The startViewTransition() method of the View Transitions API starts a new view transition and returns a ViewTransition object to represent it.

When startViewTransition() is invoked, a sequence of steps is followed as explained in The view transition process.

callback

A callback function typically invoked to update the DOM during the view transition process, which returns a Promise. The callback is invoked once the API has taken a screenshot of the current page. When the promise returned by the callback fulfills, the view transition begins in the next frame. If the promise returned by the callback rejects, the transition is abandoned.

A ViewTransition object instance.

In our Basic View Transitions demo, the updateView() function handles both browsers that do and don't support the View Transitions API. In supporting browsers, we invoke startViewTransition() to set off the view transition process without worrying about the return value.

function updateView(event) {
  // Handle the difference in whether the event is fired on the <a> or the <img>
  let targetIdentifier;
  if (event.target.firstChild === null) {
    targetIdentifier = event.target;
  } else {
    targetIdentifier = event.target.firstChild;
  }

  const displayNewImage = () => {
    const mainSrc = `${targetIdentifier.src.split("_th.jpg")[0]}.jpg`;
    galleryImg.src = mainSrc;
    galleryCaption.textContent = targetIdentifier.alt;
  };

  // Fallback for browsers that don't support View Transitions:
  if (!document.startViewTransition) {
    displayNewImage();
    return;
  }

  // With View Transitions:
  const transition = document.startViewTransition(() => displayNewImage());
}

• Smooth and simple transitions with the View Transitions API