Used in config to create a collection (tagList) of tag names which can be iterated.

11ty_getTagList.js

/*
Usage:

eleventyConfig.addCollection("tagList", require("11ty_getTagList.js") );

This collection then produces a useful list...

for tag in collections.tagList...

which then gives access to

collections[tag]

*/
module.exports = function(collection) {
  let tagSet = new Set();
  collection.getAllSorted().forEach(function(item) {
    if( "tags" in item.data ) {
      let tags = item.data.tags;
      if( typeof tags === "string" ) {
        tags = [tags];
      }

      tags = tags.filter(function(item) {
        switch(item) {
          // this list should match the `filter` list in tags.njk
          case "all":
          case "nav":
          case "post":
          case "posts":
            return false;
        }

        return true;
      });

      for (const tag of tags) {
        tagSet.add(tag);
      }
    }
  });

  // returning an array in addCollection works in Eleventy 0.5.3
  return [...tagSet].sort();
};