这个代码为什么会还没按按钮就自动跳转到url网址？
<!DOCTYPE HTML>
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
<title>无标题文档</title>
</head>
<body>
<script type="text/javascript">
var main = document.body;
//创建链接
function jump(url){
window.open(url);
}
function createa(url,text)
{
var input=document.createElement("input");
input.type="button";
input.value=text;
input.setAttribute("onclick",jump(url));
main.appendChild(input);
}
// 调用函数创建链接
createa('http://www.imooc.com','慕课网');
</script>
</body>
</html>
