arr.reduce(callback,[initialValue])

var items = [10, 120, 1000];
// our reducer function
var reducer = function add(sumSoFar, item) { return sumSoFar + item; };
// do the job
var total = items.reduce(reducer, 0);
console.log(total); // 1130

var items = [10, 120, 1000];
// our reducer function
var reducer = function add(sumSoFar, item) {
 sumSoFar.sum = sumSoFar.sum + item;
 return sumSoFar;
};
// do the job
var total = items.reduce(reducer, {sum: 0});
console.log(total); // {sum:1130}

var manageReducers = function(reducers) {
 return function(state, item) {
  return Object.keys(reducers).reduce(
   function(nextState, key) {
    reducers[key](state, item);
    return state;
   },
   {}
  );
 }
};

var reducers = { 
 totalInEuros : function(state, item) {
  return state.euros += item.price * 0.897424392;
 },
 totalInYen : function(state, item) {
  return state.yens += item.price * 113.852;
 }
};
var manageReducers = function(reducers) {
 return function(state, item) {
  return Object.keys(reducers).reduce(
   function(nextState, key) {
    reducers[key](state, item);
    return state;
   },
   {}
  );
 }
};
var bigTotalPriceReducer = manageReducers(reducers);
var initialState = {euros:0, yens: 0};
var items = [{price: 10}, {price: 120}, {price: 1000}];
var totals = items.reduce(bigTotalPriceReducer, initialState);
console.log(totals);

var result = [
  {
    subject: 'math',
    score: 88
  },
  {
    subject: 'chinese',
    score: 95
  },
  {
    subject: 'english',
    score: 80
  }
];

var sum = result.reduce(function(prev, cur) {
  return cur.score + prev;
}, 0);

var sum = result.reduce(function(prev, cur) {
  return cur.score + prev;
}, -10);

var dis = {
  math: 0.5,
  chinese: 0.3,
  english: 0.2
}
var sum = result.reduce(function(prev, cur) {
  return cur.score + prev;
}, -10);
var qsum = result.reduce(function(prev, cur) {
  return cur.score * dis[cur.subject] + pre;
}, 0)
console.log(sum, qsum);

var arrString = 'abcdaabc';
arrString.split('').reduce(function(res, cur) {
  res[cur] ? res[cur] ++ : res[cur] = 1
  return res;
}, {})

[1, 2].reduce(function(res, cur) { 
  res.push(cur + 1); 
  return res; 
}, [])

var only = function(obj, keys){
 obj = obj || {};
 if ('string' == typeof keys) keys = keys.split(/ +/);
 return keys.reduce(function(ret, key){
  if (null == obj[key]) return ret;
  ret[key] = obj[key];
  return ret;
 }, {});
};

var a = {
  env : 'development',
  proxy : false,
  subdomainOffset : 2
}
only(a,['env','proxy'])  // {env:'development',proxy : false}