1,监听keyup事件，触发则给代理对象的text属性赋值（e.target.value）
2,数据劫持函数：Object.defineProperty(代理对象，“属性”，{get(),set(va)})
set(val)=>input.value=val,span.innerHTML=val
<!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>Document</title>
</head>
<body>
input:<input type="text" name="fname" /><br/>
数据：<span></span>
<script type="text/javascript">
let obj={}
let input = document.getElementsByTagName('input')[0]
let span = document.getElementsByTagName('span')[0]
Object.defineProperty(obj,'b',{
get(){
console.log('获取数据')
},
set(val){
console.log('修改数据')
input.value=val
span.innerHTML=val
}
})
window.addEventListener('keyup',function(e){
obj.b=e.target.value
})
</script>
</body>
</html>
