let element = document.getElementById("box");

let attr = document.createAttribute("class");
attr.nodeValue = "className";
el.setAttributeNode(attr)

<div id="btn-group">
	<div class="btn btn-active">按鈕1</div>
	<div class="btn">按鈕2</div>
	<div class="btn">按鈕3</div>
	<div class="btn">按鈕4</div>
</div>

let myEventUtil = {
	// 添加監(jiān)聽事件
	addEvent (element, type, handler) {
		if (element.addEventListener) {
			element.addEventListener(type, handler, false);
		} else if (element.attach){ // ie
			element.attach("on"+ type, handler);
		} else {
			element['on' + type] = handler;
		}
	},
	getTarget (event) {
		let event = event || window.event;
		return event.target || event.srcElement;
	}
}
let my$ = id => document.getElementById(id);

let btnGroup = my$(“btn-group”);
myEventUtil.addEvent(btnGroup, 'on', function (ev) {
	// 給所有的 btn 都移除激活的類 btn-active
	// console.log(this) ==> 是一個dom元素 btnGroup 
	// 可以通過 el.children[i]拿到具體的子元素
	// 拿到子元素了可以通過 el.classList.remove("className") 刪除類
	// el.classList.add("className") 來添加類
	
	// 刪除類
	let len = this.children.length;
	for (let i = 0; i < len; i ++) {
		this.children[i].classList.remove("btn-active");
		// this.children[i].className = "btn"; // 用其中一個就行
	}
	// 添加類, 拿到具體的 btn 給它添加類
	myEventUtil.getTarget(ev).classList.add("btn-active"); 
	// myEventUtil.getTarget(ev).className = "btn"; // 用其中一個就行
});