<div >3</div>
</div>
function getElementsByClassName(node,className){
if(node.getElementsByClassName){
/*兼容ie9*/
return node.getElementsByClassName(className);
}else{
var results = [];
var items = node.getElementsByTagName("*");
for (var i = 0; i < items.length; i++) {
if(items[i].className == className){
results.push(items[i]);
}
};
return results;
}
}
var node = document.getElementById('box');
console.log(getElementsByClassName(node,"box"));