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

    arr.reduce(callback[, initialValue])

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

// initialValue  reduce()
[{ x: 22 }, { x: 42 }].reduce(maxCallback); // 42
[{ x: 22 }].reduce(maxCallback); // { x: 22 }
[].reduce(maxCallback); // TypeError

// map/reduce  -     
[{ 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(function (
  accumulator,
  currentValue,
  currentIndex,
  array,
) {
  return accumulator + currentValue;
}, 10);

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

var total = [0, 1, 2, 3].reduce(
  (accumulator, currentValue) => accumulator + currentValue,
  0,
);

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

console.log(sum); // logs 6

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

console.log(sum); // logs 6

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

var flattened = [
  [0, 1],
  [2, 3],
  [4, 5],
].reduce((accumulator, currentValue) => accumulator.concat(currentValue), []);

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 }

var people = [
  { name: "Alice", age: 21 },
  { name: "Max", age: 20 },
  { name: "Jane", age: 20 },
];

function groupBy(objectArray, property) {
  return objectArray.reduce(function (acc, obj) {
    var key = obj[property];
    if (!acc[key]) {
      acc[key] = [];
    }
    acc[key].push(obj);
    return acc;
  }, {});
}

var groupedPeople = groupBy(people, "age");
// groupedPeople is:
// {
//   20: [
//     { name: 'Max', age: 20 },
//     { name: 'Jane', age: 20 }
//   ],
//   21: [{ name: 'Alice', age: 21 }]
// }

// 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 (accumulator, currentValue) {
    return [...accumulator, ...currentValue.books];
  },
  ["Alphabet"],
);

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

let arr = [1, 2, 1, 2, 3, 5, 4, 5, 3, 4, 4, 4, 4];
let result = arr.sort().reduce((accumulator, current) => {
  const length = accumulator.length;
  if (length === 0 || accumulator[length - 1] !== current) {
    accumulator.push(current);
  }
  return accumulator;
}, []);
console.log(result); //[1,2,3,4,5]

/**
 * Runs promises from array of functions that can return promises
 * in chained manner
 *
 * @param {array} arr - promise arr
 * @return {Object} promise object
 */
function runPromiseInSequence(arr, input) {
  return arr.reduce(
    (promiseChain, currentFunction) => promiseChain.then(currentFunction),
    Promise.resolve(input),
  );
}

// promise function 1
function p1(a) {
  return new Promise((resolve, reject) => {
    resolve(a * 5);
  });
}

// promise function 2
function p2(a) {
  return new Promise((resolve, reject) => {
    resolve(a * 2);
  });
}

// function 3  - will be wrapped in a resolved promise by .then()
function f3(a) {
  return a * 3;
}

// promise function 4
function p4(a) {
  return new Promise((resolve, reject) => {
    resolve(a * 4);
  });
}

const promiseArr = [p1, p2, f3, p4];
runPromiseInSequence(promiseArr, 10).then(console.log); // 1200

// Building-blocks to use for composition
const double = (x) => x + x;
const triple = (x) => 3 * x;
const quadruple = (x) => 4 * x;

// Function composition enabling pipe functionality
const pipe =
  (...functions) =>
  (input) =>
    functions.reduce((acc, fn) => fn(acc), input);

// Composed functions for multiplication of specific values
const multiply6 = pipe(double, triple);
const multiply9 = pipe(triple, triple);
const multiply16 = pipe(quadruple, quadruple);
const multiply24 = pipe(double, triple, quadruple);

// Usage
multiply6(6); // 36
multiply9(9); // 81
multiply16(16); // 256
multiply24(10); // 240

if (!Array.prototype.mapUsingReduce) {
  Array.prototype.mapUsingReduce = function (callback, thisArg) {
    return this.reduce(function (mappedArray, currentValue, index, array) {
      mappedArray[index] = callback.call(thisArg, currentValue, index, array);
      return mappedArray;
    }, []);
  };
}

[1, 2, , 3].mapUsingReduce(
  (currentValue, index, array) => currentValue + index + array.length,
); // [5, 7, , 10]

// ECMA-262  , 5(Edition), 15.4.4.21
// : 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;
    },
  });
}