The transitionProperty property of the CSSTransition interface returns the expanded transition property name of the transition. This is the longhand CSS property for which the transition was generated.

A string.

The transition in the following example changes the width of the box on hover. Calling Element.getAnimations() returns an array of all Animation objects. In our case this returns a CSSTransition object, representing the animation created. The transitionProperty property returns the property that the transition is created for, which is width.

.box {
  background-color: #165baa;
  color: #fff;
  width: 100px;
  height: 100px;
  transition: width 4s;
}

.box:hover {
  width: 200px;
}

const item = document.querySelector(".box");
item.addEventListener("transitionrun", () => {
  let animations = document.querySelector(".box").getAnimations();
  console.log(animations[0].propertyName);
});