多维数组怎么按列下标求和

var a = [[1,2,3],[5,7,9]];
var b = [];
var brindex=0;
for(var i=0;i<a[0].length;i++){
  for(var j=0;j<a.length;j++){
     brindex+=a[j][i];
  }  
 b.push(brindex);
 brindex=0;
}
console.log(b);var a = [[1,2,3],[5,7,9]];
var b = [];
var brin