Why Are My JavaScript-Assigned CSS Transitions Not Working?

JavaScript-Assigned CSS Transitions Not Functioning

When applying CSS3 transitions to a slideshow via JavaScript, you may encounter a puzzling issue where the specified transitions fail to work despite the correct styles being applied. This discrepancy arises because CSS transitions trigger only when the specified property changes value, not when the transition is initially defined.

To resolve this issue, ensure that the three following conditions are met:

1. Property Explicit Definition: The affected element must have the property explicitly declared in its initial style.
2. Transition Definition: The element must have the desired transition defined, such as "transition: opacity 2s;".
3. Property Value Change: After defining the transition, set the new property value, triggering the animation.

In your example, the transitions were not working because the property value change (setting "opacity: 1") occurred before the browser had a chance to process the dynamically assigned transition. To fix this, introduce a delay before assigning the "target-fadein" class:

<code class="js">window.setTimeout(function() {
  slides[targetIndex].className += " target-fadein";
}, 100);</code>

Alternatively, include the "target-fadein-begin" class in your HTML directly, allowing it to be parsed on load and ready for the transition.

Remember that the JavaScript assignment of the transition itself does not initiate the animation; it is the change in the property value that triggers the transition effect.

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