next.js/examples/with-mobx/pages/_app.js

import App, {Container} from 'next/app'
import React from 'react'
import { initializeStore } from '../store'
import { Provider } from 'mobx-react'

class MyMobxApp extends App {
    static async getInitialProps(appContext) {
        // Get or Create the store with `undefined` as initialState
        // This allows you to set a custom default initialState
        const mobxStore = initializeStore()
        // Provide the store to getInitialProps of pages
        appContext.ctx.mobxStore = mobxStore

        let appProps = await App.getInitialProps(appContext)

        return {
            ...appProps,
            initialMobxState: mobxStore
        };
    }

    constructor(props) {
        super(props)
        const isServer = typeof window === 'undefined'
        this.mobxStore = isServer ?
            props.initialMobxState:
            initializeStore(props.initialMobxState)
    }

    render() {
        const { Component, pageProps } = this.props
        return (
            <Container>
                <Provider store={this.mobxStore}>
                    <Component {...pageProps} />
                </Provider>
            </Container>
        );
    }
}
export default MyMobxApp