function sum(arr, prop){
  return arr.reduce(function(sum, obj){
    return sum + obj.stats[prop];
  }, 0);
}
function sum(arr, prop){
  return arr.reduce(function(sum, obj){
    return sum + obj.stats[prop];
  }, 0);
}
function sum ( arr , prop ) { return arr . reduce ( function ( sum , obj ) { return sum + obj . stats [ prop ] ; } , 0 ) ; }
Return sum of each `prop` in `arr`.

@param {Array} arr
@param {String} prop
@return {Number}
@api private
Return sum of each prop in arr .