const makeChar = param => `${param}_`
const makeNextChar = param => [...param.toString()].reduce((result, char, idx, self) => {
     result += idx === self.length - 1 ? +(++char) + '_' : result += +(++char)
    return result
  }, '')

let result = ''

const repeat = (param, count, total) => {
  const initCount = count
  let current = param
  
  while (count > 0) {
    if (count === initCount) {
      result += makeChar(current)
    } else {
      const nextChar = makeNextChar(current)
      result += nextChar
      current = nextChar.split('_')[0]
    }
      count--
  }
  
  if (total > 0) {
    repeat(param, count = 3, total - 1)
  }
  
  return result
}
repeat(77, 3, 3)

console.log(result)