The KeyframeEffect() constructor of the Web Animations API returns a new KeyframeEffect object instance, and also allows you to clone an existing keyframe effect object instance.

The multi-argument constructor (see above) creates a completely new KeyframeEffect object instance. Its parameters are:

target

The DOM element to be animated, or null.

keyframes

A keyframes object or null.

options Optional

Either an integer representing the animation's duration (in milliseconds), or an object containing one or more of the following:

delay Optional

The number of milliseconds to delay the start of the animation. Defaults to 0.

direction Optional

Whether the animation runs forwards (normal), backwards (reverse), switches direction after each iteration (alternate), or runs backwards and switches direction after each iteration (alternate-reverse). Defaults to "normal".

duration Optional

The number of milliseconds each iteration of the animation takes to complete. Defaults to 0. Although this is technically optional, keep in mind that your animation will not run if this value is 0.

easing Optional

The rate of the animation's change over time. Accepts an <easing-function>, such as "linear", "ease-in", "step-end", or "cubic-bezier(0.42, 0, 0.58, 1)". Defaults to "linear".

endDelay Optional

The number of milliseconds to delay after the end of an animation. This is primarily of use when sequencing animations based on the end time of another animation. Defaults to 0.

fill Optional

Dictates whether the animation's effects should be reflected by the element(s) prior to playing ("backwards"), retained after the animation has completed playing ("forwards"), or both. Defaults to "none".

iterationStart Optional

Describes at what point in the iteration the animation should start. 0.5 would indicate starting halfway through the first iteration for example, and with this value set, an animation with 2 iterations would end halfway through a third iteration. Defaults to 0.0.

iterations Optional

The number of times the animation should repeat. Defaults to 1, and can also take a value of Infinity to make it repeat for as long as the element exists.

composite Optional

Determines how values are combined between this animation and other, separate animations that do not specify their own specific composite operation. Defaults to replace.

• add dictates an additive effect, where each successive iteration builds on the last. For instance with transform, a translateX(-200px) would not override an earlier rotate(20deg) value but result in translateX(-200px) rotate(20deg).
• accumulate is similar but a little smarter: blur(2) and blur(5) become blur(7), not blur(2) blur(5).
• replace overwrites the previous value with the new one.

iterationComposite Optional

Determines how values build from iteration to iteration in this animation. Can be set to accumulate or replace (see above). Defaults to replace.

pseudoElement Optional

A string containing a pseudo-element selector, such as "::before". If present, the effect is applied to the selected pseudo-element of target, rather than to target itself.

The single argument constructor (see above) creates a clone of an existing KeyframeEffect object instance. Its parameter is as follows:

sourceKeyFrames

A KeyframeEffect object that you want to clone.

In the Follow the White Rabbit example, the KeyframeEffect constructor is used to create a set of keyframes that dictate how the White Rabbit should animate down the hole:

const whiteRabbit = document.getElementById("rabbit");

const rabbitDownKeyframes = new KeyframeEffect(
  whiteRabbit, // element to animate
  [
    { transform: "translateY(0%)" }, // keyframe
    { transform: "translateY(100%)" }, // keyframe
  ],
  { duration: 3000, fill: "forwards" }, // keyframe options
);

const rabbitDownAnimation = new Animation(
  rabbitDownKeyframes,
  document.timeline,
);

// Play rabbit animation
rabbitDownAnimation.play();

• KeyframeEffect Interface
• Web Animations API
• Animation