EffectTiming.iterationStart

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

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.

• Web Animations API
• Element.animate(), KeyframeEffectReadOnly(), and KeyframeEffect() all accept an object of timing properties including this one.
• The value of this property corresponds to AnimationEffectReadOnly.timing, KeyframeEffectReadOnly.timing, and KeyframeEffect.timing).