原生JavaScript实现点击掉落动画特效

<!DOCTYPE HTML>
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
<title>无标题文档</title>

<script src="jquery.js"></script>
<script>
window.onload = function () {
	var str = '';
	var len = 20;
	var aDiv = document.getElementsByTagName('div');
	var timer = null;
	var num = 0;
	
	for ( var i=0; i<len; i++ ) {
		str += '<div style="width:50px; height:50px; background:red; position:absolute; top:0px; left:'+ i*60 +'px;"></div>';
	}
	
	document.body.innerHTML = str;
	
	document.onclick = function () {
		clearInterval( timer );
		timer = setInterval( function (){
			DM( aDiv[num], 'top', 23, 500 );
			num ++;
			if ( num === len ) {
				clearInterval( timer );
			}
		}, 100 );
	};
};
</script>

</head>

<body>
</body>
</html>

function DM( obj, attr, dir, target, endFn ) {
	
	dir = parseInt(getStyle( obj, attr )) < target ? dir : -dir;
	
	clearInterval( obj.timer );
	
	obj.timer = setInterval(function () {
		
		var speed = parseInt(getStyle( obj, attr )) + dir;			// 步长
		
		if ( speed > target && dir > 0 || speed < target && dir < 0 ) {
			speed = target;
		}
		
		obj.style[attr] = speed + 'px';
		
		if ( speed == target ) {
			clearInterval( obj.timer );
			
			/*
			if ( endFn ) {
				endFn();
			}
			*/
			endFn && endFn();
			
		}
		
	}, 30);
}