JS拖拽：用prototype 实现

function dragElement(id) {

    this.dom = document.getElementById(id);

    this.isMouseDown = false;

    this.pos = null;

}

dragElement.prototype = {

     init: function() {

            var _this = this;
            this.moveDom.onmousedown = function(e) {

            _this.isMouseDown = true;

            _this.getPos(e);

        document.onmouseup = function() {

            _this.isMouseDown = false;

            document.onmousemove = null;

            document.onmouseup = null;

        };

        document.onmousemove = function(e) {

            _this.move(e);

         };

    },

    getPos: function(e) {

        e = e || window.event;

        this.pos = {

            x: e.clientX - this.dom.offsetLeft,

            y: e.clientY - this.dom.offsetTop

        };

    },

    move: function(e) {

        var _this = this;

        this.dom.style.opactiy

        if (this.isMouseDown) {

            this.dom.style.left = e.clientX - this.pos.x + "px";

            this.dom.style.top = e.clientY - this.pos.y  + "px";

        }

    }

}

var d = new dragElement("m");

d.init();