Element: animationend event

The 
animationend event is fired when a CSS Animation has completed. If the animation aborts before reaching completion, such as if the element is removed from the DOM or the animation is removed from the element, the 
animationend event is not fired.
Syntax
Use the event name in addEventListener():
addEventListener("animationend", (event) => { })
Alternatively, assign to the event handler property of the interface:
onanimationend = (event) => { }
Event type
An AnimationEvent. Inherits from Event.
Examples
This example gets an element that's being animated and listens for the 
animationend event:
const animated = document.querySelector(".animated");
animated.addEventListener("animationend", () => {
  console.log("Animation ended");
});
The same, but using the 
onanimationend event handler property (but note that this replaces other event listeners added using 
onanimationend, if any):
const animated = document.querySelector(".animated");
animated.onanimationend = () => {
  console.log("Animation ended");
};
Live example
HTML
<div class="animation-example">
  <div class="container">
    <p class="animation">You chose a cold night to visit our planet.</p>
  </div>
  <button class="activate" type="button">Activate animation</button>
  <div class="event-log"></div>
</div>
CSS
.container {
  height: 3rem;
}
.event-log {
  width: 25rem;
  height: 2rem;
  border: 1px solid black;
  margin: 0.2rem;
  padding: 0.2rem;
}
.animation.active {
  animation-duration: 2s;
  animation-name: slide-in;
  animation-iteration-count: 2;
}
@keyframes slide-in {
  from {
    transform: translateX(100%) scaleX(3);
  }
  to {
    transform: translateX(0) scaleX(1);
  }
}
JavaScript
const animation = document.querySelector("p.animation");
const animationEventLog = document.querySelector(
  ".animation-example>.event-log",
);
const applyAnimation = document.querySelector(
  ".animation-example>button.activate",
);
let iterationCount = 0;
animation.addEventListener("animationstart", () => {
  animationEventLog.textContent = `${animationEventLog.textContent}'animation started' `;
});
animation.addEventListener("animationiteration", () => {
  iterationCount++;
  animationEventLog.textContent = `${animationEventLog.textContent}'animation iterations: ${iterationCount}' `;
});
animation.addEventListener("animationend", () => {
  animationEventLog.textContent = `${animationEventLog.textContent}'animation ended'`;
  animation.classList.remove("active");
  applyAnimation.textContent = "Activate animation";
});
animation.addEventListener("animationcancel", () => {
  animationEventLog.textContent = `${animationEventLog.textContent}'animation canceled'`;
});
applyAnimation.addEventListener("click", () => {
  animation.classList.toggle("active");
  animationEventLog.textContent = "";
  iterationCount = 0;
  const active = animation.classList.contains("active");
  applyAnimation.textContent = active
    ? "Cancel animation"
    : "Activate animation";
});
Result
Specifications
Browser compatibility
See also
CSS Animations
Using CSS Animations
AnimationEvent
Related events: animationstart, animationcancel, animationiteration