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How can I control CSS transitions and animations with JS?

I have an image that goes from opacity 0 to 1 when a bit of text is hovered. I would like the transition to be smooth, something similar to CSS transition. Can't really figure out how to make this happen, so any help would be appreciated.
The JavaScript looks like this:
document.getElementById("text-hover").addEventListener("mouseover", imageTransition);
document.getElementById("text-hover").addEventListener("mouseout", imageTransitionOut);
function imageTransition() {
document.getElementById("pic").style.opacity = "1";
}
function imageTransitionOut() {
document.getElementById("pic").style.opacity = "0";
}

Just define the transition in css, it will trigger when you change the opacity value in javascript:
#pic {
transition: opacity .3s;
opacity: 0;
}
You don't need to change your javascript
Update
If you need to animate more than one property, it is better to define the animation in css and then trigger it from javascript by toggling a class on the element
the css:
#pic {
transition: all .3s;
opacity: 0;
transform: scale(.1);
}
#pic.animate {
opacity: 1;
transform: scale(1);
}
javascript:
var textHover = document.getElementById("text-hover");
var pic = document.getElementById("pic");
textHover.addEventListener("mouseover", function() {
pic.classList.add('animate');
});
textHover.addEventListener("mouseout", function() {
pic.classList.remove('animate');
});

CSS page transition by adding & removing a class (Vanilla JavaScript)

I'm attempting to create a transition that quickly fades in the page on load and then when a link is clicked, the page fades out and moves up slightly.
To do this I created the class 'is-loaded' which I would apply to the elements ('wrapper' and 'page-fade'). When the document is loaded, the 'is-loaded' class would be added, then when a link is clicked, the same class would be removed from both elements.
The code:
.wrapper {
transform: translateY(-25px);
transition: transform cubic-bezier(0.25, 0.46, 0.45, 0.94) 0.3s;
}
.wrapper.is-loaded {
transform: translateY(0);
}
.page-loader {
position: fixed;
top: 0;
right: 0;
bottom: 0;
left: 0;
background-color: #ffffff;
z-index: 999999;
transition: opacity cubic-bezier(0.25, 0.46, 0.45, 0.94) 0.4s, visibility cubic-bezier(0.25, 0.46, 0.45, 0.95) 0.4s;
}
.page-loader.is-loaded {
opacity: 0;
visibility: hidden;
}
document.querySelectorAll('a').forEach(e => {
e.addEventListener('click', function() {
event.preventDefault()
var i = this.getAttribute('href')
var wrapper = document.querySelector('wrapper');
var pageloader = document.querySelector('page-loader');
wrapper.classList.remove('is-loaded'),
setTimeout(function() {
pageloader.classList.remove('is-loaded');
}, 80),
setTimeout(function() {
window.location = i
}, 100)
})
})
document.addEventListener("DOMContentLoaded", function() {
var wrapper = document.querySelectorAll('wrapper');
var pageloader = document.querySelectorAll('page-loader');
wrapper.classList.add('is-loaded'),
pageloader.classList.add('is-loaded');
})
I'm new to writing in JavaScript so i'm unsure why my code does nothing. Is there a way to get this to work? No jQuery please and just vanilla js. Thank you.

You're missing the prefix selector notation . necessary to find 'wrapper' or 'page-loader' with document.querySelector()|document.querySelectorAll(). In both cases, you'd use the same kind of selector syntax as you do in your CSS.
var wrapper = document.querySelectorAll('.wrapper');
var pageloader = document.querySelectorAll('.page-loader');
Also, document.querySelectorAll() returns a(n Array-like) HTML NodeList, which doesn't have a classList property. You could convert the NodeList to an Array and iterate through the result, adding to the classList of each element or, if you know there's only one of what you're looking for on the page, swap document.querySelectorAll() for document.querySelector() — which will return a single element.

How to reset rotation after onclick execution and make it possible to call it gain

I have a spinning arrows image. On click, it refreshes a captcha. I have it defined in a short css, with a rotation function called on click.
These are the CSS, the HTML and the Javascript:
function rotatearrow() {
arrowreload.style.webkitTransform = 'rotate(360deg)';
arrowreload.style.MozTransform = 'rotate(360deg)';
arrowreload.style.msTransform = 'rotate(360deg)';
}
function reloadcaptcha() {
//does what's needed and plays perfectly
}
.reload {
max-width: 32px;
height: auto;
transform: rotate(0);
transition: all 0.8s;
}
<img id="arrowreload" class="reload" src="../images/reload.png" onclick="rotatearrow(); reloadcaptcha();">
Now the point is: rotatearrow plays at the first click and rotate the arrows, but it never plays after the first time. What am I doing wrong? What changes I have to do to the code?

Every time you click you need to increment the degree:
let degree = 0;
function rotatearrow() {
degree += 360;
arrowreload.style.webkitTransform = `rotate(${degree}deg)`;
}
.reload {
max-width: 32px;
height: auto;
transform: rotate(0);
transition: all 0.8s;
}
<img id="arrowreload" class="reload" src="http://pluspng.com/img-png/triangle-png-triangle-png-clipart-2400.png" onclick="rotatearrow();">

This will work, we just need to keep incrementing the rotation value in multiples i.e -- 360,720,1080.....
function rotatearrow() {
var transformValue = document.getElementById('arrowreload').style.webkitTransform;
var transformInteger = transformValue ? ((transformValue.match(/\d+/g).map(Number)[0]/360)+1) : 1;
document.getElementById('arrowreload').style.webkitTransform = 'rotate('+(360*transformInteger)+'deg)';
}
Hope this is helpful, also this doesn't require external variable declaration...

Flashing text on value change [duplicate]

I'm brand new to jQuery and have some experience using Prototype. In Prototype, there is a method to "flash" an element — ie. briefly highlight it in another color and have it fade back to normal so that the user's eye is drawn to it. Is there such a method in jQuery? I see fadeIn, fadeOut, and animate, but I don't see anything like "flash". Perhaps one of these three can be used with appropriate inputs?

My way is .fadein, .fadeout .fadein, .fadeout ......
$("#someElement").fadeOut(100).fadeIn(100).fadeOut(100).fadeIn(100);
function go1() { $("#demo1").fadeOut(100).fadeIn(100).fadeOut(100).fadeIn(100)}
function go2() { $('#demo2').delay(100).fadeOut().fadeIn('slow') }
#demo1,
#demo2 {
text-align: center;
font-family: Helvetica;
background: IndianRed;
height: 50px;
line-height: 50px;
width: 150px;
}
<script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.min.js"></script>
<button onclick="go1()">Click Me</button>
<div id='demo1'>My Element</div>
<br>
<button onclick="go2()">Click Me</button> (from comment)
<div id='demo2'>My Element</div>

You can use the jQuery Color plugin.
For example, to draw attention to all the divs on your page, you could use the following code:
$("div").stop().css("background-color", "#FFFF9C")
.animate({ backgroundColor: "#FFFFFF"}, 1500);
Edit - New and improved
The following uses the same technique as above, but it has the added benefits of:
parameterized highlight color and duration
retaining original background color, instead of assuming that it is white
being an extension of jQuery, so you can use it on any object
Extend the jQuery Object:
var notLocked = true;
$.fn.animateHighlight = function(highlightColor, duration) {
var highlightBg = highlightColor || "#FFFF9C";
var animateMs = duration || 1500;
var originalBg = this.css("backgroundColor");
if (notLocked) {
notLocked = false;
this.stop().css("background-color", highlightBg)
.animate({backgroundColor: originalBg}, animateMs);
setTimeout( function() { notLocked = true; }, animateMs);
}
};
Usage example:
$("div").animateHighlight("#dd0000", 1000);

You can use css3 animations to flash an element
.flash {
-moz-animation: flash 1s ease-out;
-moz-animation-iteration-count: 1;
-webkit-animation: flash 1s ease-out;
-webkit-animation-iteration-count: 1;
-ms-animation: flash 1s ease-out;
-ms-animation-iteration-count: 1;
}
#keyframes flash {
0% { background-color: transparent; }
50% { background-color: #fbf8b2; }
100% { background-color: transparent; }
}
#-webkit-keyframes flash {
0% { background-color: transparent; }
50% { background-color: #fbf8b2; }
100% { background-color: transparent; }
}
#-moz-keyframes flash {
0% { background-color: transparent; }
50% { background-color: #fbf8b2; }
100% { background-color: transparent; }
}
#-ms-keyframes flash {
0% { background-color: transparent; }
50% { background-color: #fbf8b2; }
100% { background-color: transparent; }
}
And you jQuery to add the class
jQuery(selector).addClass("flash");

After 5 years... (And no additional plugin needed)
This one "pulses" it to the color you want (e.g. white) by putting a div background color behind it, and then fading the object out and in again.
HTML object (e.g. button):
<div style="background: #fff;">
<input type="submit" class="element" value="Whatever" />
</div>
jQuery (vanilla, no other plugins):
$('.element').fadeTo(100, 0.3, function() { $(this).fadeTo(500, 1.0); });
element - class name
first number in fadeTo() - milliseconds for the transition
second number in fadeTo() - opacity of the object after fade/unfade
You may check this out in the lower right corner of this webpage: https://single.majlovesreg.one/v1/
Edit (willsteel) no duplicated selector by using $(this) and tweaked values to acutally perform a flash (as the OP requested).

You could use the highlight effect in jQuery UI to achieve the same, I guess.

If you're using jQueryUI, there is pulsate function in UI/Effects
$("div").click(function () {
$(this).effect("pulsate", { times:3 }, 2000);
});
http://docs.jquery.com/UI/Effects/Pulsate

$('#district').css({opacity: 0});
$('#district').animate({opacity: 1}, 700 );

Pure jQuery solution.
(no jquery-ui/animate/color needed.)
If all you want is that yellow "flash" effect without loading jquery color:
var flash = function(elements) {
var opacity = 100;
var color = "255, 255, 20" // has to be in this format since we use rgba
var interval = setInterval(function() {
opacity -= 3;
if (opacity <= 0) clearInterval(interval);
$(elements).css({background: "rgba("+color+", "+opacity/100+")"});
}, 30)
};
Above script simply does 1s yellow fadeout, perfect for letting the user know the element was was updated or something similar.
Usage:
flash($('#your-element'))

You could use this plugin (put it in a js file and use it via script-tag)
http://plugins.jquery.com/project/color
And then use something like this:
jQuery.fn.flash = function( color, duration )
{
var current = this.css( 'color' );
this.animate( { color: 'rgb(' + color + ')' }, duration / 2 );
this.animate( { color: current }, duration / 2 );
}
This adds a 'flash' method to all jQuery objects:
$( '#importantElement' ).flash( '255,0,0', 1000 );

You can extend Desheng Li's method further by allowing an iterations count to do multiple flashes like so:
// Extend jquery with flashing for elements
$.fn.flash = function(duration, iterations) {
duration = duration || 1000; // Default to 1 second
iterations = iterations || 1; // Default to 1 iteration
var iterationDuration = Math.floor(duration / iterations);
for (var i = 0; i < iterations; i++) {
this.fadeOut(iterationDuration).fadeIn(iterationDuration);
}
return this;
}
Then you can call the method with a time and number of flashes:
$("#someElementId").flash(1000, 4); // Flash 4 times over a period of 1 second

How about a really simple answer?
$('selector').fadeTo('fast',0).fadeTo('fast',1).fadeTo('fast',0).fadeTo('fast',1)
Blinks twice...that's all folks!

I can't believe this isn't on this question yet. All you gotta do:
("#someElement").show('highlight',{color: '#C8FB5E'},'fast');
This does exactly what you want it to do, is super easy, works for both show() and hide() methods.

This may be a more up-to-date answer, and is shorter, as things have been consolidated somewhat since this post. Requires jquery-ui-effect-highlight.
$("div").click(function () {
$(this).effect("highlight", {}, 3000);
});
http://docs.jquery.com/UI/Effects/Highlight

function pulse() {
$('.blink').fadeIn(300).fadeOut(500);
}
setInterval(pulse, 1000);

I was looking for a solution to this problem but without relying on jQuery UI.
This is what I came up with and it works for me (no plugins, just Javascript and jQuery);
-- Heres the working fiddle -- http://jsfiddle.net/CriddleCraddle/yYcaY/2/
Set the current CSS parameter in your CSS file as normal css, and create a new class that just handles the parameter to change i.e. background-color, and set it to '!important' to override the default behavior. like this...
.button_flash {
background-color: #8DABFF !important;
}//This is the color to change to.
Then just use the function below and pass in the DOM element as a string, an integer for the number of times you would want the flash to occur, the class you want to change to, and an integer for delay.
Note: If you pass in an even number for the 'times' variable, you will end up with the class you started with, and if you pass an odd number you will end up with the toggled class. Both are useful for different things. I use the 'i' to change the delay time, or they would all fire at the same time and the effect would be lost.
function flashIt(element, times, klass, delay){
for (var i=0; i < times; i++){
setTimeout(function(){
$(element).toggleClass(klass);
}, delay + (300 * i));
};
};
//Then run the following code with either another delay to delay the original start, or
// without another delay. I have provided both options below.
//without a start delay just call
flashIt('.info_status button', 10, 'button_flash', 500)
//with a start delay just call
setTimeout(function(){
flashIt('.info_status button', 10, 'button_flash', 500)
}, 4700);
// Just change the 4700 above to your liking for the start delay. In this case,
//I need about five seconds before the flash started.

Would a pulse effect(offline) JQuery plugin be appropriate for what you are looking for ?
You can add a duration for limiting the pulse effect in time.
As mentioned by J-P in the comments, there is now his updated pulse plugin.
See his GitHub repo. And here is a demo.

Found this many moons later but if anyone cares, it seems like this is a nice way to get something to flash permanently:
$( "#someDiv" ).hide();
setInterval(function(){
$( "#someDiv" ).fadeIn(1000).fadeOut(1000);
},0)

The following codes work for me. Define two fade-in and fade-out functions and put them in each other's callback.
var fIn = function() { $(this).fadeIn(300, fOut); };
var fOut = function() { $(this).fadeOut(300, fIn); };
$('#element').fadeOut(300, fIn);
The following controls the times of flashes:
var count = 3;
var fIn = function() { $(this).fadeIn(300, fOut); };
var fOut = function() { if (--count > 0) $(this).fadeOut(300, fIn); };
$('#element').fadeOut(300, fIn);

If including a library is overkill here is a solution that is guaranteed to work.
$('div').click(function() {
$(this).css('background-color','#FFFFCC');
setTimeout(function() { $(this).fadeOut('slow').fadeIn('slow'); } , 1000);
setTimeout(function() { $(this).css('background-color','#FFFFFF'); } , 1000);
});
Setup event trigger
Set the background color of block element
Inside setTimeout use fadeOut and fadeIn to create a little animation effect.
Inside second setTimeout reset default background color
Tested in a few browsers and it works nicely.

Like fadein / fadeout you could use animate css / delay
$(this).stop(true, true).animate({opacity: 0.1}, 100).delay(100).animate({opacity: 1}, 100).animate({opacity: 0.1}, 100).delay(100).animate({opacity: 1}, 100);
Simple and flexible

$("#someElement").fadeTo(3000, 0.3 ).fadeTo(3000, 1).fadeTo(3000, 0.3 ).fadeTo(3000, 1);
3000 is 3 seconds
From opacity 1 it is faded to 0.3, then to 1 and so on.
You can stack more of these.
Only jQuery is needed. :)

There is a workaround for the animate background bug. This gist includes an example of a simple highlight method and its use.
/* BEGIN jquery color */
(function(jQuery){jQuery.each(['backgroundColor','borderBottomColor','borderLeftColor','borderRightColor','borderTopColor','color','outlineColor'],function(i,attr){jQuery.fx.step[attr]=function(fx){if(!fx.colorInit){fx.start=getColor(fx.elem,attr);fx.end=getRGB(fx.end);fx.colorInit=true;}
fx.elem.style[attr]="rgb("+[Math.max(Math.min(parseInt((fx.pos*(fx.end[0]-fx.start[0]))+fx.start[0]),255),0),Math.max(Math.min(parseInt((fx.pos*(fx.end[1]-fx.start[1]))+fx.start[1]),255),0),Math.max(Math.min(parseInt((fx.pos*(fx.end[2]-fx.start[2]))+fx.start[2]),255),0)].join(",")+")";}});function getRGB(color){var result;if(color&&color.constructor==Array&&color.length==3)
return color;if(result=/rgb\(\s*([0-9]{1,3})\s*,\s*([0-9]{1,3})\s*,\s*([0-9]{1,3})\s*\)/.exec(color))
return[parseInt(result[1]),parseInt(result[2]),parseInt(result[3])];if(result=/rgb\(\s*([0-9]+(?:\.[0-9]+)?)\%\s*,\s*([0-9]+(?:\.[0-9]+)?)\%\s*,\s*([0-9]+(?:\.[0-9]+)?)\%\s*\)/.exec(color))
return[parseFloat(result[1])*2.55,parseFloat(result[2])*2.55,parseFloat(result[3])*2.55];if(result=/#([a-fA-F0-9]{2})([a-fA-F0-9]{2})([a-fA-F0-9]{2})/.exec(color))
return[parseInt(result[1],16),parseInt(result[2],16),parseInt(result[3],16)];if(result=/#([a-fA-F0-9])([a-fA-F0-9])([a-fA-F0-9])/.exec(color))
return[parseInt(result[1]+result[1],16),parseInt(result[2]+result[2],16),parseInt(result[3]+result[3],16)];if(result=/rgba\(0, 0, 0, 0\)/.exec(color))
return colors['transparent'];return colors[jQuery.trim(color).toLowerCase()];}
function getColor(elem,attr){var color;do{color=jQuery.curCSS(elem,attr);if(color!=''&&color!='transparent'||jQuery.nodeName(elem,"body"))
break;attr="backgroundColor";}while(elem=elem.parentNode);return getRGB(color);};var colors={aqua:[0,255,255],azure:[240,255,255],beige:[245,245,220],black:[0,0,0],blue:[0,0,255],brown:[165,42,42],cyan:[0,255,255],darkblue:[0,0,139],darkcyan:[0,139,139],darkgrey:[169,169,169],darkgreen:[0,100,0],darkkhaki:[189,183,107],darkmagenta:[139,0,139],darkolivegreen:[85,107,47],darkorange:[255,140,0],darkorchid:[153,50,204],darkred:[139,0,0],darksalmon:[233,150,122],darkviolet:[148,0,211],fuchsia:[255,0,255],gold:[255,215,0],green:[0,128,0],indigo:[75,0,130],khaki:[240,230,140],lightblue:[173,216,230],lightcyan:[224,255,255],lightgreen:[144,238,144],lightgrey:[211,211,211],lightpink:[255,182,193],lightyellow:[255,255,224],lime:[0,255,0],magenta:[255,0,255],maroon:[128,0,0],navy:[0,0,128],olive:[128,128,0],orange:[255,165,0],pink:[255,192,203],purple:[128,0,128],violet:[128,0,128],red:[255,0,0],silver:[192,192,192],white:[255,255,255],yellow:[255,255,0],transparent:[255,255,255]};})(jQuery);
/* END jquery color */
/* BEGIN highlight */
jQuery(function() {
$.fn.highlight = function(options) {
options = (options) ? options : {start_color:"#ff0",end_color:"#fff",delay:1500};
$(this).each(function() {
$(this).stop().css({"background-color":options.start_color}).animate({"background-color":options.end_color},options.delay);
});
}
});
/* END highlight */
/* BEGIN highlight example */
$(".some-elements").highlight();
/* END highlight example */
https://gist.github.com/1068231

Unfortunately the top answer requires JQuery UI. http://api.jquery.com/animate/
Here is a vanilla JQuery solution
http://jsfiddle.net/EfKBg/
JS
var flash = "<div class='flash'></div>";
$(".hello").prepend(flash);
$('.flash').show().fadeOut('slow');
CSS
.flash {
background-color: yellow;
display: none;
position: absolute;
width: 100%;
height: 100%;
}
HTML
<div class="hello">Hello World!</div>

Here's a slightly improved version of colbeerhey's solution. I added a return statement so that, in true jQuery form, we chain events after calling the animation. I've also added the arguments to clear the queue and jump to the end of an animation.
// Adds a highlight effect
$.fn.animateHighlight = function(highlightColor, duration) {
var highlightBg = highlightColor || "#FFFF9C";
var animateMs = duration || 1500;
this.stop(true,true);
var originalBg = this.css("backgroundColor");
return this.css("background-color", highlightBg).animate({backgroundColor: originalBg}, animateMs);
};

This one will pulsate an element's background color until a mouseover event is triggered
$.fn.pulseNotify = function(color, duration) {
var This = $(this);
console.log(This);
var pulseColor = color || "#337";
var pulseTime = duration || 3000;
var origBg = This.css("background-color");
var stop = false;
This.bind('mouseover.flashPulse', function() {
stop = true;
This.stop();
This.unbind('mouseover.flashPulse');
This.css('background-color', origBg);
})
function loop() {
console.log(This);
if( !stop ) {
This.animate({backgroundColor: pulseColor}, pulseTime/3, function(){
This.animate({backgroundColor: origBg}, (pulseTime/3)*2, 'easeInCirc', loop);
});
}
}
loop();
return This;
}

Put this together from all of the above - an easy solution for flashing an element and return to the original bgcolour...
$.fn.flash = function (highlightColor, duration, iterations) {
var highlightBg = highlightColor || "#FFFF9C";
var animateMs = duration || 1500;
var originalBg = this.css('backgroundColor');
var flashString = 'this';
for (var i = 0; i < iterations; i++) {
flashString = flashString + '.animate({ backgroundColor: highlightBg }, animateMs).animate({ backgroundColor: originalBg }, animateMs)';
}
eval(flashString);
}
Use like this:
$('<some element>').flash('#ffffc0', 1000, 3);
Hope this helps!

Here's a solution that uses a mix of jQuery and CSS3 animations.
http://jsfiddle.net/padfv0u9/2/
Essentially you start by changing the color to your "flash" color, and then use a CSS3 animation to let the color fade out. You need to change the transition duration in order for the initial "flash" to be faster than the fade.
$(element).removeClass("transition-duration-medium");
$(element).addClass("transition-duration-instant");
$(element).addClass("ko-flash");
setTimeout(function () {
$(element).removeClass("transition-duration-instant");
$(element).addClass("transition-duration-medium");
$(element).removeClass("ko-flash");
}, 500);
Where the CSS classes are as follows.
.ko-flash {
background-color: yellow;
}
.transition-duration-instant {
-webkit-transition-duration: 0s;
-moz-transition-duration: 0s;
-o-transition-duration: 0s;
transition-duration: 0s;
}
.transition-duration-medium {
-webkit-transition-duration: 1s;
-moz-transition-duration: 1s;
-o-transition-duration: 1s;
transition-duration: 1s;
}

just give elem.fadeOut(10).fadeIn(10);

This is generic enough that you can write whatever code you like to animate. You can even decrease the delay from 300ms to 33ms and fade colors, etc.
// Flash linked to hash.
var hash = location.hash.substr(1);
if (hash) {
hash = $("#" + hash);
var color = hash.css("color"), count = 1;
function hashFade () {
if (++count < 7) setTimeout(hashFade, 300);
hash.css("color", count % 2 ? color : "red");
}
hashFade();
}

you can use jquery Pulsate plugin to force to focus the attention on any html element with control over speed and repeatation and color.
JQuery.pulsate() * with Demos
sample initializer:
$(".pulse4").pulsate({speed:2500})
$(".CommandBox button:visible").pulsate({ color: "#f00", speed: 200, reach: 85, repeat: 15 })

How to make a div fade on hover?

Hi I currently have span that displays over an image on hover, however I want to use a bit of javascript or css transitions to make this div fade in to about 0.8 opacity on hover then back to 0 when the mouse is not hovering.
Here is an example of how I have it setup so far, now all thats needed is the fade and 0.8 opacity:
How its setup - Jsfiddle
Im sure there is a simple bit of code that someone has to do this
Help is much appreciated thanks!

So... here's the CSS3 / HTML5-way to do this. This won't work in IE though: it will fall back on the regular, immediate way (so it does work, it just isn't as smooth as it is in the real browsers).
div.yourDiv {
-webkit-transition: .4s ease-in-out opacity;
-moz-transition: .4s ease-in-out opacity;
-o-transition: .4s ease-in-out opacity;
transition: .4s ease-in-out opacity;
}
div.yourDiv:hover {
opacity: 0.8;
}
Since CSS3-transitions are using hardware-accerelation, this really is very smooth! Besides that, you don't even need any Javascript or jQuery for this =)!

You can use CSS's :hover pseudo-class, unless you need to support IE6:
.image-hover:hover {
opacity: .8;
}
* html .image-hover:hover { /* For IE7 and higher */
filter: alpha(opacity=80);
}
That won't fade to 80%, though, it'll just go there immediately. To do that, you can use jQuery's hover and animate functions (edit: or fadeTo, which is just a convenience wrapper for animate on opacity as shown below):
$(".image-hover").hover(
function() {
$(this).stop().animate({opacity: "0.8"});
},
function() {
$(this).stop().animate({opacity: "1"});
}
);
It's not clear from your question what the text in the span is supposed to be doing, but those are the tools to get you started.
Here's an updated version of your fiddle showing the animation; I've used 0.6 rather than 0.8 just so it's more obvious.

.classa
{
opacity:0.8;
}
you can addClass and removeClass like
$("div.image-hover").hover(
function(){
//fadein
$(this).addClass("classa");
},
function(){
//fadeout
$(this).removeClass("classa");
}
);
here is the fiddle http://jsfiddle.net/2RN6E/8/
EDITED after the comment below
you can use fadeTo
$("div.image-hover").hover(
function(){
//fadein
$(this).fadeTo( "2000", "0.8");
},
function(){
//fadeout
$(this).fadeTo( "2000","1");
}
here is the fiddle http://jsfiddle.net/2RN6E/14/
);

You could do:
function fadein() {
$('.desc').animate({
opacity: 0.8,
}, 1000, function() {
// Animation complete.
})
}
function fadeout() {
$('.desc').animate({
opacity: 0,
}, 1000, function() {
// Animation complete.
})
}
$('.image-hover').hover(fadein, fadeout);
fiddle here: http://jsfiddle.net/nicolapeluchetti/2RN6E/9/

This code retains the block display for the description element: http://jsfiddle.net/2RN6E/11/
It just uses the animate function of jQuery:
$(".image-hover").hover(function() {
$(".desc").animate({opacity: '0.75'},'slow');
}, function() {
$(".desc").animate({opacity: '0'},'slow');
});