With electron-is-dev

electron.js

// https://gist.github.com/lsenta/36fa6d913dffa1d3218d212e0067056f#file-electron-js
...
const isDev = require("electron-is-dev")
...

function createWindow() {
  ...
  
  if (isDev) {
    // connect to create-react-app dev server
    mainWindow.loadURL("http://localhost:3000")
  }
  else {
    // use package build file
    mainWindow.loadURL(`file://${path.join(__dirname, "../build/index.html")}`)
  }
  
  ...
}