Passing Data to Windows in Electron

You need the ipcMain module to achieve this... As stated in the API "When used in the main process, it handles asynchronous and synchronous messages sent from a renderer process (web page). Messages sent from a renderer will be emitted to this module."

API Docs for the ipcMain module: https://electronjs.org/docs/api/ipc-main

To use the ipcMain you need to have nodeIntegration enabled on webPreferences

win = new BrowserWindow({
    webPreferences: {
        nodeIntegration: true,
    }
})

Be careful this may cause security issues.

For example: Let's say we want to pass a configuration (json) file to the web page

(Triple dots (...) represent your code that is already placed inside the file, but is not relevant to this example)

main.js

...

const { readFileSync } = require('fs') // used to read files
const { ipcMain } = require('electron') // used to communicate asynchronously from the main process to renderer processes.
...

// function to read from a json file
function readConfig () {
  const data = readFileSync('./package.json', 'utf8')
  return data
}

...
// this is the event listener that will respond when we will request it in the web page
ipcMain.on('synchronous-message', (event, arg) => {
  console.log(arg)
  event.returnValue = readConfig()
})
...

index.html

...    
<script>
    <!-- import the module -->
    const { ipcRenderer } = require('electron')

    <!-- here we request our message and the event listener we added before, will respond and because it's JSON file we need to parse it -->
    var config = JSON.parse(ipcRenderer.sendSync('synchronous-message', ''))

    <!-- process our data however we want, in this example we print it on the browser console -->
    console.log(config)

     <!-- since we read our package.json file we can echo our electron app name -->
     console.log(config.name)
</script>

To see the console of the browser you need to open the dev tools, either from the default Electron menu or from your code. e.g. inside the createWindow() function

 win.webContents.openDevTools()

To send events to particular window you can use webContents.send(EVENT_NAME, ARGS) (see docs). webContents is a property of a window instance:

// main process
storeWindow.webContents.send('store-data', store);

To listen for this event being sent, you need a listener in a window process (renderer):

// renderer process
var ipcRenderer = require('electron').ipcRenderer;
ipcRenderer.on('store-data', function (event,store) {
    console.log(store);
});