vercel__next.js/examples/with-overmind/pages/_app.js

import React from "react";
import App from "next/app";
import { createOvermind, createOvermindSSR, rehydrate } from "overmind";
import { Provider } from "overmind-react";
import { config } from "../overmind";

class MyApp extends App {
  // CLIENT: On initial route
  // SERVER: On initial route
  constructor(props) {
    super(props);

    const mutations = props.pageProps.mutations || [];

    if (typeof window !== "undefined") {
      // On the client we just instantiate the Overmind instance and run
      // the "changePage" action
      this.overmind = createOvermind(config);
      this.overmind.actions.changePage(mutations);
    } else {
      // On the server we rehydrate the mutations to an SSR instance of Overmind,
      // as we do not want to run any additional logic here
      this.overmind = createOvermindSSR(config);
      rehydrate(this.overmind.state, mutations);
    }
  }
  // CLIENT: After initial route, on page change
  // SERVER: never
  componentDidUpdate() {
    // This runs whenever the client routes to a new page
    this.overmind.actions.changePage(this.props.pageProps.mutations || []);
  }
  // CLIENT: On every page change
  // SERVER: On initial route
  render() {
    const { Component } = this.props;

    return (
      <Provider value={this.overmind}>
        <Component />
      </Provider>
    );
  }
}

export default MyApp;