Refactored

function solve() {

  let textToManipulateElement = document.getElementById('text').value;
  let namingConventionElement = document.getElementById('naming-convention').value;
  let resultElement = document.getElementById('result');


  let resultString = '';

  let textToManipulateElementArr = textToManipulateElement.split(' ');;

  if(namingConventionElement === 'Camel Case'){


    for(let i = 0 ; i < textToManipulateElementArr.length ; i++){
      if(i === 0){
        resultString += textToManipulateElementArr[i].toLowerCase();

      }else{

        let word = textToManipulateElementArr[i];
        let firstLetter = word[0].toUpperCase();

        word = word.toLowerCase();
        resultString += (firstLetter + word.slice(1));



      }
    }

    resultElement.textContent = resultString;


  }else if(namingConventionElement === 'Pascal Case'){

    for(let i = 0 ; i < textToManipulateElementArr.length ; i++){

        let word = textToManipulateElementArr[i];
        let firstLetter = word[0].toUpperCase();

        word = word.toLowerCase();
        resultString += (firstLetter + word.slice(1));


    }
    resultElement.textContent = resultString;


  }else{
    resultElement.textContent = 'Error!'

  }
}