Build A Simple CLI Tool using Deno

What is Deno?

Deno is a JavaScript/TypeScript runtime with secure defaults and great developer experience. It's built on V8, Rust, and Tokio. I suggest you watch these talks by Ryan: He talks about his mistakes with Nodejs here and a more in-depth look into deno here

Features

• Secure by default. No file, network, or environment access (unless explicitly enabled).
• Supports TypeScript out of the box.
• Ships a single executable (deno).
• Has built-in utilities like a dependency inspector (deno info) and a code formatter (deno fmt).
• Has a set of reviewed (audited) standard modules that are guaranteed to work with Deno. Scripts can be bundled into a single JavaScript file.

Lets Build Something!!!

In this article, we're going to build a simple cli tool to demonstrate some of the features of deno. Our cli will be interacting with a COVID API to fetch live data.

Requirement: make sure you have deno installed. If you don't, refer to this link. It's pretty straightforward.

Deno has the entry file mod.ts so we will follow the same in this article if you are following this article along with the coding you can create a folder named covid-cli, inside that folder you can create a file called mod.ts and copy the below code there.

const { args } = Deno;
import { parse } from "https://deno.land/std/flags/mod.ts";

console.dir(parse(args));

Here the parse(args, options = {}); contains two parameters where args is an Array and options is an object, let try to run the above code using this cmd.

deno run mod.ts arg1 -f hello --flag World --booleanFlag

After running the above code you will see the output as

{ _: [ "arg1" ], f: "hello", flag: "World", booleanFlag: true }

The first property in the object is always an array containing all arguments that did not have an option associated with them(i.e it doesn't match -f or --flag). If you do not pass a corresponding value to a flag, it defaults to true.

I will be using the this postman doc for all the COVID related API and we will perform the below to action through our CLI.

1. A summary of new and total cases globally updated daily.
2. A summary of new and total cases per country updated daily.

Let's write out the function for our first command, so our mod.ts file will look like this.

const { args } = Deno;
import { parse } from "https://deno.land/std/flags/mod.ts";

// flags:
// -h, --help: display help message
// -g, --global: display global stats
// -c, --country: get data of mentioned country

const BASE_URL: string = "https://api.covid19api.com/"
const parsedArgs = parse(args)

async function getGlobalStats() {
    const res = await fetch(`${BASE_URL}summary`)
    const data = await res.json();
    console.log(data["Global"])
}

Here, we have an async function that returns the data from our API call. We're making a fetch request (yes, deno has browser functionality in-built) to the API endpoint to get the global stat of covid19.

The function for the second command looks very similar, just we need to filter our data only for a particular country which is provided.

async function getCountryStats(country: string) {
  if (country) {
    const res = await fetch(`${BASE_URL}summary`);
    const data = await res.json();
    const index = data["Countries"].findIndex((c: any) => c.Country.toLowerCase() === country.toLowerCase())
    if (index !== -1) {
        console.log(data["Countries"][index])
    } else {
        console.log("Country Not Found")
    }
  } else {
    console.log("Country Name is needed")
  }
}

We will now write our main() function. We have a switch statement to check the first flag that was passed and calls the appropriate function. The default case simply displays a welcome message. Let add the below code in our mod.ts file

async function main() {
  switch (Object.keys(parsedArgs)[1]) {
    case "help":
    case "h":
      console.log(displayHelpMsg());
      break;
    case "global":
    case "g":
      await getGlobalStats();
      break;
    case "country":
    case "c":
       let country = parsedArgs.c || parsedArgs.country || ""
       await getCountryStats(country)
       break;
    default:
       console.log(displayHelpMsg());
  }
}

main()

And our displayHelpMsg() will look something like this

function displayHelpMsg() {
  return "flags:\n-h, --help: display help message\n-g, --global: display global stats\n-c, --country: get data of mentioned country ";
}

Testing Time!!!

To test our program, we're going to run $ deno run --allow-net mod.ts -g. We should get the following result:

• For Global Records

• For Country Wise Record

• For Help

That's all there is for our cli tool. If you'd like to see the full code, the repository is linked here. In conclusion, Deno is a very exciting project especially because you get all the benefits of typescript out of the box without the need to compile your files to js.

You can build a lot with it, ranging from cli Programs to HTTP servers. Do have a look at one of my blogwhere I have built a basic calculator app using the abc module.