Confirm the Ending (freeCodeCamp challenge)

confirm_ending.txt

/*
Check if a string (first argument, str) ends with the given target string (second argument, target).
This challenge can be solved with the .endsWith() method, which was introduced in ES2015. 
But for the purpose of this challenge, we would like you to use one of the JavaScript substring methods instead.
*/


function confirmEnding(str, target) {

  var lastLetter = str.substr(-1);                      //get last character of str by its index (-1)
  var lastWord = str.substr(str.lastIndexOf(target));   // returns the characters in a string beginning at the specified location through the specified number of characters 
                                                        //str.lastIndexOf returns indices of (target) from a str.
  if (lastLetter == target || lastWord == target){
    return true;
  }
  else {
    return false;
  }
}

confirmEnding("are frozen", "n");