How to read lines in Deno

line_reader.ts

import { readLines } from "https://deno.land/std@v0.36.0/io/mod.ts";
import { parse } from "https://deno.land/std@v0.36.0/flags/mod.ts";
import { basename } from "https://deno.land/std@v0.36.0/path/mod.ts";

export {};

if (import.meta.main) {
  const args = parse(Deno.args, {
    boolean: ["h"],
    alias: {
      h: ["help"]
    }
  });

  if (args.h) {
    printUsage();
    Deno.exit(0);
  }

  const [filename] = args._;
  if (!filename) {
    printUsage();
    Deno.exit(1);
  }

  const file = filename === "-"
    ? Deno.stdin
    : await Deno.open(String(filename));

  let lineCount = 0;
  for await (const line of readLines(file)) {
    // do something with `line`
    lineCount += 1;
  }
  file.close();
  console.log(`${lineCount} lines read.`);

  function printUsage() {
    console.error(
      `Usage: deno --allow-read ${basename(import.meta.url)} <filename>`
    );
  }
}