stupid-recursion.js

var sections = [{
  id: "A",
  flag: false,
  subsections: [{
    id: "A1",
    subsections: [{
      id: "A1a",
      flag: true
    }, {
      id: "A1b"
    }]
  }, {
    id: "A2"
  }, {
    id: "A3",
    flag: false
  }]
}, {
  id: "B",
  flag: true
}];

var item = {
  name: "Foo",
  sectionId: "A1b"
};

  /**
   * Recursively dig through subsections until we find one that matches `id`.
   * When a match is found, return the flag preference for that section or its
   * nearest ancestor.
   * 
   * `id` is a String section id
   * `current` is a single Section like the ones above
   * `siblings` is an array of other sections at the same level as `current`
   * `preferred` is a Bool, tracking the current preference
   * 
   **/
  function getSectionPreferredType(id, current, siblings, preferred) {
    var sectionPreferred = current.flag != null ? current.flag : preferred;

    if (current.id == id) {
      // if we found the matching section, return the closest section's flag
      return sectionPreferred;
    }

    if (current.subsections != null && current.subsections.length > 0) {
      // depth-first, look through each child section for a match
      var found = getSectionPreferredType(id, current.subsections[0], current.subsections.slice(1), sectionPreferred);
      if (found != null) return found;
    }

    if (siblings != null && siblings.length > 0) {
      // if we've found no matches so far, check the next child, using the
      // `preferred` that was passed in from the parent
      return getSectionPreferredType(id, siblings[0], siblings.slice(1), preferred);
    }

    return null;
  }
  
  getSectionPreferredType("A1b", sections[0], sections.slice(1))