Next.js: Unexpected missing ctx.req in _document

Sat Feb 5, 2022 Today I Learned

This week I was trying to access the ctx.req property on a _document, but the value kept showing up as undefined. I tried the most basic example that they showed in their docs:

class MyDocument extends Document {
  static async getInitialProps(ctx) {
    const initialProps = await Document.getInitialProps(ctx);

    // Expecting headers, cookies, etc, but it's undefined.

    return initialProps;

  render() {
    return (
        <Head />
          <Main />
          <NextScript />

export default MyDocument;

Why is ctx.req undefined on a server-side only process? The Next.js docs say about the custom _document:

This file is only rendered on the server

💡 Well, it turns out that another page rendered lower in the tree by <Main /> must invoke getServerSideProps first. For example, adding this to pages/index.tsx:

export const getServerSideProps = async (_ctx: GetServerSidePropsContext) => {
   * Yep, it's empty, but needed to force reading cookies and request
   * headers on server-side so _document can read it
  return { props: {} };

Yep, and now all of a sudden, the full request object is available on ctx.req inside the custom _document!

It matters which route/page you’re accessing. So, adding getServerSideProps to pages/index.tsx won’t mean that _document will behave the same on pages/login.tsx. You’d need to add getServerSideProps on that page as well.