main.js

console.log("Unit Converter:");

/* 
    This Function ask for 3 parameters, `distance`, `from`, `to`
    
        distance: the distance in number to convert
        Example: 1000, 10, 999, or another number

        from: the unit that you want to convert from
        Example: m, dm, cm, km, or another unit

        to: the unit that you want to convert to
        Example: m, dm, cm, km, or another unit

        Available Units:
        mm ==> milimeters
        cm ==> centimeters
        dm ==> decimeters
        m ==> meters
        dam ==> decameter
        hm ==> hetometer
        km ==> kilometer

*/
let distanceConversion = (distance = 0, from = "", to = "") => {
  // First, lets handle some errors
  if (isNaN(distance)) {
    return "Error: Distance, needs to be a number";
  } else if (typeof from !== "string" || typeof to !== "string") {
    return "Error: The type of From, and To parameters, has to be string";
  } else if (from.length === 0 || to.length === 0) {
    return "Error: You need to set a value for From and To";
  }
  // Now, lets make the logic
  // I'm going to store all the distance units in an array
  let units = ["mm", "cm", "dm", "m", "dam", "hm", "km"];
  // Now, let's know if we have to multiply or divide
  if (units.indexOf(from) < units.indexOf(to)) {
    // We have to divide
    let howManyZeroes = units.indexOf(to) - units.indexOf(from);
    let divideBy = "1";
    for (let i = 0; i < howManyZeroes; i++) {
      divideBy += "0";
    }

    return Number(`${distance / divideBy}`);
  } else if (units.indexOf(from) > units.indexOf(to)) {
    // we have to divide
    let howManyZeroes = units.indexOf(from) - units.indexOf(to);
    let multiplyBy = "1";
    for (let i = 0; i < howManyZeroes; i++) {
      multiplyBy += "0";
    }

    return Number(`${distance * multiplyBy}`);
  } else {
    //we dont need to do nothing, from and to have the same value
    return "From and To have to be different";
  }
};

const distancia = process.argv[2];
const from = process.argv[3];
const to = process.argv[4];

console.log(distanceConversion(distancia, from, to));