Object.defineProperty()实现双向数据绑定

<div id="app">
    <input type="text" name="txt" id="txt" value="" />
    <p id="show_txt"></p>
</div>
<script type="text/javascript">
    var obj = {};
    Object.defineProperty(obj, 'value', {
        get: function() {
        　　return obj;
        },
        set: function(newValue) {
        　　document.getElementById('txt').value = newValue;
           document.getElementById('show_txt').innerHTML = newValue;
        }
    });
    document.addEventListener('keyup', function(e) {
        obj.value = e.target.value;
    });
</script>