Why am I getting undefined in the output?

Question

unction spinWords(string){
    let newArr = string.split(' ')
    let reversedWord = ""; 
    for(let i = 0; i < newArr.length; i++){
        let curr = newArr[i]; 
        if (curr.length >= 4){
            for(let j = curr.length; j >= 0; j--){
               reversedWord += curr[j];
            }

        }
    }
    return reversedWord;
  }
  console.log(spinWords("Hey fellow warriors"))
output:  undefinedwollefundefinedsroirraw

I am trying to retun the reverseWord string concatenated with the words has more than 5 letters.