请问下面代码中的this.handle.mousedown和return部分是啥意思

Question

define(['jquery'],function($){
function Rotate(){

    this.cfg = {
        rotateBox : null,
        handle : null
    };
}

Rotate.prototype = {
    convertAngle : function(a){
        return a/Math.PI*180
    },

    onMove : function(){
        //console.log(this);
        this.angle2 = this.convertAngle(Math.atan2(this.oy - this.my, this.mx - this.ox));      //计算鼠标和元素中心连线 与 水平线夹角
        this.angle = this.angle1 - this.angle2 + this.oldDeg;                                        //计算旋转角度
        this.rotateBox.css({
            transform : "rotate(" + this.angle + "deg)"
        });
    },

    init : function(cfg){
        //console.log(this);
        $.extend(this.cfg,cfg,{});
        this.rotateBox = this.cfg.rotateBox;
        if(this.cfg.handle){
            this.handle = this.cfg.handle;
        }else{

        }
        this.handle.mousedown(function(e){
            var e = window.event || e;
            this.ox = this.rotateBox.width()/2+this.rotateBox.offset().left;    //计算元素中心坐标
            this.oy = this.rotateBox.height()/2+this.rotateBox.offset().top;
            this.x1 = this.handle.offset().left+this.handle.width()/2;          //计算手柄坐标
            this.y1 = this.handle.offset().top+this.handle.height()/2;
            this.angle1 = this.convertAngle(Math.atan2(this.oy-this.y1,this.x1-this.ox));//计算元素中心和手柄连线 与 水平线夹角
            if(this.rotateBox[0].style.transform){
                this.oldDeg = Number(this.rotateBox[0].style.transform.slice(7,-4)) % 360;      //获取元素初始角度
            }else{
                this.oldDeg = 0;
            }

            this.moving = setInterval(this.onMove.bind(this),40);
        }.bind(this));

        document.addEventListener('mousemove',function(e){
            var e = window.event || e;
            this.mx = e.pageX;                  //获取移动时鼠标位置坐标
            this.my = e.pageY;
        }.bind(this),false);

        document.addEventListener('mouseup',function() {
            clearInterval(this.moving);
        }.bind(this),false);
    }
};

return {
    Rotate : Rotate
}

});