build: use typescript for internal Electron JS code (#16441)

default_app/default_app.ts

@@ -0,0 +1,38 @@
+import { app, BrowserWindow, BrowserWindowConstructorOptions } from 'electron'
+import * as path from 'path'
+
+let mainWindow: BrowserWindow | null = null
+
+// Quit when all windows are closed.
+app.on('window-all-closed', () => {
+  app.quit()
+})
+
+export const load = async (appUrl: string) => {
+  await app.whenReady()
+
+  const options: BrowserWindowConstructorOptions = {
+    width: 900,
+    height: 600,
+    autoHideMenuBar: true,
+    backgroundColor: '#FFFFFF',
+    webPreferences: {
+      contextIsolation: true,
+      preload: path.resolve(__dirname, 'renderer.js'),
+      webviewTag: false
+    },
+    useContentSize: true,
+    show: false
+  }
+
+  if (process.platform === 'linux') {
+    options.icon = path.join(__dirname, 'icon.png')
+  }
+
+  mainWindow = new BrowserWindow(options)
+
+  mainWindow.on('ready-to-show', () => mainWindow!.show())
+
+  mainWindow.loadURL(appUrl)
+  mainWindow.focus()
+}