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.

Web Animations API's AnimationEffectTimingProperties dictionary's  iterationStart property specifies the repetition number which repetition the animation begins at and its progress through it.

Element.animate(), KeyframeEffectReadOnly.KeyframeEffectReadOnly(), and KeyframeEffect.KeyframeEffect() all accept an object of timing properties including iterationStart. The value of iterationStart corresponds directly to AnimationEffectTimingReadOnly.iterationStart in timing objects returned by AnimationEffectReadOnly, KeyframeEffectReadOnly, and KeyframeEffect.


var timingProperties = {
  iterationStart = iterationNumber

timingProperties.iterationStart = iterationNumber;


A floating-point value whose value is at least 0 and is not +Infinity, indicating the offset into the number of iterations the animation sequence is to run at which to start animating. iterationStart represents the iteration index at which the animation effect begins as well as its progress through that iteration.

Usually you'll use a value between 0.0 and 1.0 to indicate an offset into the first run of the animation at which to begin the animation performance, but any positive, non-infinite value is allowed. Since all animations' iteration indexes start at 0, a value of 0.5 would start the animation halfway through its first iteration or loop. Meanwhile, a value of 1.2 means the animation will begin playback 20% of the way through its second iteration, and so forth.

It's currently undefined what happens if you specify a value of iterationStart which is greater than the value of AnimationEffectTimingProperties.iterations. See issue 170 in the Web Animations API specification's issue tracker for details and status of any changes to the specification in this regard.


Specification Status Comment
Web Animations
The definition of 'iterationStart' in that specification.
Working Draft Editor's draft.

Browser compatibility

Feature Chrome Firefox (Gecko) Internet Explorer Opera Safari (WebKit)
Basic support (Yes) 45 (45)[1] No support (Yes) No support
Feature Android Android Webview Chrome for Android Firefox Mobile (Gecko) Firefox OS IE Mobile Opera Mobile Safari Mobile
Basic support ? ? ? 45.0 (45)[1] ? No support No support No support

[1] The Web Animations API is only enabled by default in Firefox Developer Edition and Nightly builds. You can enable it in beta and release builds by setting the preference dom.animations-api.core.enabled to true, and can disable it in any Firefox version by setting this preference to false.

See also