reduce method javascript

const sum = array.reduce((accumulator, element) => {
  return accumulator + element;
}, 0);
// An example that will loop through an array adding
// each element to an accumulator and returning it
// The 0 at the end initializes accumulator to start at 0
// If array is [2, 4, 6], the returned value in sum
// will be 12 (0 + 2 + 4 + 6)

const product = array.reduce((accumulator, element) => {
  return accumulator * element;
}, 1);
// Multiply all elements in array and return the total
// Initialize accumulator to start at 1
// If array is [2, 4, 6], the returned value in product
// will be 48 (1 * 2 * 4 * 6)

var array = [36, 25, 6, 15];

array.reduce(function(accumulator, currentValue) {
  return accumulator + currentValue;
}, 0); // 36 + 25 + 6 + 15 = 82

Executes a reducer function on each element of the array, resulting in single output value.

const numbers = [1, -1, 2, 3];

const sum = numbers.reduce((accumulator, currentValue) => accumulator + currentValue);

// accumulator = 1, currentValue = -1 => accumulator = 0
// accumulator = 0, currentValue = 2 => accumulator = 2
// accumulator = 2, currentValue = 3 => accumulator = 5

// expected output 5

var age = [ { Id: 16 }, { Id: 24 }, { Id: 32 } ];

var ageById = age.reduce((byId, age) => (byId[age.Id] = age, byId), {});
console.log(ageById);
would print out:
`{16:{Id: 16},24:{Id: 24},32:{Id: 32}}`

// Reduce() method executes a callback function that is passed in
// on each element of the array, resulting in single output value.
const array1 = [1, 2, 3, 4];
const callback = (accumulator, currentValue) => accumulator + currentValue;

// 1 + 2 + 3 + 4
console.log(array1.reduce(callback));
// expected output: 10

// 5 + 1 + 2 + 3 + 4
console.log(array1.reduce(callback, 5));
// expected output: 15 because the initial value is 5.

// This is how Reduce works.
// This is a myReduce method which takes a callback and an optional argument 
// of a default accumulator. If myReduce only receives one argument, then 
// myReduce will use the first element as the accumulator.

Array.prototype.myReduce = function(callback, acc) {
    if (!acc) {
        acc = this.shift();
    }
    this.forEach(function(element) {
        acc = callback(acc, element)
    })
    return acc;
}

array.reduce(function(acumulador, elementoAtual, indexAtual, arrayOriginal), valorInicial)

[1,2,3,4,5].reduce((acc, current)=>acc+current, 0)

/* this is our initial value i.e. the starting point*/
const initialValue = 0;

/* numbers array */
const numbers = [5, 10, 15];

/* reducer method that takes in the accumulator and next item */
const reducer = (accumulator, item) => {
  return accumulator + item;
};

/* we give the reduce method our reducer function
  and our initial value */
const total = numbers.reduce(reducer, initialValue)

function flattenArray(data) {
  // our initial value this time is a blank array
  const initialValue = [];

  // call reduce on our data
  return data.reduce((total, value) => {
    // if the value is an array then recursively call reduce
    // if the value is not an array then just concat our value
    return total.concat(Array.isArray(value) ? flattenArray(value) : value);
  }, initialValue);
}