js求助

 var BounceBall = function() {
                var ctx, W, H;

//对象中的对象
                var init = function(id) {
                    var canvas = document.getElementById(id);
                    ctx = canvas.getContext('2d');
                    /*W = canvas.width || 600;
                    H = canvas.height || 400;*/
                    W = canvas.width ;
                    H = canvas.height;

                    this.circle(200, 300, 8);
                    this.rect(300, 300, 100, 50);
                }

//给对象init添加属性：
                init.prototype = {
                    clear: function() {
                        ctx.clearRect(0, 0, W, H);
                    },
                    circle: function(x, y, r) {
                        ctx.beginPath();
                        ctx.arc(x, y, r, 0, Math.PI * 2, true);
                        ctx.closePath();
                        ctx.fill();
                    },
                    rect: function(x, y, w, h) {
                        ctx.beginPath();
                        ctx.rect(x, y, w, h);
                        ctx.closePath();
                        ctx.fill();
                    }
                }

                return init;
            } ();
我想知道最后一行的“｝”后面还加了个（）是什么意思？ 高手指教下，为什么这样用？谢谢，坐等