Why Do CSS Transitions Fail When Applied Through JavaScript?

Fixing CSS Transitions when Assigned Through JavaScript

When attempting to apply CSS3 transitions through JavaScript, you may encounter a situation where the specified transitions fail to execute, despite the correct styles being applied. This behavior can be puzzling, especially if the transitions work when applied directly through the browser's inspector.

The Underlying Cause

To fully comprehend the issue, one must understand the mechanics of CSS transitions. Three crucial steps must occur for a transition to take effect:

1. The targeted element must have the transition property explicitly defined (e.g., transition: opacity 2s;).
2. The element must have the initial value for the transitioning property set (e.g., opacity: 0;).
3. The new value for the transitioning property must be applied (e.g., opacity: 1;).

When JavaScript assigns classes dynamically, as in the case of the provided example, there is a potential issue. The browser needs time to process these changes, and dynamically assigning the target class (e.g., ".target-fadein") may not provide the necessary delay.

Solution

To solve this issue, a delay is required before assigning the target class. This ensures that the browser has ample time to apply the transition property and prepare for the animation. The following code snippet illustrates the use of a delay when applying the target class:

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

Alternately, you can add the target class ".target-fadein-begin" into the HTML directly, so it is parsed on load and ready for the transition.

Additional Insights

It is important to recognize that simply adding a transition to an element does not trigger the animation. The actual animation is triggered by changing the transitioning property's value. This explains why manually setting the initial and new property values in the inspector works.

For further clarification, consider the following code snippet:

// 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)

In this scenario, the first block works because all three conditions for a transition are met. The element has a transition defined, the initial opacity is set to 0, and the new opacity is set to 1. The second block fails because the initial opacity value is not explicitly defined before the transition class is applied. The third block works because a delay allows the browser to process the initial value setting before the transition is applied.

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