Javascript indexOf() not working

Javascript indexOf() not working

I am working on a service that randomly generates a URL and then uploads
some pasted HTML code to the URL with PHP fwrite(). As a precation, I
added a system to check if the URL has already been taken:
var URL = "thehtmlworkshop.com/test4.html";
$('#existingurls').load('existingurls.txt');
var existing = $('#existingurls').html();
var isunique = existing.indexOf( URL );
if (isunique == -1) {
alert('Form submit');
} else {
alert('Whoops! Looks like the randomly generated URL was already taken.
Please try again (this will be automatic in future).');
}
existingurls.txt contains all the created URLs. When I first tried it, to
test what would happen when the URL was a duplicate of a current URL,
instead of using the random 7 letter string generator I just put one of
the URLs already in the txt file.
This is the contents of existingurls.txt:
thehtmlworkshop.com/test1.html
thehtmlworkshop.com/test2.html
thehtmlworkshop.com/test3.html
thehtmlworkshop.com/test4.html
thehtmlworkshop.com/test5.html
Anyway, what should happen is that indexOf searches for all occurances of
'thehtmlworkshop.com/test4.html' and return it's position as 91 or
whatever its position is and it would then tell the user that the randomly
generated URL was taken. However, it seems to return -1 each time because
it always goes to the submit form dialog.
NOTE: Yes, I am using jQuery.