javascript如何获取鼠标的坐标

function Arrow(canvas){

	this.canvas = canvas;

	this.ctx = canvas.getContext("2d");

	this.x = 250;

	this.y = 200;

	this.rotation = 0;

		

	Arrow.prototype.drawArrow = function(){					

		this.ctx.beginPath();

		this.ctx.moveTo(-50,-25);

		this.ctx.lineTo(0,-25);

		this.ctx.lineTo(0,-50);

		this.ctx.lineTo(50,0);

		this.ctx.lineTo(0,50);

		this.ctx.lineTo(0,25);

		this.ctx.lineTo(-50,25);

		this.ctx.lineTo(-50,-25);

		this.ctx.stroke();

	}

	

}



function RotateToMouse(canvas){

	this.canvas = canvas;

	this.ctx = canvas.getContext("2d");

	this.arrow;

	

	this.mouseX;

	this.mouseY;



	RotateToMouse.prototype.init = function(){

		this.arrow = new Arrow(canvas);

		this.ctx.translate(this.arrow.x,this.arrow.y);

				

		var self = this;		

		this.intervalID = setInterval(function(){self.enterFrame();},1000/2);

		

		canvas.addEventListener("mousemove",function(e){

		return self.getMousePosition(e);

		});

	}

	

	RotateToMouse.prototype.getMousePosition = function(event){

	  event = event || window.event;

		if(event.pageX || event.pageY){

			this.mouseX = event.pageX;

			this.mouseY = event.pageY;

		}else{

			this.mouseX = event.clientX + document.body.scrollLeft - document.body.clientLeft;

			this.mouseY = event.clientY + document.body.scrollTop - document.body.clientTop;	

		}

	}



	RotateToMouse.prototype.enterFrame = function(){

		this.arrow.rotation = Math.atan2(this.mouseY - arrow.y, this.mouseX - arrow.x);

		this.ctx.rotate(this.arrow.rotation * Math.PI/180);

		this.arrow.drawArrow();

	}

	this.init();

}