discourse/app/assets/javascripts/pretty-text/oneboxer.js.es6

let timeout;
const loadingQueue = [];
const localCache = {};
const failedCache = {};

function resolveSize(img) {
  $(img).addClass('size-resolved');

  if (img.width > 0 && img.width === img.height) {
    $(img).addClass('onebox-avatar');
  }
}

// Detect square images and apply smaller onebox-avatar class
function applySquareGenericOnebox($elem, normalizedUrl) {
  if (!$elem.hasClass('whitelistedgeneric')) {
    return;
  }

  let $img = $elem.find('.onebox-body img.thumbnail');
  let img = $img[0];

  // already resolved... skip
  if ($img.length !== 1 || $img.hasClass('size-resolved')) {
    return;
  }

  if (img.complete) {
    resolveSize(img, $elem, normalizedUrl);
  } else {
    $img.on('load.onebox', () => {
      resolveSize(img, $elem, normalizedUrl);
      $img.off('load.onebox');
    });
  }
}

function loadNext(ajax) {
  if (loadingQueue.length === 0) {
    timeout = null;
    return;
  }

  let timeoutMs = 150;
  let removeLoading = true;
  const { url, refresh, $elem, categoryId } = loadingQueue.shift();

  // Retrieve the onebox
  return ajax("/onebox", {
    dataType: 'html',
    data: { url, refresh, category_id: categoryId },
    cache: true
  }).then(html => {
    let $html = $(html);
    localCache[normalize(url)] = $html;
    $elem.replaceWith($html);
    applySquareGenericOnebox($html, normalize(url));
  }, result => {
    if (result && result.jqXHR && result.jqXHR.status === 429) {
      timeoutMs = 2000;
      removeLoading = false;
      loadingQueue.unshift({ url, refresh, $elem, categoryId });
    } else {
      failedCache[normalize(url)] = true;
    }
  }).finally(() => {
    timeout = Ember.run.later(() => loadNext(ajax), timeoutMs);
    if (removeLoading) {
      $elem.removeClass('loading-onebox');
      $elem.data('onebox-loaded');
    }
  });
}

// Perform a lookup of a onebox based an anchor $element.
// It will insert a loading indicator and remove it when the loading is complete or fails.
export function load({ elem , refresh = true, ajax, synchronous = false, categoryId }) {
  const $elem = $(elem);

  // If the onebox has loaded or is loading, return
  if ($elem.data('onebox-loaded')) return;
  if ($elem.hasClass('loading-onebox')) return;

  const url = elem.href;

  // Unless we're forcing a refresh...
  if (!refresh) {
    // If we have it in our cache, return it.
    const cached = localCache[normalize(url)];
    if (cached) return cached.prop('outerHTML');

    // If the request failed, don't do anything
    const failed = failedCache[normalize(url)];
    if (failed) return;
  }

  // Add the loading CSS class
  $elem.addClass('loading-onebox');

  // Add to the loading queue
  loadingQueue.push({ url, refresh, $elem, categoryId });

  // Load next url in queue
  if (synchronous) {
    return loadNext(ajax);
  } else {
    timeout = timeout || Ember.run.later(() => loadNext(ajax), 150);
  }
}

// Sometimes jQuery will return URLs with trailing slashes when the
// `href` didn't have them.
function normalize(url) {
  return url.replace(/\/$/, '');
}

export function lookupCache(url) {
  const cached = localCache[normalize(url)];
  return cached && cached.prop('outerHTML');
}