1.Scales Before the Concert

const [a, op, b] = process.argv.slice(2);   // e.g. ["12", "+", "30"]

if (a === undefined || op === undefined || b === undefined) {
  console.error("Usage: node calc.js <number> <+|-|*|/> <number>");
  process.exit(1);
}

const x = Number(a);
const y = Number(b);
for (const [raw, value] of [[a, x], [b, y]]) {
  if (Number.isNaN(value)) {
    console.error(`Error: "${raw}" is not a number`);
    process.exit(1);
  }
}

const operations = {
  "+": () => x + y,
  "-": () => x - y,
  "*": () => x * y,
  "/": () => {
    if (y === 0) { console.error("Error: cannot divide by zero"); process.exit(1); }
    return x / y;
  },
};

if (!operations[op]) {
  console.error(`Error: unknown operator "${op}"`);
  process.exit(1);
}
console.log(operations[op]());

const fs = require("fs");
const path = require("path");

const TYPES = {
  images: [".jpg", ".jpeg", ".png", ".gif", ".webp"],
  documents: [".pdf", ".docx", ".txt", ".md"],
  music: [".mp3", ".wav"],
};
const folderFor = (file) => {
  const ext = path.extname(file).toLowerCase();      // .JPG -> .jpg
  return Object.keys(TYPES).find((k) => TYPES[k].includes(ext)) || "other";
};

const dir = process.argv[2];
const dryRun = process.argv.includes("--dry-run");

for (const file of fs.readdirSync(dir)) {
  const from = path.join(dir, file);
  if (!fs.statSync(from).isFile() || file.startsWith(".")) continue;   // skip folders and hidden files
  const to = path.join(dir, folderFor(file), file);
  if (fs.existsSync(to)) { console.log(`skip (exists): ${file}`); continue; }
  console.log(`${dryRun ? "would move" : "move"}: ${file} -> ${folderFor(file)}/`);
  if (!dryRun) {
    fs.mkdirSync(path.dirname(to), { recursive: true });
    fs.renameSync(from, to);                          // move, never delete
  }
}