<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<meta http-equiv="X-UA-Compatible" content="ie=edge">
<title>Document</title>
</head>
<body>
<div id="app">
<input type="text" id="a">
<span id="b"></span>
</div>
</body>
<script>
var obj = {}; //定义一个空对象
var val = ‘‘; //赋予初始值
Object.defineProperty(obj, ‘val‘, { //定义要修改对象的属性
get: function() {
return val;
},
set: function(newVal) {
val = newVal; //定义val等于修改后的内容
document.getElementById(‘a‘).value = val; //让文本框的内容等于val
document.getElementById(‘b‘).innerHTML = val; //让span的内容等于val
}
});
document.addEventListener(‘keyup‘, function(e) { //当在文本框输入内容时让对象里你定义的val等于文本框的值
obj.val = e.target.value;
})
</script>
</html>
原文：https://www.cnblogs.com/kawayi/p/14435409.html