codewars-higher-order-1.js

function countDevelopers(list) {
  return list.filter( d => d.continent === "Europe" && d.language === "JavaScript" ).length;
}

// OLD VERSION 3
function countDevelopersOld3(list) {
  return list.filter( developer => developer.continent === "Europe" && developer.language === "JavaScript" ).length;
}

// OLD VERSION 2
function countDevelopersOld2(list) {
  const europeanJS = list.filter( developer => developer.continent === "Europe" && developer.language === "JavaScript" )
  return europeanJS.length;
}

// OLD VERSION
function countDevelopersOld(list) {
  let count = 0;
  
  for(const developer of list) {
    // if the developer is from Europe and their language is javascript
    if(developer.continent === "Europe" && developer.language === "JavaScript") {
      // increment the count
      count++;
    }
  }
  
  return count;
}

codewars-higher-order-2.js

function greetDevelopers(list) {
  list.forEach(d => d.greeting = `Hi ${ d.firstName }, what do you like the most about ${ d.language }?` )
  return list;
}

codewars-higher-order-3.js

function isRubyComing(list) {
  return list.some( developer => developer.language === "Ruby")
}