Count By

count-by.js

// count # of any object property
var getProperty = function( obj, prop ) {
  if (prop.indexOf('.') < 0) return obj[prop];
  var parts = prop.split('.')
    , last = parts.pop()
    , len = parts.length
    , idx = 1
    , current = parts[0];

  while( (obj = obj[current]) && idx < len ) {
    current = parts[idx++];
  }
  if ( obj )
    return obj[last];
  return obj;
}

  
var countBy = function(list, prop) {
  return list.reduce(function(histogram, obj) {
    var key = getProperty(obj, prop);
    if ( key ) histogram[key] = histogram[key] ? histogram[key]+1 : 1;
    return histogram;
  }, {});
}

// Example
var orders = [
  { id: 1, user: { name: 'Sam' } }
, { id: 2, user: { name: 'Dan' } }
, { id: 3, user: { name: 'John' } }
, { id: 4, user: { name: 'Gillian' } }
, { id: 5, user: { name: 'John' } }
, { id: 6, user: { name: 'Sam' } }
];

countBy(orders, 'user.name');
// { Sam: 2, Dan: 1, John: 2, Gillian: 1 }