JavaScript/String/MaxCharacter.js

/**
 * @function maxCharacter
 * @example - Given a string of characters, return the character that appears the most often. Example: input = "Hello World!" return "l"
 * @param {string} str
 * @param {RegExp} ignorePattern - ignore the char in str that is not required
 * @returns {string} - char
 */
const maxCharacter = (str, ignorePattern) => {
  // initially it's count only alphabets
  if (typeof str !== 'string') {
    throw new TypeError('Argument should be a string')
  } else if (!str) {
    throw new Error('The param should be a nonempty string')
  }

  // store all char in occurrence map
  const occurrenceMap = new Map()

  for (const char of str) {
    if (!ignorePattern?.test(char)) {
      occurrenceMap.set(char, occurrenceMap.get(char) + 1 || 1)
    }
  }

  // find the max char from the occurrence map
  let max = { char: '', occur: -Infinity }

  for (const [char, occur] of occurrenceMap) {
    if (occur > max.occur) {
      max = { char, occur }
    }
  }

  return max.char
}

export default maxCharacter