js ontouchstart 事件失效，必须要刷新一次页面才有效

Question

var _canvase=document.getElementsByTagName('canvas')[0]
_canvase.ontouchstart = function(e) {        
    _canvase.ontouchstart = function(e) {    
    console.log('55559')
};

Answer 1

没有什么问题啊，不过上面写两遍具体是为何？

    <div style="border:1px solid #ccc;">
        <canvas style="width:100px;height:100px;"></canvas>
    </div>

    <script>
        var ctx = document.getElementsByTagName("canvas")[0];
        ctx.ontouchstart = function(e){
            console.log('33333333');
        }
    </script>