Array.prototype.reduce()

// アロー関数
reduce((previousValue, currentValue) => { /* … */ } )
reduce((previousValue, currentValue, currentIndex) => { /* … */ } )
reduce((previousValue, currentValue, currentIndex, array) => { /* … */ } )

reduce((previousValue, currentValue) => { /* … */ } , initialValue)
reduce((previousValue, currentValue, currentIndex) => { /* … */ } , initialValue)
reduce((previousValue, currentValue, currentIndex, array) => { /* … */ }, initialValue)

// コールバック関数
reduce(callbackFn)
reduce(callbackFn, initialValue)

// インラインコールバック関数
reduce(function(previousValue, currentValue) { /* … */ })
reduce(function(previousValue, currentValue, currentIndex) { /* … */ })
reduce(function(previousValue, currentValue, currentIndex, array) { /* … */ })

reduce(function(previousValue, currentValue) { /* … */ }, initialValue)
reduce(function(previousValue, currentValue, currentIndex) { /* … */ }, initialValue)
reduce(function(previousValue, currentValue, currentIndex, array) { /* … */ }, initialValue)

const getMax = (a, b) => Math.max(a, b);

// コールバックは 0 の位置から配列内の全要素に対して呼び出される
[1, 100].reduce(getMax, 50); // 100
[50].reduce(getMax, 10); // 50

// コールバックは 1 の位置に対して 1 度だけ呼び出される
[1, 100].reduce(getMax); // 100

// コールバックは呼び出されない
[50].reduce(getMax); // 50
[].reduce(getMax, 1); // 1

[].reduce(getMax); // TypeError

const array = [15, 16, 17, 18, 19];

function reducer(previousValue, currentValue, index) {
  const returns = previousValue + currentValue;
  console.log(
    `previousValue: ${previousValue}, currentValue: ${currentValue}, index: ${index}, returns: ${returns}`,
  );
  return returns;
}

array.reduce(reducer);

[15, 16, 17, 18, 19].reduce(
  (previousValue, currentValue) => previousValue + currentValue,
  10,
);

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

let total = [ 0, 1, 2, 3 ].reduce(
  ( previousValue, currentValue ) => previousValue + currentValue,
  0
)

const objects = [{ x: 1 }, { x: 2 }, { x: 3 }];
const sum = objects.reduce(
  (previousValue, currentValue) => previousValue + currentValue.x,
  0,
);

console.log(sum); // logs 6

const flattened = [
  [0, 1],
  [2, 3],
  [4, 5],
].reduce(
  (previousValue, currentValue) => previousValue.concat(currentValue),
  [],
);
// flattened is [0, 1, 2, 3, 4, 5]

const names = ["Alice", "Bob", "Tiff", "Bruce", "Alice"];

const countedNames = names.reduce((allNames, name) => {
  const currCount = allNames[name] ?? 0;
  return {
    ...allNames,
    [name]: currCount + 1,
  };
}, {});
// countedNames is:
// { 'Alice': 2, 'Bob': 1, 'Tiff': 1, 'Bruce': 1 }

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

function groupBy(objectArray, property) {
  return objectArray.reduce((acc, obj) => {
    const key = obj[property];

    return { ...acc, [key]: [...acc[key], obj] };
  }, {});
}

const 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" is a list of favorite books
const 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
const allbooks = friends.reduce(
  (previousValue, currentValue) => [...previousValue, ...currentValue.books],
  ["Alphabet"],
);

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

const myArray = ["a", "b", "a", "b", "c", "e", "e", "c", "d", "d", "d", "d"];
const myArrayWithNoDuplicates = myArray.reduce(
  (previousValue, currentValue) => {
    if (!previousValue.includes(currentValue)) {
      return [...previousValue, currentValue];
    }
    return previousValue;
  },
  [],
);

console.log(myArrayWithNoDuplicates);

const numbers = [-5, 6, 2, 0];

const doubledPositiveNumbers = numbers.reduce((previousValue, currentValue) => {
  if (currentValue > 0) {
    const doubled = currentValue * 2;
    return [...previousValue, doubled];
  }
  return previousValue;
}, []);

console.log(doubledPositiveNumbers); // [12, 4]

/**
 * 一連のプロミスハンドラーを連鎖させる。
 *
 * @param {array} arr - プロミスハンドラーのリストで、各ハンドラーは前のハンドラーの
 * 解決した結果を受け取り、別のプロミスを返します。
 * @param {*} input プロミスチェーンを開始するための初期値
 * @return {Object} ハンドラーの連鎖を伴う最終プロミス
 */
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

// 合成に使用する素材
const double = (x) => 2 * x;
const triple = (x) => 3 * x;
const quadruple = (x) => 4 * x;

// パイプ機能を実現する関数の合成
const pipe =
  (...functions) =>
  (initialValue) =>
    functions.reduce((acc, fn) => fn(acc), initialValue);

// 詳細な値の乗算のための合成関数
const multiply6 = pipe(double, triple);
const multiply9 = pipe(triple, triple);
const multiply16 = pipe(quadruple, quadruple);
const multiply24 = pipe(double, triple, quadruple);

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