How to use the hexo/lib/plugins/helper/url_for.bind function in hexo

hexo.on('generateBefore', function () {
    const site = hexo.config;
    const theme = hexo.theme.config;
    const email = theme.profile && theme.profile.email || site.email || '';
    const feed = site.feed ? urlFor.call(this, site.feed.path) : '';
    const result = utils.parseConfig(configSchema, theme, {
      $email: email,
      $feed: feed,
      $copyright: `© ${new Date().getFullYear()} • <a href="${site.url}">${site.author}</a>`,
      $gravatar: gravatar(email, 160),
      $title: site.title,
      $description: site.description
    });
    const urlFn = result.static_prefix ?
      a =&gt; utils.isExternal(a) ? a : `${result.static_prefix}/${a}` :
      urlFor.bind(this);

    // override default language
    site.language = utils.localeId(site.language);

    const __ = this.theme.i18n.__(site.language);

    if (!result.data_prefix) result.data_prefix = result.data_dir;

    // attach disqus script
    if (result.comments &amp;&amp; result.comments.disqus) {
      const disqus = result.comments.disqus;
      disqus.script = disqus.script || `//${disqus.shortname}.disqus.com/embed.js`;
      delete disqus.shortname;
    }

    // convert menu to array

describe('structured_data', function () {
  const Hexo = require('hexo');
  const hexo = new Hexo();
  const ctx = {
    url_for: urlFor.bind(hexo),
    config: hexo.config,
    theme: { profile: {}, sns: {} }
  };
  const structuredData = require('../../../lib/helper/structured_data').bind(ctx);

  it('generate WebSite entry', function () {
    const $ = cheerio.load(structuredData({}));
    const json = JSON.parse($('script').html());

    expect(json.length).toBe(1);
    expect(json[0]['@type']).toBe('WebSite');
  });

  it('generate additional Article entry for post page', function () {
    const $ = cheerio.load(structuredData({ type: 'post', categories: { toArray() { return [] } } }));
    const json = JSON.parse($('script').html());

describe('theme_static', function () {
  const Hexo = require('hexo');
  const hexo = new Hexo();
  const ctx = {
    url_for: urlFor.bind(hexo),
    config: hexo.config,
    theme: { theme: {} }
  };
  const themeStatic = require('../../../lib/helper/theme_static').bind(ctx);

  it('css', function () {
    const $ = cheerio.load(themeStatic('css'));
    expect($('link').eq(0).attr('href')).toMatch(/^\/styles\.\w*\.css$/);
  });

  it('js', function () {
    const $ = cheerio.load(themeStatic('js'));
    expect($('script').eq(0).attr('src')).toMatch(/^\/runtime/);
    expect($('script').eq(1).attr('src')).toMatch(/^\/polyfills/);
    expect($('script').eq(2).attr('src')).toMatch(/^\/polyfills/);
    expect($('script').eq(3).attr('src')).toMatch(/^\/main/);