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This is an experimental technology
Because this technology's specification has not stabilized, check the compatibility table for usage in various browsers. Also note that the syntax and behavior of an experimental technology is subject to change in future versions of browsers as the specification changes.

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


var keyframes = new keyframeEffectReadOnly(element, keyframeSet, keyframeOptions);
var keyframes = new keyframeEffectReadOnly(sourceKeyFrames);


The first type of constructor (see above) creates a completely new KeyframeEffectReadOnly object instance. Its parameters are:

The DOM element to be animated, or null.
A keyframe object or null.
keyframeOptions 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 the pre-defined values "linear", "ease", "ease-in", "ease-out", and "ease-in-out", or a custom "cubic-bezier" value like "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.

The following options are currently not shipped anywhere, but will be added in the near future:

Determines how values are combined between this animation and the element's underlying values.
Determines how values build from iteration to iteration in the current animation.

The second type of constructor (see above) creates a clone of an existing  KeyframeEffectReadOnly object instance. Its parameter is as follows:

A KeyframeEffectReadOnly object that you want to clone.


Specification Status Comment
Web Animations
The definition of 'KeyframeEffectReadOnly.keyframeEffectReadOnly()' in that specification.
Working Draft Editor's draft.

Browser compatibility

Feature Chrome Edge Firefox (Gecko) Internet Explorer Opera Safari (WebKit)
Basic support (Yes) No support (Yes) No support No support No support
composite and iterationComposite options No support No support No support No support No support No support
Clone constructor (2nd type) (Yes) No support No support[1] No support No support No support
Feature Android Android Webview Chrome for Android Firefox Mobile (Gecko) IE Mobile Opera Mobile Safari Mobile
Basic support (Yes) (Yes) (Yes) (Yes) No support No support No support
composite and iterationComposite options No support No support No support No support No support No support No support
Clone constructor (2nd type) (Yes) (Yes) (Yes) No support[1] No support No support No support

[1] Only enabled in Firefox 52 Nightly and Dev edition. Turned off in Beta/release.

See also

Document Tags and Contributors

 Contributors to this page: chrisdavidmills, birtles, rachelnabors
 Last updated by: chrisdavidmills,