Why Are My CSS Transitions Not Working With My JavaScript Slideshow?

CSS Transitions Failure with JavaScript Application

In an attempt to enhance a slideshow with CSS3 transitions, a user encountered a puzzling issue: the transitions refused to function despite the correct styles being applied through JavaScript.

The JavaScript retrieves the slideshow slides and assigns predefined CSS classes to control animation. However, the specified transitions fail to activate. This phenomenon is not observed when the styles and transitions are manually applied using the developer tools console.

Through analysis, it was determined that for transitions to occur, three conditions must be met:

1. The element's initial property must be explicitly defined, e.g., opacity: 0.
2. A transition must be assigned to the element, e.g., transition: opacity 2s.
3. The new property value must be applied, e.g., opacity: 1.

In the problematic JavaScript implementation, conditions 1 and 2 are dynamically assigned. As a result, there is no time delay for the browser to process and register the changes.

To resolve this issue, it is recommended to insert a delay before applying condition 3. This delay allows the browser to process the previous changes, ensuring that the transition property is properly recognized when its value is changed:

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

Alternatively, condition 3 can be applied directly within the HTML, ensuring that the transition property is defined during page load, eliminating the need for a delay in JavaScript:

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

Once these conditions are satisfied, the animation triggered by the CSS3 transition will function as expected.

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