"Parsing error: Assigning to rvalue" when linting MDX files with YAML frontmatter

[sebthom]

I am trying to configure 'eslint-plugin-mdx' in an Astro project to lint mdx files with YAML frontmatter. But don't get it working.

My eslint.config.mjs looks like this:

// @ts-check
import * as mdx from 'eslint-plugin-mdx'

/** @typedef { import("eslint").Linter.Config } FlatConfig */

/** @type {FlatConfig[]} */
export default [
  {
    ...mdxPlugin.flatCodeBlocks,
    files: ["src/**/*.{md,mdx}"],
    processor: mdx.createRemarkProcessor({
      lintCodeBlocks: true,
    }),
  },
];

When I execute pnpm eslint I get:

astrosite/src/data/blog/article.mdx
  1:3  error  Parsing error: Assigning to rvalue

So I guess the frontmatter is parsed wrong.

Any hints are greatly appreciated!

[ChristianMurphy]

@sebthom have you enabled the frontmatter plugin in your remark config file?
mdx-js/eslint-mdx#147

/cc @JounQin

[JounQin]

A runnable reproduction is required to check.

[sebthom]

Here is a reproducer: https://stackblitz.com/edit/withastro-astro-bpwbfw1j

pnpm dev to run the dev server and render the page works. Same for pnpm build to generate a static page from the mdx file.

But running pnpm eslint results in:

❯ pnpm eslint

/home/projects/withastro-astro-bpwbfw1j/src/pages/index.mdx
  1:3  error  Parsing error: Assigning to rvalue

✖ 1 problem (1 error, 0 warnings)

[sebthom]

I just realized the error only occurs for mdx.flatCodeBlocks but not for mdx.flat

[sebthom]

With mdx.flat I am getting other errors that I don't understand in this context: Expected an assignment or function call and instead saw an expression no-unused-expressions when referencing a variable in the mdx content like {frontmatter.author}

[JounQin]

mdx.flatCodeBlocks is for virtual filenames like */.mdx/0_x.js, mostly you don't need to override it, see source codes here https://github.com/mdx-js/eslint-mdx/blob/d6fc093fb32ab58fb226e8cf42ac77399b8a4758/packages/eslint-plugin-mdx/src/configs/code-blocks.ts#L3-L34

Please use according the document https://github.com/mdx-js/eslint-mdx#flat-config