const gradeCalculator = (
  grade, 
  gradeMap = [ [90, "A"], [80, "B"], [70, "C"], [60, "D"] ], 
  defaultGrade = "F"
) =>
  gradeMap.find(([lv]) => grade >= lv)?.[1] || defaultGrade

function arrayBuilder(obj) {
  const array = [];
  
  for (let k in obj) {
    for (let i = 0; i < obj[k]; i += 1) {
      array.push(k);
    }
  }
  return array;
}

console.log(arrayBuilder({'cats': 2, 'dogs': 1})); //=> ['cats', 'cats', 'dogs']
console.log(arrayBuilder({})); //=> []

const arrayBuilder = obj => Object
  .entries(obj)
  .reduce((a, [k, c]) => 
    [...a, ...Array(c).fill(k)], [])

let left = (value) => ({
    swap: () => right(value),
    flatMap: () => left(value),
    getOrElse: (defaultValue) => defaultValue
});

let right = (value) => ({
    swap: () => left(value),
    flatMap: (fn) => fn(value),
    getOrElse: () => value
});

let select = (rate) => {
    let either = right(rate);

    let context = {
        option: (value, predicate) => {
            either = (
                either
                    .flatMap((rate) => (
                        predicate(rate, value) ? left(value) : right(rate)
                    ))
            );

            return context;
        },
        default: (value) => (
            either
                .swap()
                .getOrElse(value)
        )
    };

    return context;
};

let moreThan = (a) => (b) => b >= a;

let gradeCalculator = (grade) => (
    select(grade)
        .option('A', moreThan(90))
        .option('B', moreThan(80))
        .option('C', moreThan(70))
        .option('D', moreThan(60))
        .default('F')
);