js中用事件委托实现单选下拉列表

<!DOCTYPE html>
<html>
	<head>
		<meta charset="utf-8">
		<title></title>
		<style type="text/css">
			* {
				margin: 0;
				padding: 0;
			}

			.wrap {
				width: 200px;
				margin: 20px auto;
			}

			#box {
				height: 30px;
				line-height: 30px;
				border: 1px solid #0000FF;
			}

			ul {
				list-style: none;
				border: 1px solid #000000;
				display: none;
			}

			ul>li:hover {
				background-color: #0000FF;
				color: #fff;
				cursor: pointer;
			}

			.wrap ul .active {
				background-color: orange;
				color: #fff;
			}
		</style>
		<script type="text/javascript">
			window.onload = function() {
				//获取
				var box = document.getElementById('box');
				var ul = document.querySelector('.wrap ul');
				var liS = document.querySelectorAll('.wrap ul>li');
				// console.log(box,ul,liS)
				//显示隐藏
				var t = true;
				box.onclick = function(e) {
					var ev = window.event || e;
					//阻止事件冒泡
					ev.stopPropagation ? ev.stopPropagation() : ev.cancelBubble = true;
					if (t) {
						ul.style.display = 'block';
						t = false;
					} else {
						ul.style.display = 'none';
						t = true;
					}
					// t = !t;
				}
				//选中
				ul.onclick = function(e) {
					var ev = window.event || e;
					//实际触发的元素
					console.log(ev.target);
					if (ev.target.nodeName == 'LI') {
						for (var i = 0; i < liS.length; i++) {
							liS[i].className = '';
						}
						ev.target.className = 'active';
						ul.style.display = 'none';
						box.innerText = ev.target.innerText;
						t = true;
					}
				}
				//取消
				document.onclick = function() {
					ul.style.display = 'none';
					t = true;
				}
			}
		</script>
	</head>
	<body>
		<div class="wrap">
			<div id="box"></div>
			<ul>
				<li>html</li>
				<li>css</li>
				<li>javascript</li>
				<li>css3</li>
				<li>html5</li>
			</ul>
		</div>
	</body>
</html>

普通方法实现

<!DOCTYPE html>
<html>
	<head>
		<meta charset="utf-8">
		<title></title>
		<style type="text/css">
			* {
				margin: 0;
				padding: 0;
			}

			.wrap {
				width: 220px;
				margin: 20px auto;
			}

			ul {
				list-style: none;
				border: 1px solid #000;
				display: none;
			}

			li:hover {
				cursor: pointer;
				background-color: #0000FF;
				color: white;
			}

			#box {
				border: 1px solid blue;
				height: 30px;
				line-height: 30px;
			}

			.wrap ul .active {
				background-color: #0000FF;
				color: white;
			}

			.wrap ul .bgactive {
				background-color: coral;
				color: white;
			}
		</style>
		<script type="text/javascript">
			window.onload = function() {
				var box = document.getElementById('box');
				var ul = document.querySelector('.wrap ul');
				var liS = document.querySelectorAll('.wrap ul li');
				// console.log(box,ul,liS);
				//设置下拉选中值
				var index = null;
				// box.innerText = liS[index].innerText;
				//设置下拉框状态
				var t = true;
				//box的点击
				box.onclick = function(e) {
					var ev = window.event || e;
					//阻止冒泡	兼容
					ev.stopPropagation ? ev.stopPropagation() : ev.cancelBubble = true;
					// setActive();
					//判断下拉框状态
					if (t) {
						ul.style.display = 'block';
						setActive();
						t = false;
					} else {
						ul.style.display = 'none';
						t = true;
					}
					// t = !t;
				}

				//li鼠标悬浮效果
				for (var i = 0; i < liS.length; i++) {
					//自定义下标
					liS[i].index = i;
					// liS[i].onmouseover = function() {
					// 	//清空所有的class
					// 	for (var k = 0; k < liS.length; k++) {
					// 		liS[k].className = '';
					// 	}
					// 	//给当前悬浮li加active
					// 	this.className =' active';
					// }
					//点击li添加
					liS[i].onclick = function() {
						//设置内容
						box.innerText = this.innerText;
						//修改显示项
						index = this.index;
					}

				}

				//根据下拉选中值显示
				function setActive() {
					if (index !== null) {
						for (var i = 0; i < liS.length; i++) {
							liS[i].className = '';
						}
						//设置当前值
						liS[index].className = 'bgactive';
					}
				}

				//点击空白处取消选择
				document.onclick = function() {
					ul.style.display = 'none';
					t = true;
				}

			}
		</script>
	</head>
	<body>
		<div class="wrap">
			<div id="box"></div>
			<ul>
				<li>HTML</li>
				<li>CSS</li>
				<li>JavaScript</li>
				<li>HTML5</li>
				<li>CSS3</li>
			</ul>
		</div>
	</body>
</html>