1function count (string) {  
2  var count = {};
3  string.split('').forEach(function(s) {
4     count[s] ? count[s]++ : count[s] = 1;
5  });
6  return count;
7}

1 const recorrences = ['a', 'b', 'c', 'a', 'b','a']
2                .map(i => !!~i.indexOf('a'))
3                .filter(i => i)
4                .length;
5console.log(`recorrences ${recorrences}`) 
6//recorrences 3

1let counter = str => {
2  return str.split('').reduce((total, letter) => {
3    total[letter] ? total[letter]++ : total[letter] = 1;
4    return total;
5  }, {});
6};
7
8counter("aabsssd"); // => { a: 2, b: 1, s: 3, d: 1 }