gistfile1.txt

Here's how I built a sidebar with sections in Jekyll:
```{% comment %}
  The sections for the sidebar are managed through _data/sections.yml. We load
  them in the order they're declared (`site.data.sections`), then loop through
  each section and list any pages.

  To allow pages to be ordered without explicitly requiring it, we sort by
  weight. The lower the weight, the higher a page floats in the order (get it?).
{% endcomment %}
{% assign sections = site.data.sections %}
{% assign collection = site.documents | sort: "weight" %}

{% for section_data in sections %}

  {% comment %}
    When we loop, the section_data is an array with two items:
    - 0: the identifier for the section (e.g. "introduction")
    - 1: the section data (e.g. the title)
  {% endcomment %}
  {% assign section_id = section_data[0] %}
  {% assign section = section_data[1] %}

  {% comment %}
    Sections can be hidden by setting `hidden: true`. Helpful for appendix-style
    sections that would be really long and/or confusing.
  {% endcomment %}
  {% if section.hidden != true %}
    <div class="sidebar__group links js__collapsible">
      <h3 class="links__heading u-icon u-icon--before u-icon--{{ section.icon }}">
        {{ section.title }}
      </h3>
      <ul class="links__list">

        {% comment %}
          Set up a loop through every page on the site. It's not particularly
          efficient, but since we're using a static site generator, we don't
          have an option. On the bright side, this only affects the build,
          which users never experience.
        {% endcomment %}
        {% for currentPage in collection %}
          {% if currentPage.section %}

            {% comment %}
              By default, use the section set in the page's front matter.
            {% endcomment %}
            {% assign parent_section = currentPage.section %}
          {% else %}

            {% comment %}
              To allow pages to be sorted by folder structure rather than
              requiring front matter, we use the URL structure as a fallback.

              Example:
              /super/cool/page
                      |    └── the page name
                      └─────── the immediate parent — used as the "section"
            {% endcomment %}
            {% assign parent_section_arr = currentPage.url | split: "/" | reverse | slice: 1 %}
            {% assign parent_section = parent_section_arr.first %}
          {% endif %}

          {% comment %}
            If the section matches, add the current page in the sidebar.
          {% endcomment %}
          {% if section_id == parent_section %}
            <li class="links__list-item">
              <a href="{{ site.github.url}}{{ currentPage.url }}"
                 class="links__link {% if page.url == currentPage.url %}links__link--selected{% endif %}">
                {{ currentPage.title }}
              </a>
            </li>
          {% endif %}
        {% endfor %}
      </ul>
    </div>
  {% endif %}
{% endfor %}
```


Jason Lengstorf 
[7 minutes ago] 
So however you're building that sidebar, you can check:
```{% if page.url === sidebarPage.url %}active-class{% endif %}```