Document: startViewTransition() メソッド

startViewTransition()
startViewTransition(updateCallback)

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());
}