Web/API/GlobalEventHandlers/onanimationstart

An event handler for the animationstart event. This event is sent when a CSS Animation starts to play.

Syntax

var animStartHandler = target.onanimationstart;

target.onanimationstart = Function

Value

A Function to be called when an animationstart event occurs indicating that a CSS animation has begun on the target, where the target object is an HTML element (HTMLElement), document (Document), or window (Window). The function receives as input a single parameter: an AnimationEvent object describing the event which occurred.

Example

HTML content

<div class="main">
  <div id="box">
    <div id="text">Box</div>
  </div>
</div>

<div class="button" id="play">
  Play Animation
</div>

<pre id="log"></pre>

CSS content

:root {
  --boxwidth:50px;
}

.main {
  width: 300px;
  height:300px;
  border: 1px solid black;
}

.button {
  cursor: pointer;
  width: 300px;
  border: 1px solid black;
  font-size: 16px;
  text-align: center;
  margin-top: 0;
  padding-top: 2px;
  padding-bottom: 4px;
  color: white;
  background-color: darkgreen;
  font: 14px "Open Sans", "Arial", sans-serif;
} 

#text {
  width: 46px;
  padding: 10px;
  position: relative;
  text-align: center;
  align-self: center;
  color: white;
  font: bold 1.4em "Lucida Grande", "Open Sans", sans-serif;
}

Leaving out some bits of the CSS that don't matter for the discussion here, let's take a look at the styles for the box that we're animating. First is the box itself. We set its size, position, color, and layout. Note that there's nothing there about animation. That's because we don't want the box to start animating right away. We'll add the animation style later to start animating the box.

#box {
  width: var(--boxwidth);
  height: var(--boxwidth);
  left: 0;
  top: 0;
  border: 1px solid #7788FF;
  margin: 0;
  position: relative;
  background-color: #2233FF;
  display: flex;
  justify-content: center;
}

The animation sequence is described next. First, the "slideAnimation" class, which establishes the animation that will cause the box to move over the course of five seconds, one time, using the "slideBox" keyframe set. The keyframes are defined next; they describe an animation which causes the box to migrate from the top-left corner of the container to the bottom-right corner.

.slideAnimation {
  animation: 5s ease-in-out 0s 1 slideBox;
}

@keyframes slideBox {
  from {
    left:0;
    top:0;
  }
  to {
    left:calc(100% - var(--boxwidth));
    top:calc(100% - var(--boxwidth))
  }
}

Since the CSS describes the animation but doesn't connect it to the box, we'll need some JavaScript code to do that. We'll get to that shortly.

JavaScript content

Before we get to the animation code, we define a function which logs information to a box on the user's screen. We'll use this to show information about the events we receive. Note the use of AnimationEvent.animationName and AnimationEvent.elapsedTime to get information about the event which occurred.

function log(msg, event) {
  let logBox = document.getElementById("log");
 
  logBox.innerHTML += msg;
 
  if (event) {
    logBox.innerHTML += " <code>"+ event.animationName +
        "</code> at time " + event.elapsedTime.toFixed(2) +
        " seconds.";
  }
 
  logBox.innerHTML += "\n";
};

Then we set up the event handlers for the animationstart and animationend events:

let box = document.getElementById("box");

box.onanimationstart = function(event) {
  log("Animation started", event);
}

box.onanimationend = function(event) {
  log("Animation stopped", event);
};

Finally, we set up a handler for a click on the button that runs the animation:

document.getElementById("play").addEventListener("click", function(event) {
  document.getElementById("box").className = "slideAnimation";
  event.target.style.display = "none";
}, false);

This sets the class of the box we want to animate to the class that contains the animation description, then hides the play button because this example will only run the animation once. For information about why, and how to support running an animation more than once, see Run an animation again in CSS Animations tips and tricks.

Result

Assembled together, you get this:

Specification

Specification	Status	Comment
CSS AnimationsThe definition of 'onanimationstart' in that specification.	Working Draft

Browser compatibility

Update compatibility data on GitHub

	Desktop						Mobile
	Chrome	Edge	Firefox	Internet Explorer	Opera	Safari	Android webview	Chrome for Android	Firefox for Android	Opera for Android	Safari on iOS	Samsung Internet
`onanimationstart`	Chrome Full support Yes Full support Yes Full support Yes Alternate Name' Alternate Name' Uses the non-standard name: `onwebkitanimationstart`	Edge Full support ≤79 Full support ≤79 Full support ≤79 Alternate Name' Alternate Name' Uses the non-standard name: `onwebkitanimationstart`	Firefox Full support 51	IE ?	Opera ?	Safari Full support 9	WebView Android Full support Yes Full support Yes Full support Yes Alternate Name' Alternate Name' Uses the non-standard name: `onwebkitanimationstart`	Chrome Android Full support Yes Full support Yes Full support Yes Alternate Name' Alternate Name' Uses the non-standard name: `onwebkitanimationstart`	Firefox Android Full support 51	Opera Android ?	Safari iOS Full support 9	Samsung Internet Android Full support Yes Full support Yes Full support Yes Alternate Name' Alternate Name' Uses the non-standard name: `onwebkitanimationstart`

Legend

Full support: Full support
Compatibility unknown: Compatibility unknown
Uses a non-standard name.': Uses a non-standard name.