When override _app.js what is getInitialProps used for?

Question

what is this really do ?

pageProps = await Component.getInitialProps(ctx)

it looks "pageProps" this is just an empty object

import App, {Container} from 'next/app'
import React from 'react'

export default class MyApp extends App {
  static async getInitialProps ({ Component, router, ctx }) {
    let pageProps = {}

    if (Component.getInitialProps) {
      pageProps = await Component.getInitialProps(ctx)
    }

    return {pageProps}
  }

  render () {
    const {Component, pageProps} = this.props
    return <Container>
      <Component {...pageProps} />
    </Container>
  }
}

Answer 1

getInitialProps allows you to call out to get the props that you would like the component to have when rendered on the server.

For instance, I might need to display the current weather and I want Google to index my pages with that information for SEO purposes.

To achieve that, you'd do something like this:

import React from 'react'
import 'isomorphic-fetch'
const HomePage = (props) => (
  <div>
    Weather today is: {weather}
  </div>
)
HomePage.getInitialProps = async ({ req }) => {
  const res = await fetch('https://my.weather.api/london/today')
  const json = await res.json()
  return { weather: json.today }
}
export default HomePage 

The line pageProps = await Component.getInitialProps(ctx) calls that initial function so that the HomePage component is instantiated with the initial props that result from that call to the weather API.