calculateAgeFix.js

// cheap fix
function calculateAge(bornYear, currentYear){
  if(bornYear !== undefined && currentYear !== undefined){
     console.log(currentYear - bornYear);
     } else {
       console.log("provide both year values")
     }
}

calculateAge(2000, 2018); // 18
calculateAge(); // "provide both year values"

// another cheap fix with one default
function calculateAge2(bornYear, currentYear){
  if(arguments.length == 1 && currentYear === undefined){  
    currentYear = 2018; 
    console.log(currentYear - bornYear);
  } else {
      console.log(currentYear - bornYear); 
  }
}

calculateAge2(2000); // 18
calculateAge2(2010, 2020); // 10