Using vite-plugin-ssr with mdx-js, solving ESM only library problems, understanding vite configuration and writing vite plugins

vite-plugin-ssr is vite plugin which allows us to build websites with Server Side Rendering, Client Side Rendering, Single Page Applications and Static Site Generation all in one. This plugin is like Next.js but provides more control over each page and for any of your favorite frontend framework. Please visit the website to learn how to use this plugin.

In this tutorial we'll learn how to setup mdx-js library for vite project for building markdown based websites and to prerender them using vite-plugin-ssr to generate static websites.

The vite-plugin-ssr github repo contains example projects which you can clone and start with. For example react-full example already provides a setup for working with mdx-js library. The intention of this tutorial is to show how to solve some of the problems I encountered while using the mdx-js library and vite-plugin-ssr prerender feature.

Project setup

First of all, we need to setup a vite + vite-plugin-ssr based project. To scaffold a vite-plugin-ssr project simply execute

npm init vite-plugin-ssr

Give your project a name (I named it nn-blog) and select the frontend framework (in this example react) you would like to use. Once the command runs simply go to your project folder and install all dependencies.

cd nn-blog
npm install

Then run the dev server with npm run dev. Congratulations, you've just setup a vite + vite-plugin-ssr based project. The setup comes initialized with a git repo, so you can start modifying the code around. And you'll notice how blazingly fast the vite dev server is.

Once you understand the filesystem routing concepts of vite-plugin-ssr, create some pages and experiment. When you're ready let's start with adding mdx-js.

Adding mdx-js to vite project

mdx-js is a library which converts markdown content to jsx compatible content that you can then use with your jsx based libraries such as react, preact, vue.

MDX allows you to use JSX in your markdown content. You can import components, such as interactive charts or alerts, and embed them within your content.
vite uses rollup under the hood to build bundles for production. So for installing mdx-js to a vite project, we should use @mdx-js/rollup and for handling custom MDX components we can use @mdx-js/react for react based projects.

npm install @mdx-js/rollup @mdx-js/react

Once the libraries are installed, add mdx-js to vite plugins in vite.config.js file and config the mdx plugin to use @mdx-js/react as an proiderImportSource.

import react from '@vitejs/plugin-react'
import ssr from 'vite-plugin-ssr/plugin'
+import mdx from "@mdx-js/rollup"

export default {
- plugins: [react(), ssr()]
+ plugins: [react(), mdx({
+   providerImportSource: "@mdx-js/react"
+ }), ssr()],
}

Solving problem 1 - require() of ES Module is not supported

Now after updating the vite.config.js if we try to run npm run dev we'll be given this confusing error

failed to load config from /workspace/example/nn-blog/vite.config.js
/workspace/example/nn-blog/vite.config.js:61509
undefined
            ^

Error [ERR_REQUIRE_ESM]: require() of ES Module /workspace/example/nn-blog/node_modules/@mdx-js/rollup/index.js from /workspace/example/nn-blog/vite.config.js not supported.

This problems occurs in the following order.

• npm run dev runs node ./server/index.js file which is a commonjs file
• The script creates vite dev server using vite.createServer
• The vite dev server converts vite.config.js to CJS module first and then loads the config from this file.
• As CJS module tries to require("@mdx-js/rollup") plugin which is a ESM only module the error will be generated.

To solve this problem, we should inform vite to skip building config file to CJS. This can be achieved by adding

+ "type": "module",
}

to package.json file.

Solving problem 2 - require() is not defined in ES module scope

Once we inform node to enable ES modules, we cannot use require syntax in .js files. This is exactly what you'll get when you run npm run dev

file:///workspace/example/nn-blog/server/index.js:1
const express = require('express')
                ^

ReferenceError: require is not defined in ES module scope, you can use import instead
This file is being treated as an ES module because it has a '.js' file extension and '/workspace/example/nn-blog/package.json' contains "type": "module". To treat it as a CommonJS script, rename it to use the '.cjs' file extension.

Luckily, the error itself gave us a solution. But you need to first stop scratching your head and learn to read those lines in to identify the solution. If you look carefully what we need is just to rename our index.js file to index.cjs and 💣

Solving problem 3 - Cannot find module

node:internal/modules/cjs/loader:936
  throw err;
  ^

Error: Cannot find module '/workspace/example/nn-blog/server'
    at Function.Module._resolveFilename (node:internal/modules/cjs/loader:933:15)
    at Function.Module._load (node:internal/modules/cjs/loader:778:27)
    at Function.executeUserEntryPoint [as runMain] (node:internal/modules/run_main:81:12)
    at node:internal/main/run_main_module:17:47 {
  code: 'MODULE_NOT_FOUND',
  requireStack: []
}

Wait, where is our file gone? Node says it can't find it, but it's there right in the server folder.

May be if you're patient enough or highly talented nerd enough, you'll understand that node is trying to load server module and not server/index.js. The /index.js file comes into picture as part of the CJS module loading sequence of node. So, we need to add a package.json file with the following value

{
  "main": "index.cjs"
}

And ✨ congratulations, you are now ready to go.

Adding a markdown page

Now go to pages directory and any markdown content with .md or .mdx extention. For example, for creating a /naveennamani root, add pages/naveennamani.page.mdx or pages/naveennamani/index.page.mdx or pages/index/naveennamani.page.mdx file. (I prefer the last filename for this example).

Once you create the file add any markdown content, hit [localhost:3000/naveennamani] url to see your markdown content getting converted into html. For using react components inside your mdx files simply import them and use.

# Hello world

import { Counter } from './Counter'

<Counter />

This will show a heading with an interactive counter that is also shown on home page.

Prerendering and inventing new problems

When you stop the dev server and want to build your awesome website as a static content, you can use vite-plugin-ssr prerender feature. Just add the following script to package.json

"scripts": {
   ...
   "prerender": "npm run build && vite-plugin-ssr prerender"
}

Now when you run npm run prerender, you'll see that dist\client and dist\server folders are created and build files are populated there. But prerendering is failing with

/workspace/example/nn-blog/dist/server/assets/naveennamani.page.04918628.js:4
var react = require("@mdx-js/react");
            ^

Error [ERR_REQUIRE_ESM]: require() of ES Module /workspace/example/nn-blog/node_modules/@mdx-js/react/index.js from /workspace/example/nn-blog/dist/server/assets/naveennamani.page.04918628.js not supported.

Isn't that the same problem we solved earlier? Yes. But why again? 😢
This time the problem is created in the following order.

• When you run npm run build it runs vite build and vite build --ssr with the first command building assets for dist\client and second command for dist\server.
• While dist\client assets are all esm modules, dist\client build output are cjs modules.
• So, again @mdx-js/react which is a ESM only module is failed to import through require.

This time, we can generate ES modules instead of CJS modules by configuring build options in vite.config.js as follows

  import react from '@vitejs/plugin-react'
  import ssr from 'vite-plugin-ssr/plugin'
  import mdx from "@mdx-js/rollup"
+ import { defineConfig } from 'vite'

+ export default defineConfig({
    plugins: [react(), mdx({
      providerImportSource: "@mdx-js/react"
    }), ssr()],
+   build: {
+     rollupOptions: {
+       output: {
+         format: "es"
+       }
+     }
+   }
+ })

When you run npm run prerender again, you can see that dist\server folder contains files which are ES modules. But you still get this complicated error.

Error [ERR_MODULE_NOT_FOUND]: Cannot find module '/workspace/example/nn-blog/node_modules/react/jsx-runtime' imported from /workspace/example/nn-blog/dist/server/assets/index.page.0262694b.js
Did you mean to import react/jsx-runtime.js?

Writing a vite plugin to solve our problems

At first sight, the error seems like a spelling mistake. But if you google, there is a long list of comments in the official react repo (issue #20235). The problem can be simply solved by adding .js extension to the import, but how to do that automatically?

Let's write a vite plugin to do that for us. Writing a vite plugin is very simple if you follow the Vite plugin API.

This is what I come with.

export default function fix_ssr_esm_modules(replacements) {
  function transform(code, id, ssr) {
    if (ssr) // ssr is true when `vite build --ssr` is run
      return replacements.reduce((prevCode, { find, replacement }) => {
        return prevCode.replaceAll(find, replacement);
      }, code);
  }

  return { // configuration of our plugin used by vite
    name: "vite-plugin-fix-ssr-esm-modules",
    apply: "build", // execute only for build tasks
    enforce: "post", // execute after build finished
    transform: transform, // transformation function that returns transformed code
  };
}

Now place the code in fix_ssr_esm_modules.js file and then import and use this plugin in vite.config.js file as follows.

+ import fix_ssr_esm_modules from "./fix_ssr_esm_imports.js";

export default defineConfig({
  plugins: [
    react(),
    mdx({
      providerImportSource: "@mdx-js/react",
    }),
    ssr(),
+   fix_ssr_esm_modules([
+     { find: "react/jsx-runtime", replacement: "react/jsx-runtime.js" },
+     { find: "react-dom/server", replacement: "react-dom/server.js" },
+   ]),
  ],
  build: {
    rollupOptions: {
      output: {
        format: "es",
      },
    },
  },
});

The plugin transforms the build files and replaces the import as given as options to the plugin.

Now you can run npm run prerender and serve the files in dist\client statically using npx serve. Congratulations 🌟, you just built a static site using vite-plugin-ssr.

Final touch

The final version of the source code of the project is available in github naveennamani/vite-ssr-mdx.

There is a small inconsistency with server/index.js file, but that's an alternative I found while I'm writing this article.

Sorry for the long post, if you come here after all, here is a potato for you.