为什么输出结果会有两个undefined
<!DOCTYPE HTML>
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
<title>函数传参</title>
<script type="text/JavaScript">
function add3(x,y,z)
{
sum = x + y +z;
document.write(x+"、"+y+"、"+z+"和:"+sum+"<br/>");
}
document.write(add3(5,8,3));
document.write(add3(7,1,4));
</script>
</head>
<body>
</body>
</html>
