Quick function that returns the character with the highest count from the string.

get-character-with-highest-count.js

/**
 * Returns the character seen the most in the string
 */
const getCharacterWithHighestCount = str =>
  Array.from(
    str
      .split('')
      .reduce(
        (map, char) =>
          map.has(char)
             // if the map already has the character, we increment how many times we have seen it
            ? map.set(char, map.get(char) + 1)
             // if it doesn't have it, then we set the default to 1
            : map.set(char, 1),
        new Map()
      )
      .entries()
    // we go over all entires in the map to get the highestCount for each char.
  ).reduce(
    ([highestChar, highestCount], [char, count]) =>
      count > highestCount ? [char, count] : [highestChar, highestCount]
    // finally we return the character from the highest
  )[0];

console.log(getCharacterWithHighestCount('hello world')) // l;