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What is Deno?

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

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Features
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Let's Build Something!!!

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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.

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

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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.

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const { args } = Deno;\nimport { parse } from "https://deno.land/std/flags/mod.ts";\n\nconsole.dir(parse(args));
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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.

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deno run mod.ts arg1 -f hello --flag World --booleanFlag
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After running the above code you will see the output as

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

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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.

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  1. A summary of new and total cases globally updated daily.
    2. \n
  2. A summary of new and total cases per country updated daily.
    3. \n
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Let's write out the function for our first command, so our mod.ts file will look like this.

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const { args } = Deno;\nimport { parse } from "https://deno.land/std/flags/mod.ts";\n\n// flags:\n// -h, --help: display help message\n// -g, --global: display global stats\n// -c, --country: get data of mentioned country\n\nconst BASE_URL: string = "https://api.covid19api.com/"\nconst parsedArgs = parse(args)\n\nasync function getGlobalStats() {\n    const res = await fetch(`${BASE_URL}summary`)\n    const data = await res.json();\n    console.log(data["Global"])\n}
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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.

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

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

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async function main() {\n  switch (Object.keys(parsedArgs)[1]) {\n    case "help":\n    case "h":\n      console.log(displayHelpMsg());\n      break;\n    case "global":\n    case "g":\n      await getGlobalStats();\n      break;\n    case "country":\n    case "c":\n       let country = parsedArgs.c || parsedArgs.country || ""\n       await getCountryStats(country)\n       break;\n    default:\n       console.log(displayHelpMsg());\n  }\n}\n\nmain()
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And our displayHelpMsg() will look something like this

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

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

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\n 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.

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

\n","headings":[{"value":"What is Deno?","depth":3},{"value":"Features","depth":5},{"value":"Let's Build Something!!!","depth":3},{"value":"Testing Time!!!","depth":3}],"fields":{"slug":"/engineering/build-a-cli-tool-using-deno/"},"frontmatter":{"metatitle":null,"metadescription":null,"description":null,"title":"Build A Simple CLI Tool using Deno","canonical":null,"date":"June 08, 2020","updated_date":null,"tags":["Deno","CLI","Programming"],"coverImage":{"childImageSharp":{"fluid":{"aspectRatio":1.7857142857142858,"src":"/static/313a3b2de31b5d15db618e72b377a867/03979/cli-tool.png","srcSet":"/static/313a3b2de31b5d15db618e72b377a867/f5f11/cli-tool.png 200w,\n/static/313a3b2de31b5d15db618e72b377a867/6d133/cli-tool.png 400w,\n/static/313a3b2de31b5d15db618e72b377a867/03979/cli-tool.png 800w,\n/static/313a3b2de31b5d15db618e72b377a867/47c9a/cli-tool.png 860w","sizes":"(max-width: 800px) 100vw, 800px"}}},"author":{"id":"Mohammed Modi","github":"mohammed786","bio":"Mohammed Modi is a software developer at LoginRadius and have an experience in working in most of the javascript framework including React, NodeJS, Angular etc.","avatar":null}}}},"pageContext":{"id":"8ea36e93-04d5-54e6-a5d7-7fe5d7dd8707","fields__slug":"/engineering/build-a-cli-tool-using-deno/","__params":{"fields__slug":"engineering"}}},"staticQueryHashes":["1171199041","1384082988","1711371485","1753898100","2100481360","229320306","23180105","528864852"]}