canvas画线

<canvas id="c1" width="600" height="400"></canvas>
 var c1=document.getElementById('c1');
 var ctx=c1.getContext('2d');
 c1.onmousedown = unciton(ev){
    var _this = this;
    var x = ev.pageX - this.offsetLeft;
    var y = ev.pageY - this.offsetTop;
    ctx.moveTo(x,y);
    document.onmousemove = function(ev){
            var x = ev.pageX - this.offsetLeft;
            var y = ev.pageY - this.offsetTop;
            ctx.lineTo(x,y);
            ctx.stroke();
    };
    document.onmouseup = function(){
        document.onmousemove = null;
        document.onmouseup = null;
    };
 };