Electron Fiddle Gist

## index.html
<!DOCTYPE html>
<html>
  <head>
    <meta charset="UTF-8">
    <title>Context Bridge Example</title>
    <link rel="stylesheet" type="text/css" href="./styles.css">
  </head>
  <body>
    <main class="app">
      <button onclick="window.app.setFullscreen(true)">Enter fullscreen</button>
      <button onclick="window.app.setFullscreen(false)">Exit fullscreen</button>
    </main>
    <script>
      // You can also require other files to run in this process
      require('./renderer.js')
    </script>
  </body>
</html>

## main.js
// Modules to control application life and create native browser window
const {app, BrowserWindow, ipcMain} = require('electron')
const path = require('path')

function createWindow () {
  // Create the browser window.
  const mainWindow = new BrowserWindow({
    width: 800,
    height: 600,
    webPreferences: {
      allowRunningInsecureContent: false,
      contextIsolation: true,
      enableRemoteModule: false,
      nodeIntegration: false,
      sandbox: true,
      preload: path.join(app.getAppPath(), 'preload.js'),
    }
  })

  // and load the index.html of the app.
  mainWindow.loadFile('index.html')

  // Open the DevTools.
  // mainWindow.webContents.openDevTools()

  ipcMain.handle('setFullscreen', (event, flag) => {
    if (mainWindow) {
      mainWindow.setFullScreen(flag)
    }
  })
}

// This method will be called when Electron has finished
// initialization and is ready to create browser windows.
// Some APIs can only be used after this event occurs.
app.on('ready', createWindow)

// Quit when all windows are closed.
app.on('window-all-closed', function () {
  // On OS X it is common for applications and their menu bar
  // to stay active until the user quits explicitly with Cmd + Q
  if (process.platform !== 'darwin') {
    app.quit()
  }
})

app.on('activate', function () {
  // On OS X it's common to re-create a window in the app when the
  // dock icon is clicked and there are no other windows open.
  if (BrowserWindow.getAllWindows().length === 0) {
    createWindow()
  }
})

## preload.js
const { ipcRenderer, contextBridge } = require('electron')

contextBridge.exposeInMainWorld(
  'app',
  {
    setFullscreen: (flag) => ipcRenderer.invoke('setFullscreen', flag),
  }
)

## renderer.js
// Empty

## styles.css
.app {
  text-align: center;
  margin: 10rem;
}
	<!DOCTYPE html>
	<html>
	<head>
	<meta charset="UTF-8">
	<title>Context Bridge Example</title>
	<link rel="stylesheet" type="text/css" href="./styles.css">
	</head>
	<body>
	<main class="app">
	<button onclick="window.app.setFullscreen(true)">Enter fullscreen</button>
	<button onclick="window.app.setFullscreen(false)">Exit fullscreen</button>
	</main>
	<script>
	// You can also require other files to run in this process
	require('./renderer.js')
	</script>
	</body>
	</html>
	// Modules to control application life and create native browser window
	const {app, BrowserWindow, ipcMain} = require('electron')
	const path = require('path')

	function createWindow () {
	// Create the browser window.
	const mainWindow = new BrowserWindow({
	width: 800,
	height: 600,
	webPreferences: {
	allowRunningInsecureContent: false,
	contextIsolation: true,
	enableRemoteModule: false,
	nodeIntegration: false,
	sandbox: true,
	preload: path.join(app.getAppPath(), 'preload.js'),
	}
	})

	// and load the index.html of the app.
	mainWindow.loadFile('index.html')

	// Open the DevTools.
	// mainWindow.webContents.openDevTools()

	ipcMain.handle('setFullscreen', (event, flag) => {
	if (mainWindow) {
	mainWindow.setFullScreen(flag)
	}
	})
	}

	// This method will be called when Electron has finished
	// initialization and is ready to create browser windows.
	// Some APIs can only be used after this event occurs.
	app.on('ready', createWindow)

	// Quit when all windows are closed.
	app.on('window-all-closed', function () {
	// On OS X it is common for applications and their menu bar
	// to stay active until the user quits explicitly with Cmd + Q
	if (process.platform !== 'darwin') {
	app.quit()
	}
	})

	app.on('activate', function () {
	// On OS X it's common to re-create a window in the app when the
	// dock icon is clicked and there are no other windows open.
	if (BrowserWindow.getAllWindows().length === 0) {
	createWindow()
	}
	})
	const { ipcRenderer, contextBridge } = require('electron')

	contextBridge.exposeInMainWorld(
	'app',
	{
	setFullscreen: (flag) => ipcRenderer.invoke('setFullscreen', flag),
	}
	)