const reviews = [4.5, 4.0, 5.0, 2.0, 1.0, 5.0, 3.0, 4.0, 1.0, 5.0, 4.5, 3.0, 2.5, 2.0];

const reviewsLetter = ['c', 'd', 'f', 'a', 'g', 'a', 'd', 'f', 'e', 'b'];

​

// 1. Using the reduce function, create an object that

// has properties for each review value, where the value

// of the property is the number of reviews with that score.

// for example, the answer should be shaped like this:

// { 4.5: 1, 4: 2 ...}

​

const countGroupedByReview = reviewsLetter.reduce(groupBy, {});

​

function groupBy (acc, review){

  const count = acc[review] || 0;

  return {...acc, [review]: count + 1}

}

​

console.log(countGroupedByReview);