Get the most occurring character in a string

getHighestRepeatingChar.js

String.prototype.getHighestRepeatingChar = function() {
  const charCountHash = {}
  let highestOccurence = 0;
  let highestRepeatingChar;
  for (let charIndex = 0; charIndex < this.length; charIndex++) {
    const currentChar = this[charIndex]
    const prevCharCount = charCountHash[currentChar] || 0
    const currentCharCount = prevCharCount + 1;
    charCountHash[currentChar] = currentCharCount;
    if (highestOccurence === 0) {
      highestOccurence = 1;
      highestRepeatingChar = currentChar;
    }
    if (currentCharCount > highestOccurence) {
      highestOccurence = currentCharCount;
      highestRepeatingChar = currentChar;
    }
  }
  return highestRepeatingChar;
}
sample = 'testing ground is growling when tesla test is tried out';
console.log(`Most repeating character in "${sample}" is "${sample.getHighestRepeatingChar()}"`)