String compression

string_compression.js

function compressString(str) {
  const arrayOfChars = str.split('')

  let currentChar = arrayOfChars[0]
  let timeOfAppear = 1

  const mapOfChars = arrayOfChars.slice(1).reduce((result, value, index) => {
    if (value === currentChar) {
      timeOfAppear++
      return result
    }

    result = result + timeOfAppear + value

    currentChar = value
    timeOfAppear = 1

    return result
  }, currentChar)

  const compressString = mapOfChars + timeOfAppear

  if (compressString.length > str.length && compressString !== str) {
    return str
  }

  return compressString
}

console.log(compressString('aaaaaabbbbbccccc')) // a6b5c5
console.log(compressString('aaaaaaaaaa')) // a10
console.log(compressString('abcabcabc')) // abcabcabc
console.log(compressString('aaaaaasdddddd')) // a6s1d6