Secondly, look through the your code for any variables you are not actually using - there is one that you declare that is never used for anything, so you can safely delete it.
Finally, look again at your logic in that if block. For those three cases ((popped === target) || (lastString === target) || (chkString === target)). Think about whether any of these are actually just redundant because one of the other ones implicitly includes their logic. You will find that you only need one of them, not all three
On preview: Damnit, @P1xt beat me to the punch (with better code)
Spoiler - show code
function confirmEnding(str, target) {
var firstString = str.split(" ");
var popped = firstString.pop();
var oneWordCount = target.length;
var lastString = target.substring(oneWordCount);
var chkString = str.substring(str.length - oneWordCount);
return ((chkString === target));
}
confirmEnding("Bastian", "an");