Updating Password with Ajax

What am I doing wrongly in these code? I want to update user password without refreshing the page, but this is not working as my code stops at  

_("pmessage").innerHTML = 'please wait ...';

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on the jquery/ajax code. The Ajax seems not to be firing as my DB is not updated with the new password. Below are my codes:

HTML

<form id='pform' method='post'>
<div id='entercurrent'>Enter Current Password:</div>
<input id='current' name='current' type='password'>
<div id='enternew'>Enter New Password:</div>
<input id='newpwd' name='newpwd' type='password'>
<div id='enterconfirm'>Confirm New Password:</div>
<input id='confirmpwd' name='confirmpwd' type='password'>
<input id='passsubmit' type='submit' value='Update Password'>
</form>
<div id='pmessage'></div>

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Ajax

$(document).ready(function() {
    $("#passsubmit").click(function(event) {
        event.preventDefault();
        var cp = $("#current").val();
        var np = $("#newpwd").val();
        var cf = $("#confirmpwd").val();
        dataString = "cp=" + cp + "&np=" + np + "&cf=" + cf;
        _("passsubmit").style.display = "none";
        _("pmessage").innerHTML = 'please wait ...';

        $.ajax({
            type: "POST",
            url: "functions/changePwd.php",
            data: dataString,
            success: function() {}
        });
    });
});

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PHP -  functions/changePwd.php

if(isset($_POST['cp'])) {
    $currentpwd = $_POST['cp'];
    $newpwd = password_hash($_POST['np'], PASSWORD_DEFAULT);
    $confirmnewpwd = password_hash($_POST['cf'], PASSWORD_DEFAULT);
}
$sql = "UPDATE users SET password='$newpwd' WHERE username='$log_username'";
$query = mysqli_query($db_connect, $sql);

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