基于继承的拖拽

    window.onload = function() {
        new Drag("div1");
        new LimitDrag("div2");
    };

    //父类
    function Drag(id) {
        var _this = this;

        this.disX = 0;
        this.disY = 0;
        this.oDiv = document.getElementById(id);

        this.oDiv.onmousedown = function() {
            _this.fnDown();
        };
    }

    Drag.prototype.fnDown = function(ev) {
        var _this = this;

        var oEvent = ev || event;
        this.disX = oEvent.clientX - this.oDiv.offsetLeft;
        this.disY = oEvent.clientY - this.oDiv.offsetTop;

        document.onmousemove = function() {
            _this.fnMove();
        };

        document.onmouseup = function() {
            _this.fnUp();
        };

        //兼容FF
        return false;
    };

    Drag.prototype.fnMove = function(ev) {
        var oEvent = ev || event;

        var l = oEvent.clientX - this.disX;
        var t = oEvent.clientY - this.disY;

        this.oDiv.style.left = l + "px";
        this.oDiv.style.top = t + "px";
    };

    Drag.prototype.fnUp = function(ev) {
        document.onmousemove = null;
        document.onmouseup = null;
    };

    //子类
    function LimitDrag(id) {
        Drag.call(this, id);
    }

    //继承父类的属性和方法
    for (var i in Drag.prototype) {
        LimitDrag.prototype[i] = Drag.prototype[i];
    }

    //重写父类的方法
    LimitDrag.prototype.fnMove = function(ev) {
        var oEvent = ev || event;

        var l = oEvent.clientX - this.disX;
        var t = oEvent.clientY - this.disY;

        if (l < 0) {
            l = 0;
        } else if (l > document.documentElement.clientWidth - this.oDiv.offsetWidth) {
            l = document.documentElement.clientWidth - this.oDiv.offsetWidth;
        }

        if (t < 0) {
            t = 0;
        } else if (t > document.documentElement.clientHeight - this.oDiv.offsetHeight) {
            t = document.documentElement.clientHeight - this.oDiv.offsetHeight;
        }

        this.oDiv.style.left = l + "px";
        this.oDiv.style.top = t + "px";
    };