dom对象

document.getElementById();根据id找
document.getElementsByclassName();根据类名class找
document.getElementsByName();根据名称找
document.getElementsByTagName();根据标签p,h1的名字找

 可以把bbc显示在h1标签里面

<h1 id="aa"></h1>
<script>
var hh = document.getElementById('aa');
hh.innerHTML = 'bbc';
</script>

var hh = document.getElementsByTagName('h1')[0];
hh.innerHTML = 'ok';