A very simple modal dialog

index.html

DEMO: https://jsfiddle.net/k3zfzxgx/

<!-- HTML -->
<div id="loginModal" class="modal">
    <div class="modal-content modal-login">
        <div class="modal-header">
            <span class="modal-close">×</span>
            <h2>My title</h2>
        </div>
        <div class="modal-body">
            <p>My modal dialog</p>
        </div>
        <div class="modal-footer">
            Modal footer
        </div>
    </div>
</div>

modal.js

function ModalDialog(options) {
    var self = this;
    this.modalEl = options.el;

    // Get the modal
    this.modal = document.getElementById(this.modalEl);
    this.close = document.getElementsByClassName("modal-close")[0];
    
    // When the user clicks anywhere outside of the modal, close it
    window.onclick = function(event) {
        if (event.target == self.modal) {
            self.modal.style.display = "none";
        }
    };

    this.close.onclick = function() {
        self.modal.style.display = "none";
    };
}

ModalDialog.prototype.open = function() {
    this.modal.style.display = "block";
};

$(function() {
    var loginDialog = new ModalDialog({
        'el': 'loginModal'
    });
    
    $('.open-modal-element').on('click', function(){
        loginDialog.open();
    });
});

style.css

.modal .modal-login {
    max-width: 400px;
}

.modal-header h2{
    margin: 0;
}
/* The Modal (background) */
.modal {
    display: none; /* Hidden by default */
    position: fixed; /* Stay in place */
    z-index: 1; /* Sit on top */
    padding-top: 100px; /* Location of the box */
    left: 0;
    top: 0;
    width: 100%; /* Full width */
    height: 100%; /* Full height */
    overflow: auto; /* Enable scroll if needed */
    background-color: rgb(0,0,0); /* Fallback color */
    background-color: rgba(0,0,0,0.4); /* Black w/ opacity */
}

/* Modal Content */
.modal-content {
    position: relative;
    background-color: #fefefe;
    margin: auto;
    padding: 0;
    border: 0px solid #888;
    width: 80%;
    box-shadow: 0 4px 8px 0 rgba(0,0,0,0.2),0 6px 20px 0 rgba(0,0,0,0.19);
    -webkit-animation-name: animatetop;
    -webkit-animation-duration: 0.4s;
    animation-name: animatetop;
    animation-duration: 0.4s;
    border-radius: 4px;
}

/* Add Animation */
@-webkit-keyframes animatetop {
    from {top:-300px; opacity:0} 
    to {top:0; opacity:1}
}

@keyframes animatetop {
    from {top:-300px; opacity:0}
    to {top:0; opacity:1}
}

/* The Close Button */
.modal-close {
    color: white;
    float: right;
    font-size: 28px;
    font-weight: bold;
}

.modal-close:hover,
.modal-close:focus {
    color: #000;
    text-decoration: none;
    cursor: pointer;
}

.modal-header {
    padding: 2px 16px;
    background-color: #313334;
    color: white;
}

.modal-body {padding: 2px 16px; color: #9fa1a1}

.modal-footer {
    padding: 2px 16px;
    background-color: #ccc;
    color: white;
}