条件渲染
image.png
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<meta http-equiv="X-UA-Compatible" content="IE=edge">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<title>计数器</title>
</head>
<body>
<div id="root"></div>
<script src="../lib/react.js"></script>
<script src="../lib/react-dom.js"></script>
<script src="../lib/babel.js"></script>
<script type="text/babel">
// 1.定义App根组件
class App extends React.Component {
constructor() {
super()
this.state = {
message: "Hello World",
isReady: false,
friend: undefined
}
}
render() {
const { isReady, friend } = this.state
// 1.条件判断方式一: 使用if进行条件判断
let showElement = null
if (isReady) {
showElement = <h2>准备开始比赛吧</h2>
} else {
showElement = <h1>请提前做好准备!</h1>
}
return (
<div>
{/* 1.方式一: 根据条件给变量赋值不同的内容 */}
<div>{showElement}</div>
{/* 2.方式二: 三元运算符 */}
<div>{ isReady ? <button>开始战斗!</button>: <h3>赶紧准备</h3> }</div>
{/* 3.方式三: &&逻辑与运算 */}
{/* 场景: 当某一个值, 有可能为undefined时, 使用&&进行条件判断 */}
<div>{ friend && <div>{friend.name + " " + friend.desc}</div> }</div>
</div>
)
}
}
// 2.创建root并且渲染App组件
const root = ReactDOM.createRoot(document.querySelector("#root"))
root.render(<App/>)
</script>
</body>
</html>
还有一个可选链。
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<meta http-equiv="X-UA-Compatible" content="IE=edge">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<title>计数器</title>
</head>
<body>
<div id="root"></div>
<script src="../lib/react.js"></script>
<script src="../lib/react-dom.js"></script>
<script src="../lib/babel.js"></script>
<script type="text/babel">
// 1.定义App根组件
class App extends React.Component {
constructor() {
super()
this.state = {
message: "Hello World",
isShow: true
}
}
changeShow() {
this.setState({ isShow: !this.state.isShow })
}
render() {
const { message, isShow } = this.state
// let showElement = null
// if (isShow) {
// showElement = <h2>{message}</h2>
// }
return (
<div>
<button onClick={() => this.changeShow()}>切换</button>
{ isShow && <h2>{message}</h2> }
{/* v-show的效果 */}
<h2 style={{display: isShow ? 'block': 'none'}}>哈哈哈哈</h2>
</div>
)
}
}
// 2.创建root并且渲染App组件
new App()
const root = ReactDOM.createRoot(document.querySelector("#root"))
root.render(<App/>)
</script>
</body>
</html>
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