
I spent some time this weekend updating a remark utility I wrote a while back. I wanted to add an 'overview' section to the beginning of every post -- basically a table of contents generated from post headers. There's a great little plugin that does this called [`remark-toc`](https://github.com/remarkjs/remark-toc), which uses `mdast-util-toc` to do just this. It did present a few problems for my workflow:

1. It requires you to define where the table of contents went in the markdown. For other folks, I'm sure this is a useful feature, but I didn't want to have to bother.
2. It removed all content between between the table of contents and the first header. Again, maybe not a problem for some folks, but I often have an introductory section that lacks a header.

Because of this, I ended up spinning up a custom implementation of `mdast-util-toc`.

### remark plugin

The [`unified`](https://github.com/unifiedjs/unified) ecosystem has a fairly robust language for describing itself. At it's core, '**unified** is an interface for processing text using syntax trees' (unified documentation). For our purposes, we can think of it this way:

- we have an `.mdx` or `.md` file
- it is run through a `parser` and converted into a syntax tree
- that syntax tree can be transformed by any number of plugins or `transformers`
- the new syntax tree is run through a `compiler` and output as html

We want to write a plugin that returns a `transformer` that modifies our syntax tree. A really basic version would look like this:

``` js
module.exports = parse;

function parse(options = {}) {
  return transformer;

  function transformer(tree) {
    // modify tree here
  }
}
```

We can pass this module to the `remarkPlugins` argument of `renderToString` to transform the tree.

### generating overview section

We'll update the above generic `parse` function to use `mdast-util-toc`, which does the heavy lifting of generating a list of headers with relevant links. `mdast-util-toc` returns a [node](https://github.com/syntax-tree/unist#node) with a `type` of `list` that we can pass to the `children` array of the syntax tree. If that `node` is returned, we'll update the `children` array in the syntax tree so the map comes first.

``` js
const tocUtil = require('mdast-util-toc');

module.exports = parse;

function parse(options = {}) {
  return transformer;

  function transformer(tree) {
    const { map } = tocUtil(tree);
    if (map) {
      tree.children = [].concat(map, tree.children)
    }
  }
}
```

This works, however, I want to generate a `div` with the class-name `toc` that includes both this table of contents as well as an `overview` heading. We'll need to generate this [`parent` node](https://github.com/syntax-tree/unist#parent) as well as the header node. I wrote up a really generic function for this:

``` js
const genOverview = map => {
  const header = {
    type: 'heading',
    depth: 3,
    children: [{ type: 'text', value: 'overview' }]
  };

  return {
    type: 'parent',
    children: [header, map],
    data: {
      hProperties: { className: 'toc' }
    }
  };
};
```

And update the `transformer` function:

``` js
const { map } = tocUtil(tree);
if (map) {
  const overview = genOverview(map);
  tree.children = [].concat(overview, tree.children);
}
```

While not the most exciting thing in the world, this should offer a brief overview for thinking about handling syntax tress. Looking forward to doing more in the future.

### resources

- [unified documentation](https://github.com/unifiedjs/unified)
- [remark plugin documentation](https://github.com/unifiedjs/unified#plugin)

### up next

- [ ] templating


generating a table of contents for markdown posts

generating theme tokens and falling back to defaults

posts tagged 'table of contents'

remark generated table of contents