Why Aren\'t My CSS Transitions Working When Applied Through JavaScript?

CSS Transitions Not Applied When Assigned Through JavaScript

Despite applying CSS3 transitions with JavaScript, they fail to work as expected. This issue arises when dynamically assigning CSS classes containing transition properties.

To trigger transitions effectively, the prerequisites are:

• Explicit definition of the property (e.g., opacity: 0;)
• Defined transition (e.g., transition: opacity 2s;)
• Setting the new property (e.g., opacity: 1;)

In JavaScript, the problem stems from the browser's processing time. The correct styles must be applied first, followed by a slight delay before setting the CSS class responsible for the transition. This delay allows the browser to register the applied styles before the transition is applied.

To implement this delay, use window.setTimeout() to postpone adding the CSS class containing the transition:

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

Alternatively, include a transition-triggering class (target-fadein-begin) in the HTML when it loads:

<code class="html"><div class="fadeable target-fadein-begin"></div></code>

By meeting these criteria, CSS transitions triggered through JavaScript can function as expected, allowing for seamless animations.

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