work in progress solution.

work.js

let input = require('sync-input');

console.log("Welcome to Currency Converter!");
console.log(`1 USD equals  1 USD
1 USD equals  113.5 JPY
1 USD equals  0.89 EUR
1 USD equals  74.36 RUB
1 USD equals  0.75 GBP`);
console.log("What do you want to convert?");

let fromCurrency = input("From:");
let toCurrency = input("To:");

let currencies = [
  {
    name: "JPY",
    val: 113.5,
  },
  {
    name: "EUR",
    val: 0.89,
  },
  {
    name: "RUB",
    val: 74.36,
  },
  {
    name: "GBP",
    val: 0.75,
  },
  {
    name: "USD",
    val: 1,
  },
];

(function checkCurrencies(fCurr, tCurr) {
  let isFound = false;
  let fromCurrVal;
  let toCurrVal;
  
  currencies.forEach(function(itm, ind, arr) {
    if(itm.name == fCurr) {
      // true
      arr.forEach(function(item) {
        if(item.name == tCurr) {
          // true
          isFound = true;
          fromCurrVal = itm.val;
          toCurrVal = item.val;
        }
      });
    }
  });
  
  (!(isFound == true)) ? console.log("Unknown currency") : convertCurrency(fromCurrVal, toCurrVal);
}(fromCurrency, toCurrency));

function convertCurrency(fCurrVal, tCurrVal) {
  let amount = input("Amount:");
  let result = "";
  
  if(amount < 1) {
    result = "The amount can not be less than 1";
  } else if(isNaN(amount)) {
    result = "The amount has to be a number";
  } else {
    // all good! convert the currency
    result = amount * tCurrVal / fCurrVal;
  }
  
  isNaN(result) ? console.log(result) : console.log("Result: " + amount + " " + fromCurrency.toUpperCase() + " equals " + parseFloat(result).toFixed(4) + " " + toCurrency.toUpperCase());
}