Array.prototype.reduce()

const array1 = [1, 2, 3, 4];

// 0 + 1 + 2 + 3 + 4
const initialValue = 0;
const sumWithInitial = array1.reduce(
  (accumulator, currentValue) => accumulator + currentValue,
  initialValue,
);

console.log(sumWithInitial);
// Expected output: 10

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

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

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

var initialValue = 0;
var sum = [{ x: 1 }, { x: 2 }, { x: 3 }].reduce(function (
  accumulator,
  currentValue,
) {
  return accumulator + currentValue.x;
}, initialValue);
// sum == 6

var initialValue = 0;
var sum = [{ x: 1 }, { x: 2 }, { x: 3 }].reduce(
  (accumulator, currentValue) => accumulator + currentValue.x,
  initialValue,
);
// sum == 6

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

// friends -   ()
//   "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 - ,       +
//     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"]

//   ECMA-262, 5- , 15.4.4.21
//  (en): http://es5.github.io/#x15.4.4.21
//  (ru): http://es5.javascript.ru/x15.4.html#x15.4.4.21
if (!Array.prototype.reduce) {
  Array.prototype.reduce = function (callback /*, initialValue*/) {
    "use strict";
    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");
    }
    var t = Object(this),
      len = t.length >>> 0,
      k = 0,
      value;
    if (arguments.length >= 2) {
      value = arguments[1];
    } else {
      while (k < len && !(k in t)) {
        k++;
      }
      if (k >= len) {
        throw new TypeError("Reduce of empty array with no initial value");
      }
      value = t[k++];
    }
    for (; k < len; k++) {
      if (k in t) {
        value = callback(value, t[k], k, t);
      }
    }
    return value;
  };
}