双向绑定的极简实现

<input id="a" type="text" />
<span id="b"></span>

var obj = {}；

Object.defineProperty(obj,'txt',{
    set:function(val){
        document.getElementById('a').value = val;
        document.getElementById('b').innerHTML = val;
    }
})

document.getElementById('a').addEventListener('input',function(e){
    obj.txt = e.target.value;
});