count value a to b character javascript

                                    let counter = str => {
  return str.split('').reduce((total, letter) => {
    total[letter] ? total[letter]++ : total[letter] = 1;
    return total;
  }, {});
};

counter("aabsssd"); // => { a: 2, b: 1, s: 3, d: 1 }

                                     const recorrences = ['a', 'b', 'c', 'a', 'b','a']
                .map(i => !!~i.indexOf('a'))
                .filter(i => i)
                .length;
console.log(`recorrences ${recorrences}`) 
//recorrences 3