React的渲染element（做最少的DOM操作）
Rendering Elements
An element describes what you want to see on the screen.
原则：尽可能少的操作DOM
如下面react官网的例子就是很好的说明：
function tick() { const element = ( <div> <h1>Hello, world!</h1> <h2>It is {new Date().toLocaleTimeString()}.</h2> </div> ); ReactDOM.render(element, document.getElementById('root'));} setInterval(tick, 1000);
ven though we create an element describing the whole UI tree on every tick, only the text node whose contents have changed gets updated by React DOM.
