Deno app that unpacks a file containing msgpack data into JSON

msgunpack.ts

#!/usr/bin/env deno run

/**
    Unpacks a file containing msgpack data into JSON.
    @param filePath The path to the file to unpack.
    @param outputPath The path to the file to write the output to.
    @param pretty Whether to pretty-print the output.
*/

import * as msgpack from "https://unpkg.com/@msgpack/msgpack@2.7.2/mod.ts";
import { parse } from "https://deno.land/std@0.140.0/flags/mod.ts";
import { stripIndent } from "https://deno.land/x/deno_tags@1.8.0/tags/stripIndent.ts";

const args = parse(Deno.args, {
  string: ["output"],
  boolean: ["help", "pretty"],
  alias: {
    h: "help",
    // "?": "help",
    p: "pretty",
    o: "output",
  },
});

function errorExit(msg?: string): never {
  if (msg) console.error(`%c${msg}`, "color: red");
  Deno.exit(1);
}

const filename = args._[0];
if (args.help || typeof filename !== "string") {
  console.log(stripIndent`
    Usage:
        msgunpack [OPTIONS] <FILE>
    Options:
        -h, --help
            Show this help message.
        -p, --pretty
            Pretty-print the output.
        -o, --output <FILE>
            Write the output to a file.
    `);
  errorExit();
}

let file: Uint8Array;
try {
  file = await Deno.readFile(filename);
} catch (e) {
  if (e instanceof Deno.errors.NotFound)
    errorExit(`File not found: ${filename}`);
  else if (e instanceof Deno.errors.PermissionDenied)
    errorExit("Permission denied to read file");
  else throw e;
}

const data = msgpack.decode(file);
const space = args.pretty ? "  " : "";

const results = JSON.stringify(data, null, space);
if (args.output) {
  try {
    await Deno.writeTextFile(args.output, results);
    console.log("%cSaved to output", "color: green");
  } catch (e) {
    if (e instanceof Deno.errors.PermissionDenied)
      errorExit("Permission denied to write output");
    else throw e;
  }
} else {
  console.log(results);
}