JS数组求和结果显示NAN
我确实是给sum赋值了啊，为什么还是NAN？
```html
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<meta http-equiv="X-UA-Compatible" content="IE=edge">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<title>实验三练习一</title>
</head>
<body align="center">
<script>
var arry=[1,2,3,4,5,6,7,8,9,10];
function sum()
{
var sum;
for(var i=0;i<arry.length;i++)
{
sum+=arry[i];
}
return sum;
}
function show()
{
document.write(sum());
}
</script>
<table border="1" align="center">
<thead>
<th>数组</th>
<th>元素</th>
</thead>
<tbody>
<tr>
<td>arry[0]</td>
<td>1</td>
</tr>
<tr>
<td>arry[1]</td>
<td>2</td>
</tr>
<tr>
<td>arry[2]</td>
<td>3</td>
</tr>
<tr>
<td>arry[3]</td>
<td>4</td>
</tr><tr>
<td>arry[4]</td>
<td>5</td>
</tr><tr>
<td>arry[5]</td>
<td>6</td>
</tr><tr>
<td>arry[6]</td>
<td>7</td>
</tr><tr>
<td>arry[7]</td>
<td>8</td>
</tr>
<tr>
<td>arry[8]</td>
<td>9</td>
</tr><tr>
<td>arry[9]</td>
<td>10</td>
</tr>
</tbody>
</table>
<button onclick="show()">求和</button>
</body>
</html>
```
