CSS Animation when closing modal

Question

I have the following code, extracted from this w3Schools tutorial. If you run it, you will see that when the "open model" is clicked, the modal is opened with a CSS animation. It's beautiful and simple. However, when you click the close button, the modal closes suddenly, instead of running a inverse animation. How can I define in CSS a way to make the animation run when closing the modal? Thanks.

// Get the modal
var modal = document.getElementById('myModal');

// Get the button that opens the modal
var btn = document.getElementById("myBtn");

// Get the <span> element that closes the modal
var span = document.getElementsByClassName("close")[0];

// When the user clicks on the button, open the modal
btn.onclick = function() {
    modal.style.display = "block";
}

// When the user clicks on <span> (x), close the modal
span.onclick = function() {
    modal.style.display = "none";
}

// When the user clicks anywhere outside of the modal, close it
window.onclick = function(event) {
    if (event.target == modal) {
        modal.style.display = "none";
    }
}

 /* The Modal (background) */
.modal {
    display: none; /* Hidden by default */
    position: fixed; /* Stay in place */
    z-index: 1; /* Sit on top */
    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/Box */
.modal-content {
    background-color: #fefefe;
    margin: 15% auto; /* 15% from the top and centered */
    padding: 20px;
    border: 1px solid #888;
    width: 80%; /* Could be more or less, depending on screen size */
}

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

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

/* Modal Header */
.modal-header {
    padding: 2px 16px;
    background-color: #5cb85c;
    color: white;
}

/* Modal Body */
.modal-body {padding: 2px 16px;}

/* Modal Footer */
.modal-footer {
    padding: 2px 16px;
    background-color: #5cb85c;
    color: white;
}

/* Modal Content */
.modal-content {
    position: relative;
    background-color: #fefefe;
    margin: auto;
    padding: 0;
    border: 1px 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
}

/* 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}
}

 <!-- Trigger/Open The Modal -->
<button id="myBtn">Open Modal</button>

<!-- The Modal -->
<div id="myModal" class="modal">

   <!-- Modal content -->
<div class="modal-content">
  <div class="modal-header">
    <span class="close">&times;</span>
    <h2>Modal Header</h2>
  </div>
  <div class="modal-body">
    <p>Some text in the Modal Body</p>
    <p>Some other text...</p>
  </div>
  <div class="modal-footer">
    <h3>Modal Footer</h3>
  </div>
</div>

</div>

Answer 1

Use transition instead of animation, and then toggle a class instead of change the inline style

Note, you might need some update to deal with whether to animate the gray background or not

// Get the modal
var modal = document.getElementById('myModal');

// Get the button that opens the modal
var btn = document.getElementById("myBtn");

// Get the <span> element that closes the modal
var span = document.getElementsByClassName("close")[0];

// When the user clicks on the button, open the modal
btn.onclick = function() {
    modal.classList.add('show');
}

// When the user clicks on <span> (x), close the modal
span.onclick = function() {
    modal.classList.remove('show');
}

// When the user clicks anywhere outside of the modal, close it
window.onclick = function(event) {
    if (event.target == modal) {
      modal.classList.remove('show');
    }
}

/* The Modal (background) */
.modal {
    position: fixed; /* Stay in place */
    z-index: 1; /* Sit on top */
    left: 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 */
    top: -100%;
    opacity: 0;
    transition: top 0.4s, opacity 0.4s;
}
.modal.show {
    top: 0;
    opacity: 1;
}

/* Modal Content/Box */
.modal-content {
    background-color: #fefefe;
    margin: 15% auto; /* 15% from the top and centered */
    padding: 20px;
    border: 1px solid #888;
    width: 80%; /* Could be more or less, depending on screen size */
}

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

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

/* Modal Header */
.modal-header {
    padding: 2px 16px;
    background-color: #5cb85c;
    color: white;
}

/* Modal Body */
.modal-body {padding: 2px 16px;}

/* Modal Footer */
.modal-footer {
    padding: 2px 16px;
    background-color: #5cb85c;
    color: white;
}

/* Modal Content */
.modal-content {
    position: relative;
    background-color: #fefefe;
    margin: auto;
    padding: 0;
    border: 1px 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);
}

<!-- Trigger/Open The Modal -->
<button id="myBtn">Open Modal</button>

<!-- The Modal -->
<div id="myModal" class="modal">

   <!-- Modal content -->
<div class="modal-content">
  <div class="modal-header">
    <span class="close">&times;</span>
    <h2>Modal Header</h2>
  </div>
  <div class="modal-body">
    <p>Some text in the Modal Body</p>
    <p>Some other text...</p>
  </div>
  <div class="modal-footer">
    <h3>Modal Footer</h3>
  </div>
</div>

</div>