index.js

/**
 * The function below has no parameters. Instead, access the arguments from the command line.
 * The first argument passed after the filename should be either "plus" or "minus", which represents the kind of calculation that will be done. Every argument afterwards should be a number.
 * Depending on the operation, either add up all of the numbers or subtract all of the numbers, from left to right.
 * @returns {number} The result of either adding all numbers or subtracting all numbers, depending on the arguments added to the command line.
 */
function calculator() {

  let result = 0;
  let message = '';

  if (process.argv[2] === "plus" && process.argv[3] !== undefined && process.argv[4] !== undefined) {
    for (let i = 3; i < process.argv.length; i++) {
      result += Number(process.argv[i]);
    }
  } 
  else if (process.argv[2] === "minus" && process.argv[3] !== undefined && process.argv[4] !== undefined) {
    result = process.argv[3];
    for (let i = 4; i < process.argv.length; i++) {
      result -= Number(process.argv[i]);
    }
  } 
  else if (process.argv[2] === undefined) {
    message = "No operation provided...";
    return message;
  } 
  else if (process.argv[3] === undefined) {
    message = "No numbers provided...";
    return message;
  } 
  else if (process.argv[2] !== "plus" && process.argv[2] !== "minus") {
    message = `Invalid operation: ${process.argv[2]}`;
    return message;
  }
  
  return result;
}

// Don't change anything below this line.
module.exports = calculator;