Html5 canvas 画带箭头的线

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

        function Line(x1,y1,x2,y2){
            this.x1=x1;
            this.y1=y1;
            this.x2=x2;
            this.y2=y2;
        }
        Line.prototype.drawWithArrowheads=function(ctx){

            // arbitrary styling
            ctx.strokeStyle="blue";
            ctx.fillStyle="blue";
            ctx.lineWidth=1;
            
            // draw the line
            ctx.beginPath();
            ctx.moveTo(this.x1,this.y1);
            ctx.lineTo(this.x2,this.y2);
            ctx.stroke();

            // draw the starting arrowhead
            var startRadians=Math.atan((this.y2-this.y1)/(this.x2-this.x1));
            startRadians+=((this.x2>this.x1)?-90:90)*Math.PI/180;
            this.drawArrowhead(ctx,this.x1,this.y1,startRadians);
            // draw the ending arrowhead
            var endRadians=Math.atan((this.y2-this.y1)/(this.x2-this.x1));
            endRadians+=((this.x2>this.x1)?90:-90)*Math.PI/180;
            this.drawArrowhead(ctx,this.x2,this.y2,endRadians);

        }
        Line.prototype.drawArrowhead=function(ctx,x,y,radians){
            ctx.save();
            ctx.beginPath();
            ctx.translate(x,y);
            ctx.rotate(radians);
            ctx.moveTo(0,0);
            ctx.lineTo(5,20);
            ctx.lineTo(-5,20);
            ctx.closePath();
            ctx.restore();
            ctx.fill();
        }

        // create a new line object
        var line=new Line(50,50,250,275);
        // draw the line
        line.drawWithArrowheads(context);