Provide a way to populate field values from Markdown/MDX processing pipeline

この issue では実装ファイルやテストが指定されていないため、まず Markdown/MDX 処理パイプラインをたどり、plugin が生成した vFile.data が現在どのように扱われているかを確認してください。frontmatter、code、_raw と並んでこれらの値が生成されたコンテンツオブジェクトに公開され、searchMeta の例を対象としたカバレッジがあることを完了条件とします。

Feature:
I'd like to use an MDX plugin to create programmatic meta (variable declarations) that can then be exposed in the final object contentlayer provides.

Use case:
An example of here this would be useful is generating a TOC, creating a list of unique keywords, etc.

Work around:
Use computed fields (which don't expose the the raw AST)

Related issues: https://github.com/kentcdodds/mdx-bundler/issues/169

Consider the following config structure

export default makeSource({
  contentDirPath: "content",
  documentTypes: [Posts, Things, People],
  mdx: {
    rehypePlugins: [rehypeSlug, rehypeAutolinkHeadings, searchMeta],
  },
});

Where searchMeta looks at paragraph nodes of mhast, grabs a list of unique words, and adds them to the metadata as searchMeta.

A markdown file with the structure of:

---
title: Hello World
slug: hello-world
---
Hello World! Please say Hello!

Would generate a final object of:

{
"title": "Hello World",
"slug": "hello-world",
"searchMeta": ["hello", "world", "please", "say"],
"code": "().....",
"_raw": "..."
}

For sake of complete, if not ugly code, here's a working example of the plugin that adds searchMeta to the data attribute of the vFile in the rehype plugin chain.

import { visit } from "unist-util-visit";

export default function searchMeta() {
  return (tree, file) => {
    visit(tree, { tagName: "p" }, (node) => {
      let words = node.children.reduce((collector, current) => {
        if (typeof current.value === "string") {
          let wordList = current.value
            .split(" ")
            .filter((word) => !word.includes(":"))
            .map((word) => word.toLowerCase().replace(/[^a-z0-9]/gi, ""))
            .filter((word) => word.length > 3);
          let newCollector = new Set([...wordList, ...collector]);
          return newCollector;
        } else {
          return collector;
        }
      }, new Set());

      file.data.searchMeta = [...words];
    });
  };
}