Find the substring triples count that contain the same amount of "a"

solution.js

const getAllPartSets = s => {
  const partSets = [];

  // pivot in 0
  for (let i = 2; i < s.length; i++) {
    partSets.push([
      s[0],
      s.slice(1, i),
      s.slice(i)
    ]);
  }

  // pivot in I
  for (let i = 1; i < s.length - 1; i++) {
    partSets.push([
      s.slice(0, i),
      s.slice(i, i + 1),
      s.slice(i + 1)
    ]);
  }

  // pivot in last
  // - we start in 2 as the first part set is the same
  //   as the last one in the pivot 0
  // - we finish in s.length - 2 as the last part set is
  //   the same as the last one in pivot I
  for (let i = 2; i < s.length - 2; i++) {
    partSets.push([
      s.slice(0, i),
      s.slice(i, -1),
      s[s.length - 1]
    ]);
  }


  return partSets;
};

const getLetterCount = (str, letter) =>
  str
    .split("")
    .filter(curLetter => curLetter === letter)
    .length;

const solution = s => {
  const filteredLetter = "a";

  const allPartSets = getAllPartSets(s);

  const filteredPartSets = allPartSets.filter(
    partSet => {
      const firstPartLetterCount = getLetterCount(partSet[0], filteredLetter);

      return partSet
         .slice(1)
         .every(part => getLetterCount(part, filteredLetter) === firstPartLetterCount);
    }
  );

   return filteredPartSets.length;
};