jQuery.Deferred exception: toastr is not defined when using

Hello!I'm trying to invoke alert message each time when user fails to login. So I written this function which must be executed when document loads:

$(document).ready(function()

{

toastr.error('Wrong login credentials. Please, try again...');

});

But in js console i get this errors:

jQuery.Deferred exception: toastr is not defined @http://127.0.0.1:8000/login/:63:9j@http://127.0.0.1:8000/static/js/jquery-3.2.1.min.js:2:29997g/</k<@http://127.0.0.1:8000/static/js/jquery-3.2.1.min.js:2:30313undefined

ReferenceError: toastr is not defined

My scripts in html:

<script src="js/jquery-3.2.1.min.js"></script>

<script src="js/popper.min.js"></script>

<script src="js/bootstrap.min.js"></script>

<script src="js/mdb.min.js"></script>

<script type='text/javascript'>

$(document).ready(function(){

toastr.error('Wrong login credentials. Please, try again...');

});

</script>

All js files I've downloaded from Get Started page. Can you tell what is wrong with my code?