vue bidirectional data binding

vue 双向数据绑定模拟（查看示例）
<!DOCTYPE html>
<html>
    <head>
        <meta charset="utf-8">
        <title>test html load</title>
    </head>
    <body>
        var obj = {value: <input id="xss" type="text" value="1">};
        <div id="content" class="">
            hello world
        </div>

        <script type="text/javascript">
            function Observer(obj, key, value){
                var dep = new Dep();
                if(Object.prototype.toString.call(value).slice(8, -1) == 'Object'){
                    Object.keys(value).forEach((key) => {
                        new Observer(obj, key, value[key]);
                    });
                }

                Object.defineProperty(obj, key, {
                    enumerable: true,
                    configurable: true,
                    get: () => {
                        if(Dep.target){
                            dep.addSub(Dep.target);
                        }
                        return value;
                    },
                    set: (newValue) => {
                        value = newValue;
                        dep.notify();
                    }
                })
            }

            function Dep(){
                this.subs = [];

                this.addSub = function(watcher){
                    var index = this.subs.indexOf(watcher);
                    if(index == -1){
                        this.subs.push(watcher);
                    }
                }

                this.removeSub = function(watcher){
                    var index = this.subs.indexOf(watcher);
                    if(index != -1){
                        this.subs.splice(index, 1);
                    }
                }

                this.notify = function(){
                    this.subs.forEach((watcher) => {
                        watcher.update();
                    });
                }
            }

            function Watcher(fn){
                this.update = function(){
                    Dep.target = this;
                    fn();
                    Dep.target = null;
                }
                this.update();
            }

            var obj = {value: 1};
            Object.keys(obj).forEach((key) => {
                new Observer(obj, key, obj[key]);
            })

            new Watcher(() => {
                document.getElementById('content').innerHTML = obj.value;
            });

            document.getElementById('xss').addEventListener('input', () => {
                obj.value = document.getElementById('xss').value;
            }, false);

        </script>

    </body>
</html>