CSS transitions do not work when assigned trough JavaScript

Question

I'm having some major headache trying to apply CSS3 transitions to a slideshow trough JavaScript.

Basically the JavaScript gets all of the slides in the slideshow and applies CSS classes to the correct elements to give a nice animated effect, if there is no CSS3 transitions support it will just apply the styles without a transition.

Now, my 'little' problem. All works as expected, all slides get the correct styles, the code runs without bugs (so far). But the specified transitions do not work, even though the correct styles where applied. Also, styles and transitions work when I apply them myself trough the inspector.

Since I couldn't find a logical explanation myself I thought someone here could answer it, pretty please?

I've put together a little example of what the code is right now: http://g2f.nl/38rvma Or use JSfiddle (no images): http://jsfiddle.net/5RgGV/1/

Answer 1

To make transition work, three things have to happen.

1. the element has to have the property explicitly defined, in this case: opacity: 0;
2. the element must have the transition defined: transition: opacity 2s;
3. the new property must be set: opacity: 1

If you are assigning 1 and 2 dynamically, like you are in your example, there needs to be a delay before 3 so the browser can process the request. The reason it works when you are debugging it is that you are creating this delay by stepping through it, giving the browser time to process. Give a delay to assigning .target-fadein:

window.setTimeout(function() {
  slides[targetIndex].className += " target-fadein";
}, 100); 

Or put .target-fadein-begin into your HTML directly so it's parsed on load and will be ready for the transition.

Adding transition to an element is not what triggers the animation, changing the property does.

// Works
document.getElementById('fade1').className += ' fade-in'

// Doesn't work
document.getElementById('fade2').className = 'fadeable'
document.getElementById('fade2').className += ' fade-in'

// Works
document.getElementById('fade3').className = 'fadeable'

window.setTimeout(function() {
  document.getElementById('fade3').className += ' fade-in'
}, 50)

.fadeable {
  opacity: 0;
}

.fade-in {
  opacity: 1;
  transition: opacity 2s;
}

<div id="fade1" class="fadeable">fade 1 - works</div>
<div id="fade2">fade 2 - doesn't work</div>
<div id="fade3">fade 3 - works</div>