Reduce with a Map object

code 

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);