How to Map String Characters

Question

I have the following array with a string inside.

let fruta = ['banana'];

fruit is the name of my array and banana is the string. my intention is to create a code that Mapeie the string 'banana' showing the position of each letter getting;

banana

 b: 0

 a: 1,3,5

 n: 2,4

The first line is the entire string The second line is the position where the letter 'b' should be The third line are the positions where the letters 'a' should be The last line is the positions where the letters 'n' should be

I created the following command to perform the action I want;

//console.log(fruta);

let fruta = [
  'banana'
];

function mapString(fruta, ) {
  let map = {};
  for (let i = 0; i < fruta.length; i++) {
    let atual = fruta[i];
    if (map[atual]) {
      map[atual].push(i);
    } else {
      map[atual] = [i];
    }
  }
  return map;
};
let stringMap = mapString(fruta);
for (let letter in stringMap) {
  console.log(letter + ': ' + stringMap[letter]);
}

But when I spin the code the return is not;

banana
b: 0
a: 1,3,5
n: 2,4

The return is;

banana: 0

Because this is going wrong?