concatenation

index.ts

import { PathLike, readFile, appendFile } from "fs";
import path from "path";
import { fileURLToPath } from "url";
import { dirname } from "path";
const __filename = fileURLToPath(import.meta.url);
const __dirname = dirname(__filename);

const files = process.argv.slice(2);

/**
 * Sequential Iteration pattern with callbacks:
 *
 * Pseudo:
 * iterate() {
 *  if (index === tasks.length) return
 *
 *  task = tasks[index];
 *
 *  task(() => iterate(index++));
 * }
 *
 * iterate(0);
 */

function concatFiles(
  files: string[],
  dest: PathLike,
  cb: (err: NodeJS.ErrnoException | null) => void
) {
  function iterate(index: number) {
    if (index === files.length) {
      return cb(null);
    }

    readFileContentsAndAppend(files[index], dest, (err) => {
      if (err) {
        console.log("Error reading the file");
        return cb(err);
      }

      iterate(++index);
    });
  }

  iterate(0);
}

function readFileContentsAndAppend(
  file: PathLike,
  dest: PathLike,
  cb: (err: NodeJS.ErrnoException | null) => void
) {
  const filePath = path.join(__dirname, file.toString());

  readFile(filePath, { encoding: "utf-8" }, (err, data) => {
    if (err) {
      if (err.code === "ENOENT") {
        console.error(`The file "${file}" doesn't exist. Exiting`);
        process.exit(1);
      }

      return cb(err);
    }

    appendFile(dest, data, (err) => {
      if (err) {
        return console.error("Error appending to dest file");
      }
      return cb(null);
    });
  });
}

concatFiles([...files], path.join(__dirname, "dest"), (err) => {
  if (err) {
    return console.error(err);
  }
  console.log("Files concatenated at destination");
});