• 被移动元素必须为绝对定位
  position:absolute;
• Dom

 <div class = "move-container" >
      <div class = "move" style="position:absolute; width:100px; height:100px; background:gray">
      </div>
  </div> 

• Javasrtipt

var moveElem = document.querySelector('.move'); //待拖拽元素      

var dragging; //是否激活拖拽状态
var tLeft, tTop; //鼠标按下时相对于选中元素的位移

//监听鼠标按下事件
document.addEventListener('mousedown', function(e) {
    if (e.target == moveElem) {
        
        dragging = true; //激活拖拽状态
       var moveElemRect = moveElem.getBoundingClientRect();
        tLeft = e.clientX - moveElemRect.left; //鼠标按下时和选中元素的坐标偏移:x坐标
        tTop = e.clientY - moveElemRect.top; //鼠标按下时和选中元素的坐标偏移:y坐标
    }
});

//监听鼠标放开事件
document.addEventListener('mouseup', function(e) {
    dragging = false;
});

//监听鼠标移动事件
document.addEventListener('mousemove', function(e) {
    if (dragging) {
        var moveX = e.clientX - tLeft, 
              moveY = e.clientY - tTop;

        moveElem.style.left = moveX + 'px';
        moveElem.style.top = moveY + 'px';
      
    }
});

下面是一个封装好的函数

• 封装拖拽函数

var funDrag = function(element, callback) {
    callback = callback || function() {};
    var params = {
        left: 0,
        top: 0,
        currentX: 0,
        currentY: 0,
        flag: false
    };
    //获取相关CSS属性
    var getCss = function(o,key){
        return o.currentStyle? o.currentStyle[key] : document.defaultView.getComputedStyle(o,false)[key];     
    };
    
    //拖拽的实现
    if(getCss(element, "left") !== "auto"){
        params.left = getCss(element, "left");
    }
    if(getCss(element, "top") !== "auto"){
        params.top = getCss(element, "top");
    }
    //o是移动对象
    element.onmousedown = function(event){
        params.flag = true;
        event = event || window.event;
        params.currentX = event.clientX;
        params.currentY = event.clientY;
    };
    document.onmouseup = function(){
        params.flag = false;    
        if(getCss(element, "left") !== "auto"){
            params.left = getCss(element, "left");
        }
        if(getCss(element, "top") !== "auto"){
            params.top = getCss(element, "top");
        }
        callback();
    };
    document.onmousemove = function(event){
        event = event || window.event;
        if(params.flag){
            var nowX = event.clientX, nowY = event.clientY;
            var disX = nowX - params.currentX, disY = nowY - params.currentY;
            element.style.left = parseInt(params.left) + disX + "px";
            element.style.top = parseInt(params.top) + disY + "px";
        }
    }    
};