简单封装一个拖拽插件，以面向对象方式实现，当然还有其他很多写法，这里简单做个笔记记录一下！

Css

.box {
    position: absolute;
    width: 100px;
    height: 100px;
    background: #f00;
}

Html

<div class="box"></div>

Javascript

(function(doc){
    /**
     * [Drag 拖拽]
     * @param {[type]} el [要拖拽的元素]
     * 使用方法：
     * new Drag('.box').init();
     */
    function Drag(el){
        this.el = el;
        this.elem = doc.querySelector(this.el);
        this.disX = 0;
        this.disY = 0;
        this.isMove = false; // 判断mouseup的开关，如为true才触发mousemove事件动作
    }

    Drag.prototype = {
        constructor: Drag,   // prototype{} 覆盖的方式，要手动将constructor引用指回Drag父类
        init: function() {
            var _that = this;
            _that.elem.addEventListener('mousedown', function(ev){
                _that.down(ev, _that);

                // mousemove和mouseup放这里而不放外面的原因是当鼠标按下要拖动的元素才触发拖拽动作
                doc.addEventListener('mousemove', function(ev){
                    _that.move(ev, _that);
                });
                doc.addEventListener('mouseup', function(){
                    _that.up(_that);
                });
            });
        },
        // 鼠标按下动作
        down: function(ev, _that) {
            var ev = ev || event;
            _that.disX = ev.pageX - _that.elem.offsetLeft;
            _that.disY = ev.pageY - _that.elem.offsetTop;
            _that.isMove = true;    // 开启开关
        },
        // 鼠标拖拽动作
        move: function(ev, _that) {
            var ev = ev || event;
            if(_that.isMove) { // 判断开关是否开启
                _that.elem.style.left = ev.pageX - _that.disX + 'px';
                _that.elem.style.top = ev.pageY - _that.disY + 'px';
            }
        },
        // 鼠标提起动作
        up: function(_that) {
            _that.isMove = false;   // 关闭开关
            console.log('up...');
        }
    }

    //暴露全局对象
    window.Drag = Drag;

})(document);

调用：

var _darg = new Drag('.box').init();