next.js/examples/with-apollo-and-redux-saga/lib/initApollo.js

import { ApolloClient } from 'apollo-client'
import { HttpLink } from 'apollo-link-http'
import { InMemoryCache } from 'apollo-cache-inmemory'
import fetch from 'isomorphic-unfetch'

let apolloClient = null

// Polyfill fetch() on the server (used by apollo-client)
if (!process.browser) {
  global.fetch = fetch
}

function create(initialState) {
  return new ApolloClient({
    connectToDevTools: process.browser,
    ssrMode: !process.browser, // Disables forceFetch on the server (so queries are only run once)
    link: new HttpLink({
      uri: 'https://api.graph.cool/simple/v1/cixmkt2ul01q00122mksg82pn', // Server URL (must be absolute)
      credentials: 'same-origin' // Additional fetch() options like `credentials` or `headers`
    }),
    cache: new InMemoryCache().restore(initialState || {})
  })
}

export default function initApollo(initialState) {
  // Make sure to create a new client for every server-side request so that data
  // isn't shared between connections (which would be bad)
  if (!process.browser) {
    return create(initialState)
  }

  // Reuse client on the client-side
  if (!apolloClient) {
    apolloClient = create(initialState)
  }

  return apolloClient
}