js触屏滑动事件

var answerBar=document.getElementById("answerSelect");
var start_left;
var end_left;
answerBar.addEventListener("touchstart",function(event){
    event.preventDefault();
    if (event.targetTouches.length == 1) {
        var touch = event.targetTouches[0];
        start_left = touch.pageX;
    }
},false);
answerBar.addEventListener("touchmove",function(event){
    event.preventDefault();
    if (event.targetTouches.length == 1) {
        var touch = event.targetTouches[0];
        end_left = touch.pageX;
    }
},false);
answerBar.addEventListener("touchend",function(event){
    if(end_left-start_left>40){
        console.info("r");
    }
    else if(start_left-end_left>40){
       console.info("l");
    }
},false);