discourse/app/assets/javascripts/pretty-text/engines/discourse-markdown/inject-line-number.js.es6

export function setup(helper) {
  const opts = helper.getOptions();

  if (opts.previewing && opts.injectLineNumbersToPreview) {
    helper.whiteList([
      'p.preview-sync-line',
      'p[data-line-number]',
      'h1.preview-sync-line',
      'h1[data-line-number]',
      'h2.preview-sync-line',
      'h2[data-line-number]',
      'h3.preview-sync-line',
      'h3[data-line-number]',
      'h4.preview-sync-line',
      'h4[data-line-number]',
      'h5.preview-sync-line',
      'h5[data-line-number]',
      'h6.preview-sync-line',
      'h6[data-line-number]',
      'blockquote.preview-sync-line',
      'blockquote[data-line-number]',
      'hr.preview-sync-line',
      'hr[data-line-number]',
      'ul.preview-sync-line',
      'ul[data-line-number]',
      'ol.preview-sync-line',
      'ol[data-line-number]',
    ]);

    helper.registerPlugin(md => {
      const injectLineNumber = (tokens, index, options, env, self) => {
        let line;
        const token = tokens[index];

        if (token.map && token.level === 0) {
          line = token.map[0];
          token.attrJoin('class', 'preview-sync-line');
          token.attrSet('data-line-number', String(line));
        }

        return self.renderToken(tokens, index, options, env, self);
      };

      md.renderer.rules.paragraph_open = injectLineNumber;
      md.renderer.rules.heading_open = injectLineNumber;
      md.renderer.rules.blockquote_open = injectLineNumber;
      md.renderer.rules.hr = injectLineNumber;
      md.renderer.rules.ordered_list_open = injectLineNumber;
      md.renderer.rules.bullet_list_open = injectLineNumber;
    });
  }
}