Turn array into flat structure via Array.prototype.reduce

flattenArray

function flatten(arr) {
  if (Array.isArray(arr)) {
  return arr.reduce(function(done,curr){
    return done.concat(flatten(curr));
    }, []);
  } else {
    return arr;
  }
}

console.log(flatten([[1,2,3], [4,5,6],[7,8,9]]));