Minification not working properly

[suffle]

Bug report

When importing a library on a page (or in another file that is used on a page), the minification does not work correctly and bloats the js files.

To Reproduce

I added a very simple test.js to my pages folder that looks like this:

import React from "react";

import mdx from "@mdx-js/mdx";

const TestPage = props => {
  return <h1>This is a headline</h1>;
};

export default TestPage;

and also added some lines to my next.config.js to make the build work:

module.exports = {
  webpack(config, { isServer }) {
    if (!isServer) {
      config.node = {
        fs: "empty",
      };
    }
    return config;
};

Expected behavior

The imported mdx-Library is not used so I would expect it to be removed on minification. As you can see in the screenshot, there is a huge size dfference between the test-page with and without the import.

Screenshots

Without the import statement

With the import statement

System information

• OS: Linux
• Version of Next.js: 9.5.3, and also the latest 9.5.4-canary
• Version of Node.js:12.18.3

[timneutkens]

MDX is probably not marked as side-effect free. This is not a Next.js bug so I'll convert it to a discussion.