How to come back to the start prompt after else?

I would like the else part of the following if-statement to make that prompt pop up again. In other words, I want the prompt to come back if user writes something other than "yes" or "no".

var str = prompt("Do you want to come in?").toLowerCase();
if (srt === "yes"){
    alert("cool.");
}
else if (str === "no"){
    alert("goodbye.");
}
else {
    var str = prompt("Do you want to come in?").toLowerCase();
}