var data = [

  [1, 2, 3, 4, 5, 6],

  [10, 20, 30, 40, 50, 60]

];

​

//create the same nummber of empty arrays as there are arrays in data.

var averagedCols = new Array(data.length).fill([]);

​

for (var i = 0; i < data.length; i++) {

  for(var j = 0; j < data[i].length; j++) {

    if (j%2 === 0) {

      var average = (data[i][j] + data[i][j+1])/2;

      averagedCols[i].push(average);

    } 

  }

}

console.log(averagedCols);

//expecting [1.5, 3.5, 5.5]

console.log(averagedCols[0]);

//expecting [15, 35, 55]

console.log(averagedCols[1]);

​