html 绘制矩形轨迹，选中区域

<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8" />
<title></title>
<style type="text/css">
.box {
	border:1px solid black;
    width: 0px;
    height: 0px;
    position: absolute;
    
	z-index:-1;
}
div{
opacity:1;
border:1px solid black;
width:800px;
height:500px;

}
</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
    var startX, startY;  
    // 鼠标按下
    document.onmousedown = function(e) {
        startX = e.pageX;
        startY = e.pageY;
		// 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");
			if(e.pageX>startX){
				if(e.pageY>startY){
					active_box.style.top = startY + 'px';
					active_box.style.left = startX + 'px';
					ab.style.width = Math.abs(e.pageX - startX) + 'px';
					ab.style.height = Math.abs(e.pageY - startY) + 'px';
				}else if(e.pageY==startY){
					active_box.style.top = startY + 'px';
					active_box.style.left = startX + 'px';
					ab.style.width = Math.abs(e.pageX - startX) + 'px';
					ab.style.height = Math.abs(e.pageY - startY) + 'px';
				}else{
					active_box.style.top = e.pageY + 'px';
					active_box.style.left = startX + 'px';
					ab.style.width = Math.abs(e.pageX - startX) + 'px';
					ab.style.height = Math.abs(e.pageY - startY) + 'px';
				}
			}else if(e.pageX==startX){
				if(e.pageY>startY){
					active_box.style.top = startY + 'px';
					active_box.style.left = startX + 'px';
					ab.style.width = Math.abs(e.pageX - startX) + 'px';
					ab.style.height = Math.abs(e.pageY - startY) + 'px';
				}else if(e.pageY==startY){
					active_box.style.top = startY + 'px';
					active_box.style.left = startX + 'px';
					ab.style.width = Math.abs(e.pageX - startX) + 'px';
					ab.style.height = Math.abs(e.pageY - startY) + 'px';
				}else{
					active_box.style.top = e.pageY + 'px';
					active_box.style.left = startX + 'px';
					ab.style.width = Math.abs(e.pageX - startX) + 'px';
					ab.style.height = Math.abs(e.pageY - startY) + 'px';
				}
			}else{
				if(e.pageY>startY){
					active_box.style.top = startY + 'px';
					active_box.style.left = e.pageX + 'px';
					ab.style.width = Math.abs(e.pageX - startX) + 'px';
					ab.style.height = Math.abs(e.pageY - startY) + 'px';
				}else if(e.pageY==startY){
					active_box.style.top = e.pageY + 'px';
					active_box.style.left = e.pageX + 'px';
					ab.style.width = Math.abs(e.pageX - startX) + 'px';
					ab.style.height = Math.abs(e.pageY - startY) + 'px';
				}else{
					active_box.style.top = e.pageY + 'px';
					active_box.style.left = e.pageX + 'px';
					ab.style.width = Math.abs(e.pageX - startX) + 'px';
					ab.style.height = Math.abs(e.pageY - startY) + 'px';
				}	
			}
            
        }
    };     
    // 鼠标抬起
    document.onmouseup = function(e) {
        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
            document.body.removeChild(ab);
        }
    };
};
</script>
</head>
<body>
<div></div>
<p>点击鼠标左键并拖动绘制矩形</p>
</body>
</html>
html 绘制矩形轨迹，选中区域

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