js 输出二维数组的最大值

function num(arr){max=a[0][0];
for (var i = 0; i < a.length; i++) {
for (var j = 0; j< a[i].length; j++) {
if (max<a[i][j]) {
max=a[i][j];}
}
}
alert(max);}
var a=[[5,6,7],[15,16,8,7],[3,23,44,55,66]];
num(a);