Deno (which is considered stable) that are behind the "unstable" flag at the time of this writing. That's important to note since you might assume that Deno being "stable" means all of it is stable. That's not the case.

  • Run the program again with the --unstable --watch flags...
deno run --unstable --watch app.ts World

Check file:///home/burkeholland/dev/deno-first-look-exercises/app.ts
Hello World
Watcher Process terminated! Restarting on file change...

Modify the code in "app.ts" so that it transforms the output to uppercase

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

Save the file and notice that the terminal updates with the new output

Watcher File change detected! Restarting!
Check file:///home/burkeholland/dev/deno-first-look-exercises/app.ts
Watcher Process terminated! Restarting on file change...


There are other flags that can be passed in that deal with different aspects that are unique to Deno. We will take a closer look at these flags when we talk about concepts like dependencies and permissions. For now, the important things to know about the Deno CLI are...

  1. Pass -h after any option to see a description, sample usage and sub-options
  2. Pass in all flags before the file name
  3. Pass in your own arguments after the file name