Pascal or Camel Case JS Advanced

function solve(first, second) {
  first = document.getElementById('text').value;
  second = document.getElementById('naming-convention').value;
  let resultElement = document.getElementById('result');

  let smallLetters = first.toLowerCase();
  let arr = smallLetters.split(' ');
  let newString = [];

  if (second === "Camel Case") {
    for (let i = 1; i < arr.length; i++) {
      let word = arr[i];
      let firstLetter = word[0];
      firstLetter = firstLetter.toUpperCase();
      let remainWord = word.slice(1);
      let newWord = firstLetter + remainWord;

      newString.push(newWord);
  }
  let result = arr[0] + newString.join('');
    resultElement.textContent = result;
  } else if (second === "Pascal Case") {
    for (let i = 0; i < arr.length; i++) {
      let word = arr[i];
      let firstLetter = word[0];
      firstLetter = firstLetter.toUpperCase();
      let remainWord = word.slice(1);
      let newWord = firstLetter + remainWord;

      newString.push(newWord);
    }
    let result = newString.join('');
    resultElement.textContent = result;
  } else {
    resultElement.textContent = "Error!";
  }
}

OR

function solve(first, second) {
  first = document.getElementById('text').value;
  second = document.getElementById('naming-convention').value;
  let resultElement = document.getElementById('result');

  let smallLetters = first.toLowerCase();
  let arr = smallLetters.split(' ');

  if (second === "Camel Case") {
    let firstWord = arr.shift(arr[0]);
    let newString = arr.map(x => x.charAt(0).toUpperCase() + x.slice(1)).join('');

    let result = firstWord + newString;
    resultElement.textContent = result;
  } else if (second === "Pascal Case") {
    let newString = arr.map(x => x.charAt(0).toUpperCase() + x.slice(1)).join('');
    resultElement.textContent = newString;
  } else {
    resultElement.textContent = "Error!";
  }
}