vue.cli3的main.js修改原来的代码为现在的作用是啥，能说下么，谢谢

[chenqichun]

main.js默认生成
new Vue({
router,
store,
render: h => h(App)
}).$mount('#app')

----- vue-cli3案例的里main.js ---------
const app = new Vue({
components: {
App,
},
render: h => h(App),
});

window.mountApp = () => {
app.$mount('#app');
};

if (process.env.NODE_ENV === 'production') {
if (window.STYLE_READY) {
window.mountApp();
}
} else {
window.mountApp();
}