It's main file of electron filesystem.

index.js

const { app, BrowserWindow } = require('electron');

function createWindow (){
  // Create the browser window.
  const win = new BrowserWindow({
    width: 800,
    height: 600,
    webPreferences: {
      nodeIntegration: true
    }
  });

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

  // Open the DevTools.
  win.webContents.openDevTools()
}

app.whenReady().then(createWindow);

// Quit when all windows are closed.
app.on('window-all-closed', () => {
  if (process.platform !== 'darwin') {
    app.quit();
  }
})

app.on('activate', () => {
  if (BrowserWindow.getAllWindows().length === 0) {
    createWindow();
  }
});