Animation: startTime property

A floating-point number representing the current time in milliseconds, or null if no time is set. You can read this value to determine what the start time is currently set at, and you can change this value to make the animation start at a different time.

In the Running on Web Animations API example, we can sync all new animated cats by giving them all the same startTime as the original running cat:

const catRunning = document
  .getElementById("withWAAPI")
  .animate(keyframes, timing);

/* A function that makes new cats. */
function addCat() {
  const newCat = document.createElement("div");
  newCat.classList.add("cat");
  return newCat;
}

/* This is the function that adds a cat to the WAAPI column */
function animateNewCatWithWAAPI() {
  // make a new cat
  const newCat = addCat();

  // animate said cat with the WAAPI's "animate" function
  const newAnimationPlayer = newCat.animate(keyframes, timing);

  // set the animation's start time to be the same as the original .cat#withWAAPI
  newAnimationPlayer.startTime = catRunning.startTime;

  // Add the cat to the pile.
  WAAPICats.appendChild(newCat);
}

To offer protection against timing attacks and fingerprinting, the precision of animation.startTime might get rounded depending on browser settings. In Firefox, the privacy.reduceTimerPrecision preference is enabled by default and defaults to 2ms. You can also enable privacy.resistFingerprinting, in which case the precision will be 100ms or the value of privacy.resistFingerprinting.reduceTimerPrecision.microseconds, whichever is larger.

For example, with reduced time precision, the result of animation.startTime will always be a multiple of 0.002, or a multiple of 0.1 (or privacy.resistFingerprinting.reduceTimerPrecision.microseconds) with privacy.resistFingerprinting enabled.

// reduced time precision (2ms) in Firefox 60
animation.startTime;
// Might be:
// 23.404
// 24.192
// 25.514
// …

// reduced time precision with `privacy.resistFingerprinting` enabled
animation.startTime;
// Might be:
// 49.8
// 50.6
// 51.7
// …

• Web Animations API
• Animation
• Animation.currentTime for the current time of the animation.