// source http://jsbin.com/leseha/2

things.js

var myData = [
  {
    week: 1,
    count: 5
  },
  {
    week: 2,
    count: 1
  },
  {
    week: 1,
    count: 1
  },
  {
    week: 2,
    count: 3
  },
  {
    week: 2,
    count: 4
  }
];


//----Helper debugger-------------------------
var log = function (x,y,z) {
  console.log("eh?",x,y,z);
  return x;
};


//----Useful utility functions----------------
var compose = function (f, g) {
  return function (x) { return f( g(x) ); };
};


//----JS-tailored function--------------------
var uniqueFilter = function (item, index, array) {
  return array.indexOf(item) === index;
};


//----Ahh, real functions!--------------------
var prop = function (propName) {
  return function (obj) { return obj[propName]; };
};

var compare = function (value) {
  return function (target) { return target === value; };
};

var add = function (x, y) { return x + y; };

var uniqueValues = function (objArray, key) {
  return objArray.map( prop(key) ).filter(uniqueFilter);
};


//----The meat and potatos--------------------
function reduceByKey(data, key, value) {
 
  return uniqueValues(data, key)
    .map(function(uniqVal) {
      var sum = data
        .filter( compose(compare(uniqVal), prop(key)) )
        .map( prop(value) )
        .reduce(add)
      ;
      return {
        key: uniqVal,
        value: sum
      };
    })
   ;
}
 
var x = reduceByKey(myData, 'week', 'count');
console.log(x);