<canvas id="canva" width="500px" height="300px"></canvas>

<style type="text/css">
        *{
            margin: 0;
            padding: 0;
        }
        #canva{
                background: indianred;
        }
</style> 

<script type="text/javascript">
    window.onload=function(){
        var canva=document.getElementById('canva');
        var ctx  =canva.getContext('2d');
        var h=canva.height;     
        var w=canva.width;
        var rext={                          //定义验证块的属性
            x:Math.random()*(w-50),
            y:Math.random()*(h-50),
        }
        var hk={                           //滑块属性
            x:'0',
            y:''
        }
        Rect();                            //绘制验证块
        Hk(0,rext.y);                      //绘制滑块

        canva.addEventListener('click',function(){
                var ev=ev||event;
                var x=ev.clientX;
                var y=ev.clientY;
                if(x>=0 && x<=50 && y>=rext.y && y<=rext.y+50  ){//canvas内部监听
                   canva.addEventListener('mousemove',function(ev){
                        ev=ev||event;
                        ctx.clearRect(hk.x,hk.y,50,50);   //清除滑块            
                        hk.x=ev.clientX;    
                        Hk(hk.x,rext.y);
                        //绘制滑块
                        var hk_x=ev.clientX;
                        var yz_x=rext.x;
                        (function(x,y){
                            if(x>y && x<y+50){
                                console.log("验证成功");
                            }
                        })(hk_x,yz_x)      //判断验证的匿名函数
                   })              

                }
            })   //点击事件的处理

        function Rect(){
            ctx.fillStyle='whitesmoke';
            ctx.fillRect(rext.x,rext.y,50,50);
        }
        function Hk(x,y){
            hk.x=x;
            hk.y=y;
            ctx.fillRect(hk.x,hk.y,50,50);  
        }   
    }
</script>