SOURCE

function byId(id){

return typeof(id)==="string"?document.getElementById(id):id;
}
var a=0,

nav=byId("nav"),
navs=nav.getElementsByClassName("nav1");

function all(){

for (var i = 0; i < navs.length; i++) {
    navs[i].setAttribute("da",i);
}
}
all();

    <div class="nav" id="nav">
        <!-- 导航1 -->
        <div class="nav1 onc">
            <a href="#">
                <span>首页</span>
            </a>    
        </div>
        <!-- 导航2 -->
        <div class="nav1">
            <a href="#">
                <span>点击看看</span>
            </a>    
        </div>
        <!-- 导航3 -->
        <div class="nav1">
            <a href="#">
                <span>轮播</span>
            </a>    
        </div>
        <!-- 导航4 -->
        <div class="nav1">
            <a href="#">
                <span>我的网站</span>
            </a>    
        </div>
    </div>