console 命令行工具 X clear
console
<!DOCTYPE html>
<html>
<head>
<meta charset="gb2312" />
<title></title>
<style type="text/css">
.box {
    background: #f00;
    width: 0px;
    height: 0px;
    position: absolute;
    opacity: 0.5;
    cursor: move;
}
</style>
<script type="text/javascript">
window.onload = function(e) {
    e = e || window.event;
    // startX, startY 为鼠标点击时初始坐标
    // diffX, diffY 为鼠标初始坐标与 box 左上角坐标之差，用于拖动
    var startX, startY, diffX, diffY;
    // 是否拖动，初始为 false
    var dragging = false;
          
    // 鼠标按下
    document.onmousedown = function(e) {
        startX = e.pageX;
        startY = e.pageY;
          
        // 如果鼠标在 box 上被按下
        if(e.target.className.match(/box/)) {
            // 允许拖动
            dragging = true;
          
            // 设置当前 box 的 id 为 moving_box
            if(document.getElementById("moving_box") !== null) {
                document.getElementById("moving_box").removeAttribute("id");
            }
            e.target.id = "moving_box";
          
            // 计算坐标差值
            diffX = startX - e.target.offsetLeft;
            diffY = startY - e.target.offsetTop;
        }
        else {
            // 在页面创建 box
            var active_box = document.createElement("div");
            active_box.id = "active_box";
            active_box.className = "box";
            active_box.style.top = startY + 'px';
            active_box.style.left = startX + 'px';
            document.body.appendChild(active_box);
            active_box = null;
        }
    };
           
    // 鼠标移动
    document.onmousemove = function(e) {
        // 更新 box 尺寸
        if(document.getElementById("active_box") !== null) {
            var ab = document.getElementById("active_box");
            ab.style.width = e.pageX - startX + 'px';
            ab.style.height = e.pageY - startY + 'px';
        }
           
        // 移动，更新 box 坐标
        if(document.getElementById("moving_box") !== null && dragging) {
            var mb = document.getElementById("moving_box");
            mb.style.top = e.pageY - diffY + 'px';
            mb.style.left = e.pageX - diffX + 'px';
        }
    };
           
    // 鼠标抬起
    document.onmouseup = function(e) {
        // 禁止拖动
        dragging = false;
        if(document.getElementById("active_box") !== null) {
            var ab = document.getElementById("active_box");
            ab.removeAttribute("id");
            // 如果长宽均小于 3px，移除 box
            if(ab.offsetWidth < 3 || ab.offsetHeight < 3) {
                document.body.removeChild(ab);
            }
        }
    };
};
</script>
</head>
<body>
<p>点击鼠标左键并拖动绘制矩形</p>
</body>
</html>