createa("http://www.imooc.com/","慕课网");?? createa为什么这样输出？而不是document.write(createa)

<script type="text/javascript">

var main = document.body;

var body= document.body;

//创建链接

function createa(url,text)

{

    var a = document.createElement("a");

    a.href = url;

    a.innerHTML = text;

    a.style.color = "red";

    body.appendChild(a);

}

// 调用函数创建链接

createa("http://www.imooc.com/","慕课网");

</script>