Prettier plugin for formatting JS frontmatter

prettier-plugin-js-frontmatter.js

import prettier from 'prettier';
import babelParser from 'prettier/plugins/babel';
import htmlParser from 'prettier/plugins/html';
const frontmatterRegex = /^---js\n([\s\S]*?)\n---\n/;

function parse(text, options) {
  const match = text.match(frontmatterRegex);
  if (match) {
    const frontmatter = match[1];
    const content = text.slice(match[0].length);
    return {
      type: 'frontmatterDocument',
      frontmatter,
      content,
    };
  }
  return {
    type: 'root',
    text: text,
  };
}

async function print(path, options, print) {
  const node = path.getValue();
  if (node.type === 'frontmatterDocument') {
    // Ensure babel can interpret the frontmatter by wrapping it as an expression.
    const prependedFrontmatter = `(${node.frontmatter})`;
    const formattedFrontmatterWithReturn = await prettier.format(
      prependedFrontmatter,
      {
        ...options,
        parser: 'babel',
        plugins: [babelParser],
      },
    );
    // Switch formatted code back to object literal by removing `(` and `);\n`.
    const formattedFrontmatterWithoutReturn =
      formattedFrontmatterWithReturn.slice(1, -3);
    // Format remaining context as HTML
    const formattedContent = await prettier.format(node.content, {
      ...options,
      parser: 'html',
      plugins: [htmlParser],
    });
    return `---js\n${formattedFrontmatterWithoutReturn}\n---\n\n${formattedContent}`;
  }
  // No js frontmatter detected, just format as HTML.
  const formattedContent = await prettier.format(node.text, {
    ...options,
    parser: 'html',
    plugins: [htmlParser],
  });

  return formattedContent;
}

export default {
  parsers: {
    'js-frontmatter': {
      parse,
      astFormat: 'js-frontmatter',
      locStart: () => 0,
      locEnd: () => 0,
    },
  },
  printers: {
    'js-frontmatter': {
      print,
    },
  },
  options: {},
};

prettier.config.example.js

/** @type {import('prettier').Config} */
export default {
  plugins: ['./prettier-plugin-js-frontmatter.js'],
  overrides: [
    {
      files: '*.webc',
      options: {
        parser: 'js-frontmatter',
      },
    },
  ],
};