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)let flattened = [[0, 1], [2, 3], [4, 5]].reduce(
  function(accumulator, currentValue) {
    return accumulator.concat(currentValue)
  },
  []
)
// flattened is [0, 1, 2, 3, 4, 5]
var array = [36, 25, 6, 15];

array.reduce(function(accumulator, currentValue) {
  return accumulator + currentValue;
}, 0); // 36 + 25 + 6 + 15 = 82let nums = [1, 2, 3];

nums.reduce((curr, next) => curr + next);// 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;
}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}}`