devdocs/assets/javascripts/lib/favicon.js

/*
 * decaffeinate suggestions:
 * DS102: Remove unnecessary code created because of implicit returns
 * DS208: Avoid top-level this
 * Full docs: https://github.com/decaffeinate/decaffeinate/blob/main/docs/suggestions.md
 */
let defaultUrl = null;
let currentSlug = null;

const imageCache = {};
const urlCache = {};

const withImage = function(url, action) {
  if (imageCache[url]) {
    return action(imageCache[url]);
  } else {
    const img = new Image();
    img.crossOrigin = 'anonymous';
    img.src = url;
    return img.onload = () => {
      imageCache[url] = img;
      return action(img);
    };
  }
};

this.setFaviconForDoc = function(doc) {
  if (currentSlug === doc.slug) { return; }

  const favicon = $('link[rel="icon"]');

  if (defaultUrl === null) {
    defaultUrl = favicon.href;
  }

  if (urlCache[doc.slug]) {
    favicon.href = urlCache[doc.slug];
    currentSlug = doc.slug;
    return;
  }

  const iconEl = $(`._icon-${doc.slug.split('~')[0]}`);
  if (iconEl === null) { return; }

  const styles = window.getComputedStyle(iconEl, ':before');

  const backgroundPositionX = styles['background-position-x'];
  const backgroundPositionY = styles['background-position-y'];
  if ((backgroundPositionX === undefined) || (backgroundPositionY === undefined)) { return; }

  const bgUrl = app.config.favicon_spritesheet;
  const sourceSize = 16;
  const sourceX = Math.abs(parseInt(backgroundPositionX.slice(0, -2)));
  const sourceY = Math.abs(parseInt(backgroundPositionY.slice(0, -2)));

  return withImage(bgUrl, docImg => withImage(defaultUrl, function(defaultImg) {
    const size = defaultImg.width;

    const canvas = document.createElement('canvas');
    const ctx = canvas.getContext('2d');

    canvas.width = size;
    canvas.height = size;
    ctx.drawImage(defaultImg, 0, 0);

    const docIconPercentage = 65;
    const destinationCoords = (size / 100) * (100 - docIconPercentage);
    const destinationSize = (size / 100) * docIconPercentage;

    ctx.drawImage(docImg, sourceX, sourceY, sourceSize, sourceSize, destinationCoords, destinationCoords, destinationSize, destinationSize);

    try {
      urlCache[doc.slug] = canvas.toDataURL();
      favicon.href = urlCache[doc.slug];

      return currentSlug = doc.slug;
    } catch (error) {
      Raven.captureException(error, { level: 'info' });
      return this.resetFavicon();
    }
  }));
};

this.resetFavicon = function() {
  if ((defaultUrl !== null) && (currentSlug !== null)) {
    $('link[rel="icon"]').href = defaultUrl;
    return currentSlug = null;
  }
};