array.reduce

var total = [0, 1, 2, 3].reduce(function(sum, value) {
  return sum + value;
}, 0
// total is 6

var flattened = [[0, 1], [2, 3], [4, 5]].reduce(function(a, b) {
  return a.concat(b
}, []
// flattened is [0, 1, 2, 3, 4, 5]

arr.reduce(callback[, initialValue])

var maxCallback = ( acc, cur ) => Math.max( acc.x, cur.x 
var maxCallback2 = ( max, cur ) => Math.max( max, cur 

// reduce() without initialValue
[ { x: 22 }, { x: 42 } ].reduce( maxCallback  // 42
[ { x: 22 }            ].reduce( maxCallback  // { x: 22 }
[                      ].reduce( maxCallback  // TypeError

// map/reduce; better solution, also works for empty arrays
[ { x: 22 }, { x: 42 } ].map( el => el.x )
                        .reduce( maxCallback2, -Infinity 

[0, 1, 2, 3, 4].reduce(
  function (
    accumulator,
    currentValue,
    currentIndex,
    array
  ) {
    return accumulator + currentValue;
  }

[0, 1, 2, 3, 4].reduce( (prev, curr) => prev + curr 

[0, 1, 2, 3, 4].reduce(
  (accumulator, currentValue, currentIndex, array) => {
    return accumulator + currentValue;
  },
  10

var sum = [0, 1, 2, 3].reduce(function (a, b) {
  return a + b;
}, 0
// sum is 6

var total = [ 0, 1, 2, 3 ].reduce(
  ( acc, cur ) => acc + cur,
  0

var flattened = [[0, 1], [2, 3], [4, 5]].reduce(
  function(a, b) {
    return a.concat(b
  },
  []

// flattened is [0, 1, 2, 3, 4, 5]

var flattened = [[0, 1], [2, 3], [4, 5]].reduce(
  ( acc, cur ) => acc.concat(cur),
  []

var names = ['Alice', 'Bob', 'Tiff', 'Bruce', 'Alice'];

var countedNames = names.reduce(function (allNames, name) { 
  if (name in allNames) {
    allNames[name]++;
  }
  else {
    allNames[name] = 1;
  }
  return allNames;
}, {}
// countedNames is:
// { 'Alice': 2, 'Bob': 1, 'Tiff': 1, 'Bruce': 1 }

// friends - an array of objects 
// where object field "books" - list of favorite books 
var friends = [{
  name: 'Anna',
  books: ['Bible', 'Harry Potter'],
  age: 21
}, {
  name: 'Bob',
  books: ['War and peace', 'Romeo and Juliet'],
  age: 26
}, {
  name: 'Alice',
  books: ['The Lord of the Rings', 'The Shining'],
  age: 18
}];

// allbooks - list which will contain all friends' books +  
// additional list contained in initialValue
var allbooks = friends.reduce(function(prev, curr) {
  return [...prev, ...curr.books];
}, ['Alphabet']

// allbooks = [
//   'Alphabet', 'Bible', 'Harry Potter', 'War and peace', 
//   'Romeo and Juliet', 'The Lord of the Rings',
//   'The Shining'
// ]

// Production steps of ECMA-262, Edition 5, 15.4.4.21
// Reference: http://es5.github.io/#x15.4.4.21
// https://tc39.github.io/ecma262/#sec-array.prototype.reduce
if (!Array.prototype.reduce) {
  Object.defineProperty(Array.prototype, 'reduce', {
    value: function(callback /*, initialValue*/) {
      if (this === null) {
        throw new TypeError( 'Array.prototype.reduce ' + 
          'called on null or undefined' 
      }
      if (typeof callback !== 'function') {
        throw new TypeError( callback +
          ' is not a function'
      }

      // 1. Let O be ? ToObject(this value).
      var o = Object(this

      // 2. Let len be ? ToLength(? Get(O, "length")).
      var len = o.length >>> 0; 

      // Steps 3, 4, 5, 6, 7      
      var k = 0; 
      var value;

      if (arguments.length >= 2) {
        value = arguments[1];
      } else {
        while (k < len && !(k in o)) {
          k++; 
        }

        // 3. If len is 0 and initialValue is not present,
        //    throw a TypeError exception.
        if (k >= len) {
          throw new TypeError( 'Reduce of empty array ' +
            'with no initial value' 
        }
        value = o[k++];
      }

      // 8. Repeat, while k < len
      while (k < len) {
        // a. Let Pk be ! ToString(k).
        // b. Let kPresent be ? HasProperty(O, Pk).
        // c. If kPresent is true, then
        //    i.  Let kValue be ? Get(O, Pk).
        //    ii. Let accumulator be ? Call(
        //          callbackfn, undefined,
        //          « accumulator, kValue, k, O »).
        if (k in o) {
          value = callback(value, o[k], k, o
        }

        // d. Increase k by 1.      
        k++;
      }

      // 9. Return accumulator.
      return value;
    }
  }
}