Some examples about reduce() in Javascript

reduceExamples.js

//Example 1 - Calculate average value of an array (transform array into a single number)
  var scores = [89, 76, 47, 95]
  var initialValue = 0
  var reducer = function (accumulator, item) {
    return accumulator + item
  }
  var total = scores.reduce(reducer, initialValue)
  var average = total / scores.length
/*Explain about function

You'll notice .reduce takes in two values, a callback function and an initial value.

The very first time the reducer function is called, it's going to be passed the initialValue you gave it 
(the 2nd argument to .reduce) and the first item in the actual array. So in our example above the first time 
that our reducer function runs, accumulator is going to be 0 and item is going to be 89. Remember, the goal 
is to transform an array into a single value. We currently have two numbers, 0 and 89, and are goal is to get 
that to one value. Because we're wanting to find the sum of every item in the array, we'll add 89 + 0 to get 89. 
That brings up a very important step. The thing that gets returned from the reducer function will then be passed 
as the accumulator the next time the function runs. So when reducer runs again, accumulator will be 89 and item 
will now be the second item in the array, 76. This pattern continues until we have no more items in the array 
and we get the summation of all of our reducer functions, which is 307.
*/

// =====================
// EXAMPLE 5 - Calculate average
function average(arr){
  return arr.reduce((a,b)=>a+b)/arr.length
}
console.log(average([1,2,3,4,5])); //output: 3

// =====================
// EXAMPLE 5 - Count something
var arr=[1,2,3,4,5];
//count odd number:
console.log(arr.reduce((a,b) => a+(b%2?1:0),0)); //output: 3
//count even number:
console.log(arr.reduce((a,b) => a+(b%2?0:1),0)); //output: 2

// =====================
//EXAMPLE 2 - Transform array to an object
  var votes = [
  'tacos',
  'pizza',
  'pizza',
  'tacos',
  'fries',
  'ice cream',
  'ice cream',
  'pizza'
]
var initialValue = {}
var reducer = function(tally, vote) {
  if (!tally[vote]) {
    tally[vote] = 1;
  } else {
    tally[vote] = tally[vote] + 1;
  }
  return tally;
}
var result = votes.reduce(reducer, initialValue) // {tacos: 2, pizza: 3, fries: 1, ice cream: 2}

// And vice versa (object to array)
var maxSpeed = {
    car:300, 
    bike:60, 
    motorbike:200
}
var arr = [];
for (var vehicle in maxSpeed)
      arr.push([vehicle, maxSpeed[vehicle]])  //Output: [["bike", 60], ["motorbike", 200], ["car", 300]]
)
// =======END===========

// =====================
// EXAMPLE 3 - Counting the occurrences
//3.1 Of JavaScript array number elements
var newArr = [1,1,3,4,3,2,2]
var countObj = newArr.reduce(function (acc, curr) {
    if (typeof acc[curr] == 'undefined') {
      acc[curr] = 1;
    } else {
      acc[curr] += 1;
    }
  
    return acc;
}, {});

//3.2. Count objects that have a specific property:
people = [
    {name: 'Mary', gender: 'girl'},
    {name: 'Paul', gender: 'boy'},
    {name: 'John', gender: 'boy'},
    {name: 'Lisa', gender: 'girl'},
    {name: 'Bill', gender: 'boy'},
    {name: 'Maklatura', gender: 'girl'}
]

var numBoys = people.reduce(function(n, person) {
    return n + (person.gender == 'boy');
}, 0);

//3.3. A function for processing 3.1 & 3.2
var count = function(ary, classifier) {
    return ary.reduce(function(counter, item) {
        var p = (classifier || String)(item);
        counter[p] = counter.hasOwnProperty(p) ? counter[p] + 1 : 1;
        return counter;
    }, {})
}

If you don't provide a classifier this simply counts different elements:
  
  > count([1,2,2,2,3,1])
  {
   "1": 2,
   "2": 3,
   "3": 1
  }

With a classifier you group elements by specific property:

  > countByGender = count(people, function(item) { return item.gender })
  {
   "girl": 3,
   "boy": 3
  }

Greate ^^
// ======================

Very nice!

very helpfull . thanks for sharing