remark-mdx-frontmatter.ts

import { name as isIdentifierName } from 'estree-util-is-identifier-name'
import { valueToEstree } from 'estree-util-value-to-estree'
import { type Literal, type Root } from 'mdast'
import { parse as parseToml } from 'toml'
import { type Plugin } from 'unified'
import { parse as parseYaml } from 'yaml'

type FrontmatterParsers = Record<string, (value: string) => unknown>

export interface RemarkMdxFrontmatterOptions {
  /**
   * If specified, the YAML data is exported using this name. Otherwise, each
   * object key will be used as an export name.
   */
  name?: string

  /**
   * A mapping of node types to parsers.
   *
   * Each key represents a frontmatter node type. The value is a function that accepts the
   * frontmatter data as a string, and returns the parsed data.
   *
   * By default `yaml` nodes will be parsed using [`yaml`](https://github.com/eemeli/yaml) and
   * `toml` nodes using [`toml`](https://github.com/BinaryMuse/toml-node).
   */
  parsers?: FrontmatterParsers
}

/**
 * A remark plugin to expose frontmatter data as named exports.
 *
 * @param options Optional options to configure the output.
 * @returns A unified transformer.
 */
const remarkMdxFrontmatter: Plugin<[RemarkMdxFrontmatterOptions?], Root> = ({
  name = 'frontmatter',
  parsers
} = {}) => {
  if (!isIdentifierName(name)) {
    throw new Error(`Name should be a valid identifier, got: ${JSON.stringify(name)}`)
  }

  const allParsers: FrontmatterParsers = {
    yaml: parseYaml,
    toml: parseToml,
    ...parsers
  }

  return (ast) => {
    let data: unknown
    const node = ast.children.find((child) => Object.hasOwn(allParsers, child.type))

    if (node) {
      const parser = allParsers[node.type]

      const { value } = node as Literal
      data = parser(value)
    }

    ast.children.unshift({
      type: 'mdxjsEsm',
      value: '',
      data: {
        estree: {
          type: 'Program',
          sourceType: 'module',
          body: [
            {
              type: 'ExportNamedDeclaration',
              specifiers: [],
              declaration: {
                type: 'VariableDeclaration',
                kind: 'const',
                declarations: [
                  {
                    type: 'VariableDeclarator',
                    id: { type: 'Identifier', name },
                    init: valueToEstree(data, { preserveReferences: true })
                  }
                ]
              }
            }
          ]
        }
      }
    })
  }
}

export default remarkMdxFrontmatter