how to put a string of letters into a map object in javascript code example

Example: characterMapping js

function characterMapping(str) {
  let charMap = {}
  let max = 0; 
  let maxChar = '';
  for (let char of str) {
    if (!charMap[char]) {      
      charMap[char] = 1;
    } else {
      charMap[char]++
    }
  }
  console.log(charMap);
}