https://programmers.co.kr/learn/courses/30/lessons/12904/solution_groups?language=javascript&type=my

palindrome.js

function solution(s){
  let {length} = s;
  while(length){
      for(let i = 0; i + length <= s.length; i++){
          if(isPalin(s.slice(i,i+length))) return length;
      }
      length--
  }
}

function isPalin(s){
  for(let i = 0, {length} = s; i < length/2; i++){
      if(s[i] !== s[length-1-i]) return false
  }
  
  return true;
}

하드코딩...