getMostCommonGenres()

solution.js

function getMostCommonGenres(books) { 
  let countObj = {};
  books.forEach(aBook => {
    if (countObj[aBook.genre] != null) {
      countObj[aBook.genre]++;
    } else {
      countObj[aBook.genre] = 1;
    }
  });
  let countArray = [];
  for (const [key, value] of Object.entries(countObj)) {
    countArray.push({
      'name' : key,
      'count' : value
    }); 
  }
  countArray.sort((a,b) => b.count - a.count);
  return countArray.slice(0, 5);
}

solution2.js

function getMostCommonGenres(books) {
  let result = {};
  let genre = books.forEach((book) => {
    if (result[book.genre] == null) {
      result[book.genre] = 1; 
    } else {
      result[book.genre] += 1;
    }
  })
  let countArray = [];
  for (const [key, value] of Object.entries(result)) {
    countArray.push({
      'name' : key,
      'count' : value
    }); 
  }
  countArray.sort((a,b) => b.count - a.count);
  return countArray.slice(0, 5);
}  

function helper(books) {
 let countObj = {};
  books.forEach(aBook => {
    if (countObj[aBook.genre] != null) {
      countObj[aBook.genre]++;
    } else {
      countObj[aBook.genre] = 1;
    }
  }
 return countObj;
}

function getMostCommonGenres(books) { 
  let countObj = helper(books);
  let countArray = [];
  for (const [key, value] of Object.entries(countObj)) {
    countArray.push({
      'name' : key,
      'count' : value
    }); 
  }
  countArray.sort((a,b) => b.count - a.count);
  return countArray.slice(0, 5);
}