Edit in JSFiddle

function myReplace(str, before, after) {
  if (/[A-Z]/.test(before[0])) {
    after = after[0].toUpperCase() + after.substring(1, after.length);
  }
  var strToArray = str.split(" ");
  var elementToDelete = strToArray.indexOf(before); //Outputs index 4
  return strToArray.join(" ");
}
myReplace("He is Sleeping on the couch", "Sleeping", "sitting");
myReplace("His name is Tom", "Tom", "john") should
return "His name is John".
myReplace("This has a spellngi error", "spellngi", "spelling")
myReplace("Let us go to the store", "store", "mall")