I would suggest you use CryptoJS in this case.
Basically CryptoJS is a growing collection of standard and secure cryptographic algorithms implemented in JavaScript using best practices and patterns. They are fast, and they have a consistent and simple interface.
So if you want to calculate the MD5 hash of your password string then do as follows:
<script src="https://cdnjs.cloudflare.com/ajax/libs/crypto-js/3.1.9-1/core.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/crypto-js/3.1.9-1/md5.js"></script>
<script>
var passhash = CryptoJS.MD5(password).toString();
$.post(
'includes/login.php',
{ user: username, pass: passhash },
onLogin,
'json' );
</script>
So this script will post the hash of your password string to the server.
For further info and support on other hash calculating algorithms you can visit: