Sort characters by frequency

sortCharactersByFrequency.js

var frequencySort = function(s) {

    let charCounts = {};
    
    for(let i = 0; i < s.length; i++) {
        if(charCounts[s[i]]) {
            charCounts[s[i]] += 1;
        } else {
            charCounts[s[i]] = 1;
        }
    }
    
    let freqs = Object.keys(charCounts);
    
    freqs.sort((a,b) => {
      return charCounts[b] - charCounts[a];
    })
    
    freqs = freqs.map(item => {
      let accumulator = '';
      for(i = 0; i < charCounts[item]; i++) {
        accumulator += item;
      }
      return accumulator;
    });
    
    return freqs.join('')
    
};