var obj = {  a1 : {key1: 50, key2:60, key3:0},  a2 : {key1: 0, key2:60, key3:0},  a3 : {key1: 50, key2:0, key3:0},  a4 : {key1: 50, key2:60, key3:0} }var arr = [];for(var i in obj) { for(var j in obj[i] ) {  arr[j] += obj[i][j]; }}console.log(arr);我想取得a1,z2,z3,z4的key1（key2,key3,key4）值的和，現在打印出來的顯示NaN了是怎么回事