luandevpro/_document.js

## _document.js
import Document, { Head, Main, NextScript } from 'next/document'
// Import styled components ServerStyleSheet
import { ServerStyleSheet } from 'styled-components';
import { getServerSideToken, getUserScript } from '../auth';

export default class MyDocument extends Document {
   static async getInitialProps(ctx) {
    const sheet = new ServerStyleSheet();
    const page = ctx.renderPage((App) => (props) =>
      sheet.collectStyles(<App {...props} />),
    );
    const styleTags = sheet.getStyleElement();
    const userData = await getServerSideToken(ctx.req);
    return { ...page, styleTags , ...userData};
  }

  render () {
     var { user } = this.props
    return (
      <html>
        <Head>
          <title>My page</title>
          {this.props.styleTags}
        </Head>
        <body>
          <Main />
          // chú ý chỗ này nhé
          <NextScript />
          <script dangerouslySetInnerHTML={{__html: getUserScript(user)}}/>
        </body>
      </html>
    )
  }
}

## index.js
// auth/index.js
const WINDOW_USER = "__USER__"

export const loginUser = async (email, password) => {
   const { data } = await axios.post("/api/login", { email , password});
   if(typeof window !== undefined){
      window[WINDOW_USER] = data || {}
   }
}

export const getUserScript = (user) => {
   return `${WINDOW_USER} = ${JSON.stringify(user)}`;
}
	import Document, { Head, Main, NextScript } from 'next/document'
	// Import styled components ServerStyleSheet
	import { ServerStyleSheet } from 'styled-components';
	import { getServerSideToken, getUserScript } from '../auth';

	export default class MyDocument extends Document {
	static async getInitialProps(ctx) {
	const sheet = new ServerStyleSheet();
	const page = ctx.renderPage((App) => (props) =>
	sheet.collectStyles(<App {...props} />),
	);
	const styleTags = sheet.getStyleElement();
	const userData = await getServerSideToken(ctx.req);
	return { ...page, styleTags , ...userData};
	}

	render () {
	var { user } = this.props
	return (
	<html>
	<Head>
	<title>My page</title>
	{this.props.styleTags}
	</Head>
	<body>
	<Main />
	// chú ý chỗ này nhé
	<NextScript />
	<script dangerouslySetInnerHTML={{__html: getUserScript(user)}}/>
	</body>
	</html>
	)
	}
	}
	// auth/index.js
	const WINDOW_USER = "__USER__"

	export const loginUser = async (email, password) => {
	const { data } = await axios.post("/api/login", { email , password});
	if(typeof window !== undefined){
	window[WINDOW_USER] = data \|\| {}
	}
	}

	export const getUserScript = (user) => {
	return `${WINDOW_USER} = ${JSON.stringify(user)}`;
	}