text: // Activates the image gallery.
  // The main task is to attach an event listener to each image in the gallery
  // and respond appropriately on click.
  function activateGallery() {
    let thumbnails = document.querySelectorAll("#gallery-thumbs > div > img");
    let mainImage  = document.querySelector("#gallery-photo img");
  
    thumbnails.forEach(function(thumbnail) {
      // Preload large images.
      let newImageSrc  = thumbnail.dataset.largeVersion;
      let largeVersion = new Image();
      largeVersion.src = FILL_IN;
  
      thumbnail.addEventListener("click", function() {
        // Set clicked image as display image.
        mainImage.setAttribute("src", newImageSrc);
  
        // Change which image is current.
        document.querySelector(".current").classList.remove("current");
        thumbnail.parentNode.classList.add("current");
  
        // Update image info.
        let galleryInfo = document.querySelector("#gallery-info");
        let title       = galleryInfo.querySelector(".title");
        let description = galleryInfo.querySelector(".description");
  
        title.innerHTML       = thumbnail.dataset.title;
        description.innerHTML = thumbnail.dataset.description;
      });
    });
  }
  