const array = ["a", "a", "b", "b", "c", "c", "c", "c", "d", "d", "e"]

function arrayCompress(arr) {
  if(!arr.length) return []
  const result = []
  let current = arr.shift()
  let count = 1
  result.push(current)
  while(arr.length) {
    let next = arr.shift()
    if(current === next) count += 1
    else {
      if(count !== 1) {
        result.push(count)
        result.push(next)
      }
      else {
        result.push(next)
      }

      count = 1
    }
    current = next
  }
  if(count !== 1) result.push(count)

  return result
}
arrayCompress(array)