<!doctype html>
<html>
<title>Lazyload images can load immediately from the list of available images</title>
<link rel="author" title="Dom Farolino" href="mailto:dom@chromium.org">
<link rel="help" href="https://html.spec.whatwg.org/multipage/images.html#updating-the-image-data">
<script src="/resources/testharness.js"></script>
<script src="/resources/testharnessreport.js"></script>

<!-- A `loading=lazy` image will be placed below this div so that it is below
     the viewport -->
<div style="height: 1000vh;"></div>
<div id="below-viewport-img-container"></div>

<script>
const image_path = location.origin + '/html/semantics/embedded-content/the-img-element/resources/image.png';

promise_test(async t => {
  const eager_image_promise = new Promise((resolve, reject) => {
    const img = new Image();
    img.onload = resolve;
    img.onerror = e => { reject(new Error("The img should not fail to load")) };
    img.src = image_path;
  });

  await eager_image_promise;

  // At this point, the image fetched eagerly above exists in the "list of
  // available images". As per the spec's #updating-the-image-data algorithm [1]
  // step 6, the "list of avalable images" is consulted before we take any
  // lazyload-specific action. This means that lazyload images can load eagerly
  // if they target a resource in the list of available images, which the image
  // below is doing.
  //
  // Note that if https://github.com/whatwg/html/issues/7005 resolves in favor
  // of allowing in-flight image requests to be placed in the list of available
  // images, as opposed to just complete images, this would allow lazyload
  // images (in addition to non-lazyload ones) to coalesce with these in-flight
  // entries in the list of available images too. In that case we'd need to test
  // for this here.
  // [1]: https://html.spec.whatwg.org/multipage/images.html#updating-the-image-data
  const lazyload_image_promise = new Promise((resolve, reject) => {
    const img = new Image();
    img.loading = 'lazy';
    img.onload = resolve;
    img.onerror = e => { reject("The img should not fail to load") };

    document.querySelector('#below-viewport-img-container').append(img);
    img.src = image_path;
  });
  const timeout_promise = new Promise((resolve, reject) => {
    t.step_timeout(() => {
      reject(new Error("The `loading=lazy` image should load immediately from " +
                       "the list of available images, beating this timeout " +
                       "promise."));
    }, 1000);
  });

  // The `lazyload_image_promise` should resolve first because lazyload images
  // are able to eagerly use resources in the "list of available images".
  await Promise.race([lazyload_image_promise, timeout_promise]);
}, 'Lazyload images can load immediately from the list of available images');
</script>
