Html5 Canvas通过鼠标移动画圆，一下为Javascript画圆部分函数 有什么问题？

var canvas=document.getElementById("demo");
var cxt=canvas.getContext("2d");

function circle()
{
canvas.onmousedown=function(evt)
{
cxt.lineWidth=1;
evt=window.event||evt;

var startX=evt.pageX-this.offsetLeft;
var startY=evt.pageY-this.offsetTop;
 }

canvas.onmousemove=function(evt)
{
var x=evt.pageX-this.offsetLeft;
var y=evt.pageY-this.offsetTop;

cxt.strokeStyle="rgb(180,0,0)"; 

cxt.beginPath();
var radii=Math.sqrt((startX-x)*(startX-x)+(startY-y)*(startY-y));   

cxt.arc(startX,startY,radii,0,Math.PI*2,false);

cxt.stroke();
cxt.closePath();
}

}