Get the most frequent element from array of objects

getMostFrequent.js

/**
 * Filter an array of objects to get the most frequent element
 *
 * @param {Object[]} arr The array of objects
 * @param {String} keyname The name that will be used to filter the objects
 * @returns {Array}
 */
getMostFrequent(arr, keyname) {
  // Store how many times each element appears
  let lookup = {};
  for (let i = 0; i < arr.length; i++) {
    const key = arr[i][keyname];
    if (lookup[key] === undefined) lookup[key] = 0; // Insert if not available yet
    lookup[key]++;
  }

  // Transform that into an sortable array
  let sortableLookup = [];
  for (let o in lookup) {
    sortableLookup.push([o, lookup[o]]);
  }

  // Sort the elements
  let result = sortableLookup.sort((a, b) => b[1] - a[1]);

  // Get one of the most frequent
  let best = result[0];

  // Filter by `keyname` to get the most frequent words
  result = result.filter(el => el[1] === best[1]).map(el => el[0]);

  return result;
}

feel free to improve this code. thank you