Avancé ! map, reduce

 const languageSkills = [

  {

    language: "Spanish",

    skill: "Professional Proficiency",

  },

  {

    language: "English",

    skill: "Professional Working Proficiency",

  },

  {

    language: "German",

    skill: "Professional Proficiency",

  },

];

const output = languageSkills.reduce((map, { language, skill }) => {

  if (map.has(skill)) map.get(skill).push(language);

  else map.set(skill, [language]);

  return map;

}, new Map());

console.log(...output);

MAp en action

let tag = [];

explore = (node) => {
    for (let elt of node.children) {
            tag.push(elt.nodeName);
            explore(elt);
      }
    }

explore(document.body); //

getEleCnt = (arr) => {
      return arr.reduce(function(prev,next){
         prev.has(next) ? prev.set(next,prev.get(next) + 1) :prev.set(next,1);        
        return prev;
      },new Map());
    }
 
let allCntEle = getEleCnt(tag);

for(let [u, r] of allCntEle.entries())
console.log(`${u}: ${r}`);