index.js

/**
 * Makes ideal change based on the difference between the purchase price of an item,
 * how much cash is tendered, and the current contents of the register
 *
 */
const makeChange = (purchasePrice, cashTendered, cashRegisterContents) => {
  // Exact change tendered
  // Oops
  throw 'Insufficient Funds';

  // Another oops
  throw 'Dang!';
  return [];
}

const interface = (change) => {
  if (output.isEmpty()) {
    return 'CLOSED';
  }
  return change;
}

try {
  return interface(makeChange(/*...*/));
} catch (e) {
  return e.message;
}