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css transitions don't work when changes are made in javascript

#loading_screen {
display: none;
z-index: 1;
height: 100vh;
width: 100vw;
opacity: 0;
background-color: red;
transition: opacity 4s 0s ease;
}
<div id="loading_screen" class="page">
</div>
<script>
function hide_page() {
const loading = document.getElementById('loading_screen');
loading.style.display = 'block';
loading.style.opacity = '1';
}
hide_page()
</script>
The loading_screen div appears instantly, as if the transition didn't even exist
Is there a chance that the css is not functional immediately when I run the page?

You need to wait for the browser to update and paint the loading element first, then you can use setTimeout to change the opacity after the browser has done its paint.
function hide_page() {
const loading = document.getElementById('loading_screen');
loading.style.display = 'block';
setTimeout(() => {
loading.style.opacity = '1';
});
}
hide_page();
#loading_screen {
display: none;
z-index: 1;
height: 100vh;
width: 100vw;
opacity: 0;
background-color: red;
transition: opacity 4s ease;
}
<div id="loading_screen" class="page">
</div>

CSS transition not working when adding class via jquery

I am adding a class to show an image when the mouse is over a div, but the transition isnt working at all.
I am using opacity, I know that the visibily: hidden is not animable.
The code is in the snippet:
$(document).ready(function() {
$("#trigger").on("mouseenter", function () {
$("#imgPuffo").addClass("visible");
$("#trigger").on("mouseout", function () {
$("#imgPuffo").removeClass("visible");
});
});
});
#trigger {
height: 100px;
width: 100px;
background: red;
}
img {
opacity: 0;
animation: opacity 2s;
}
.visible {
visibility: visible;
opacity: 1;
animation: opacity 2s;
}
.imgPuffo {
height: 200px;
}
<script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.min.js"></script>
<div id="trigger"></div>
<img id="imgPuffo" class="imgPuffo" src="https://www.pinclipart.com/picdir/big/449-4499911_how-to-draw-papa-smurf-from-the-smurfs.png" alt="">

There's a bit of confusion as animation is being used, but animation will look for an #keyframes sequence to tell it what animation to run. In fact it looks as though we don't need a full CSS animation in this case, just a CSS transition.
I've added transition: all 2s in case you want to transition anything else in future, like the scale, but if you just want to stick with transitioning opacity you could do transition: opacity 2s instead.
$(document).ready(function() {
$("#trigger").on("mouseenter", function () {
$("#imgPuffo").addClass("visible");
$("#trigger").on("mouseout", function () {
$("#imgPuffo").removeClass("visible");
});
});
});
#trigger {
height: 100px;
width: 100px;
background: red;
}
img {
opacity: 0;
/* animation: opacity 2s; */
transition: all 2s;
}
.visible {
visibility: visible;
opacity: 1;
/* animation: opacity 2s; */
}
.imgPuffo {
height: 200px;
}
<script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.min.js"></script>
<div id="trigger"></div>
<img id="imgPuffo" class="imgPuffo" src="https://www.pinclipart.com/picdir/big/449-4499911_how-to-draw-papa-smurf-from-the-smurfs.png" alt="">

if you want to use animations check the docs. You need to use #keyframes
$(document).ready(function() {
$("#trigger").on("mouseenter", function () {
$("#imgPuffo").addClass("visible");
$("#trigger").on("mouseout", function () {
$("#imgPuffo").removeClass("visible");
});
});
});
#trigger {
height: 100px;
width: 100px;
background: red;
}
img {
opacity: 0;
animation: opacity 2s;
}
.visible {
visibility: visible;
opacity: 1;
transition: opacity 2s ease-in-out;
}
.imgPuffo {
height: 200px;
}
<script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.min.js"></script>
<div id="trigger"></div>
<img id="imgPuffo" class="imgPuffo" src="https://www.pinclipart.com/picdir/big/449-4499911_how-to-draw-papa-smurf-from-the-smurfs.png" alt="">

How about a solution for css using a :hover, without jquery?
#trigger {
height: 100px;
width: 100px;
background: red;
}
#trigger:hover + .imgPuffo {
opacity: 1;
}
.imgPuffo {
opacity: 0;
height: 200px;
transition: opacity 2s;
}
<script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.min.js"></script>
<div id="trigger"></div>
<img id="imgPuffo" class="imgPuffo" src="https://www.pinclipart.com/picdir/big/449-4499911_how-to-draw-papa-smurf-from-the-smurfs.png" alt="">

JavaScript / CSS hover function

Would it be possible to create something like what I have in the code below with just vanilla JavaScript or CSS? I also want it to have the hover div stay open until a button click, which would then slide the original div back down. Anything helps, cheers!
$(document).ready(function(){
$('.up-down').mouseover(function(){
$('.default').stop().animate({
height: 0
}, 200);
}).mouseout(function(){
$('.default').stop().animate({
height: 200
}, 200)
})
});
.up-down {
overflow:hidden;
height:200px;
width:200px;
}
.slide {
width:200px;
height:200px;
}
.default {
background-color:#ccc;
}
.onhover {
background-color:#1DB7CB;
}
<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>
<div class="up-down">
<div class="slide default"></div>
<div class="slide onhover"></div>
</div>

Using your code as background, you can do it like this:
Remember Jquery is just a library of Javascript, anything you can do o Jquery you can do it on Javascript.
var updown = document.querySelector('.up-down');
var def = document.querySelector('.default');
var btn = document.querySelector('.clickMe');
updown.addEventListener('mouseover', function() {
def.style.height = '0px';
});
btn.addEventListener('click', function() {
def.style.height = '200px';
});
.up-down {
overflow:hidden;
height:200px;
width:200px;
}
.slide {
width:200px;
height:200px;
}
.default {
background-color:#ccc;
transition: height .2s linear;
}
.onhover {
background-color:#1DB7CB;
}
<div class="up-down">
<div class="slide default"></div>
<div class="slide onhover"><button class="clickMe">
Click me
</button></div>
</div>

Are you looking for something like this?
.up-down {
height: 200px;
width: 200px;
}
.slide {
width: 100%;
height: 100%;
transition: all 0.5s;
}
.default {
background: blue;
}
.onhover {
background: green;
height: 0%;
}
.up-down:hover .default {
height: 0%;
}
.up-down:hover .onhover {
height: 100%;
}
<div class="up-down">
<div class="slide default"></div>
<div class="slide onhover"></div>
</div>

Use CSS3 transition for height value of the div with onhoverclass attribute:
.up-down {
position: relative;
overflow: hidden;
height: 200px;
width: 200px;
}
.slide {
width: 200px;
height: 200px;
}
.default {
background-color: #ccc;
}
.onhover {
position: absolute;
bottom: 0;
height: 0;
background-color: #1DB7CB;
-webkit-transition: height 1s ease-out 0.5s;
-moz-transition: height 1s ease-out 0.5s;
-o-transition: height 1s ease-out 0.5s;
transition: height 1s ease-out 0.5s;
}
.up-down:hover .onhover {
height: 200px;
}
<div class="up-down">
<div class="slide onhover"></div>
<div class="slide default"></div>
</div>
To change effect direction use top: 0; instead of bottom: 0;

Would it be possible to create something like what I have in the code below with just vanilla JavaScript ?
Yes, everything that's done with jquery is doable with javascript.
With css
It sure is. Here is something to get you started
.up-down {
overflow:hidden;
height:200px;
width:200px;
position: relative;
}
.slide {
width:200px;
height:200px;
position:absolute;
}
.default {
background-color:#ccc;
}
.onhover {
background-color:#1DB7CB;
z-index: 2;
top: 200px;
transition: top 0.4s;
}
.up-down:hover .onhover {
top: 0px;
}
<div class="up-down">
<div class="slide default"></div>
<div class="slide onhover"></div>
</div>
So to explain the code somewhat:
.up-down:hover .onhover {
top: 0px;
}
on hover of up-down put the .onhover back up.
.onhover {
top: 200px;
transition: top 0.4s;
}
Initial position under the grey div, the transition makes it so :hover will be done on the span of 0.4s.
If you want the div to stay blue when the mouseleave, just add a class on mouseenter. You probably still can do it with pure css but I guess it would be harder than this:
.onhover {
background-color:#1DB7CB;
z-index: 2;
top: 200px;
transition: top 0.4s;
}
addedClass{
top: 0px;
}
// target is up-down
target.addEventListener("mouseenter", e => {
// element is your onhover elem
element.classList.add("addedClass");
});
target.addEventListener("click", e => {
element.classList.remove(" addedClass");
});

You can add event listeners with addEventListener.
The animation can be implemented with the transition property so a change in height will slowly transition instead of happening immediately.
I also added the button like you requested in your edit.
var updown = document.querySelector('.up-down');
var def = document.querySelector('.default');
var btn = document.querySelector('#btn');
updown.addEventListener('mouseover', function() {
def.style.height = '0px';
});
updown.addEventListener('mouseout', function() {
def.style.height = '200px';
});
btn.addEventListener('click', function() {
var isDown = def.style.height === '200px';
def.style.height = isDown ? '0px' : '200px';
});
.up-down {
overflow:hidden;
height:200px;
width:200px;
}
.slide {
width:200px;
height:200px;
}
.default {
background-color:#ccc;
transition: height .2s linear;
}
.onhover {
background-color:#1DB7CB;
}
<button type="button" id="btn">Toggle</button>
<div class="up-down">
<div class="slide default"></div>
<div class="slide onhover"></div>
</div>

Hard transition of other properties when display property is involved

I want to fade between two differently sized elements within a container overlaying each other. The first element should be faded out, then the container resized and finally the other element faded in.
Here's the related snippet:
var layer1 = document.getElementById("layer1");
var layer2 = document.getElementById("layer2");
function switchLayers() {
layer1.addEventListener("transitionend", function() {
layer2.classList.add("fadein");
});
layer1.classList.add("fadeout");
}
#container {
position: relative;
background-color: yellow;
padding: 10px;
overflow: hidden;
}
.layer {
position: relative;
width: 400px;
}
#layer1 {
height: 100px;
float: left;
background-color: blue;
}
#layer2 {
height: 150px;
background-color: red;
display: none;
opacity: 0;
}
#layer1.fadeout {
opacity: 0;
transition: opacity 1s ease-out;
}
#layer2.fadein {
display: block;
opacity: 1;
transition: opacity 1s ease-out;
}
<button onclick="switchLayers()">Switch layers</button>
<div id="container">
<div id="layer1" class="layer"></div>
<div id="layer2" class="layer"></div>
</div>
When the second layer's display property is set to block it works as expected, i.e. the opacity is changed from 0 to 1 within a second. Though if it's set to none, the transition suddenly is discrete.
I've tried to set all within the transition value to transition all properties and also tried to include the display property in the transition like this:
transition: display 0s, opacity 1s ease-out;
Though without success. Note that because the container should resize to the size of the currently displayed layer, the visibility property can't be used (as it hides the element but still lets it occupy the space).
How to made this work?

Try using the visibility property instead of display.
For more information regarding the state changes in visibility and display, refer article.
For transitioning the parent height, you have to manually change the height property of the #container. Using display: block & display: none will never transition the parent.
Refer code:
var layer1 = document.getElementById("layer1");
var layer2 = document.getElementById("layer2");
function switchLayers() {
layer1.addEventListener("transitionend", function() {
layer2.classList.add("fadein");
document.getElementById("container").style.height = "170px";
});
layer1.classList.add("fadeout");
}
#container {
position: relative;
background-color: yellow;
padding: 10px;
height: 100px;
overflow: hidden;
transition: all 0.5s;
}
.layer {
position: relative;
width: 400px;
}
#layer1 {
height: 100px;
float: left;
background-color: blue;
}
#layer2 {
height: 150px;
background-color: red;
visibility: none;
opacity: 0;
}
#layer1.fadeout {
visibility: none;
opacity: 0;
transition: all 1s ease-out;
}
#layer2.fadein {
visibility: visible;
opacity: 1;
transition: all 1s ease-out;
}
<button onclick="switchLayers()">Switch layers</button>
<div id="container">
<div id="layer1" class="layer"></div>
<div id="layer2" class="layer"></div>
</div>

There is no straightforward way. Transitions do not work on display, nor do they work on auto height. So, visibility is a good bet.
Note that because the container should resize to the size of the
currently displayed layer, the visibility property can't be used (as
it hides the element but still lets it occupy the space).
Then, you will need to hack it out. You can make use of min-height. Give a faux min-height to your container, and then apply the height of your layer2 to it once the transition ends. Also, because display on layer2 will block the transition, you need to separate out the classes for display and opacity and space out their application using a zero timeout in between.
Here is a crude idea:
var layer1 = document.getElementById("layer1"),
layer2 = document.getElementById("layer2"),
container = document.getElementById("container"),
h = window.getComputedStyle(layer2).getPropertyValue("height");
container.addEventListener("transitionend", function(e) {
if (e.target.id === 'layer1') {
// apply layer2 height to container min-height
container.style.minHeight = h;
}
if (e.target.id === 'container') {
// First show the layer2
layer2.classList.add("show");
// Then a dummy pause to fadein
setTimeout(function(){
layer2.classList.add("fadein");
}, 0);
}
}, false);
function switchLayers() {
layer1.classList.add("fadeout");
}
#container {
position: relative;
background-color: yellow;
padding: 10px; overflow: hidden;
min-height: 1px; /* faux min-height */
transition: min-height 1s linear;
}
.layer { position: relative; width: 400px; }
#layer1 {
height: 100px; float: left;
background-color: blue;
transition: all 1s linear;
}
#layer2 {
height: 150px; background-color: red;
display: none; opacity: 0;
transition: opacity 1s linear;
}
#layer1.fadeout { opacity: 0; }
#layer2.show { display: block; } /* Separate out display */
#layer2.fadein { opacity: 1; } /* Separate out opacity */
<button onclick="switchLayers()">Switch layers</button>
<div id="container">
<div id="layer1" class="layer"></div>
<div id="layer2" class="layer"></div>
</div>

a tag showing image on hover with transition

I'm trying to display an image on the hover of an a tag which I have got complete and fully working, now I'm facing a problem...
I want the image to 'fade' in and out of the page when it is hovered rather than just appearing in order to give it a more satisfying finish.
My code at the minute is...
a.top-row:hover:after {
transition: .5s;
-webkit-transition: .5s;
-moz-transition: .5s;
display: block;
position: absolute;
z-index: 10;
top: -295px;
left: -30px; }
and to display the image I have seperate id's for each of the a tags so it's done like this..
a#a1:hover:after {
content: url(../images/cars/audi/a1.png); }
HTML -
<a class="top-row" id="a1">A1</a>

Like this? (background fixed, animating opacity)
a.top-row {
position: relative;
}
a.top-row:after {
transition: .5s;
-webkit-transition: .5s;
-moz-transition: .5s;
display: block;
position: absolute;
top: 100%;
left: 100%;
z-index: 10;
width: 70px;
height: 50px;
opacity: 0;
content: ' ';
pointer-events: none;
}
#a1:after {
background: url(https://placehold.it/70x50);
}
#r8:after {
background: url(https://placehold.it/70x50/ff00ff);
}
a.top-row:hover:after {
opacity: 1;
}
<a class="top-row" id="a1">A1</a><br/>
<a class="top-row" id="r8">R8</a>

Having images fade-in using only CSS would only make it complicated. Instead you can use the jQuery fadeIn() and fadeOut() method to achieve the same.
HTML
<div>
<a link="" id="showme">abcd</a>
<img src="image.jpg" height="200" width="300" alt="button" id="btn">
</div>
jQuery
$('#showme').hover(
function() {
$('#btn').fadeIn('slow');
},function() {
$('#btn').fadeOut('slow');
});
CSS
div {
width:305px;
height:229px;
}
#btn {
display:none;
}