Relevant usage of js array reduce

arr.reduce([callback, initialValue])

let arr = [1, 2, 3, 4, 5];

// 10代表初始值，p代表每一次的累加值，在第一次为10

// 如果不存在初始值，那么p第一次值为1

// 此时累加的结果为15

let sum = arr.reduce((p, c) => p + c, 10); // 25

// 转成es5的写法即为：

var sum = arr.reduce(function(p, c) {

 console.log(p);

 return p + c;

}, 10);

const anagrams = str => {

 if (str.length <= 2) {

  return str.length === 2 ? [str, str[1] + str[0]] : str;

 }

 return str.split("").reduce((acc, letter, i) => {

  return acc.concat(anagrams(str.slice(0, i) + str.slice(i + 1)).map(val => letter + val));

 }, []);

}

anagrams("abc"); // 结果会是什么呢？

const sum = arr => arr.reduce((acc, val) => acc + val, 0);

sum([1, 2, 3]);

const countOccurrences = (arr, value) => arr.reduce((a, v) => v === value ? a + 1 : a + 0, 0);

countOccurrences([1, 2, 3, 2, 2, 5, 1], 1);

const curry = (fn, arity = fn.length, ...args) =>

 arity <= args.length ? fn(...args) : curry.bind(null, fn, arity, ...args);

curry(Math.pow)(2)(10);

curry(Math.min, 3)(10)(50)(2);

const deepFlatten = arr =>

 arr.reduce((a, v) => a.concat(Array.isArray(v) ? deepFlatten(v) : v), []);

deepFlatten([1, [2, [3, 4, [5, 6]]]]);

const fibonacci = n => Array(n).fill(0).reduce((acc, val, i) => acc.concat(i > 1 ? acc[i - 1] + acc[i - 2] : i), []);

fibonacci(5);

const pipe = (...funcs) => arg => funcs.reduce((acc, func) => func(acc), arg);

pipe(btoa, x => x.toUpperCase())("Test");

// redux-actions/src/handleAction.js

const handleAction = (type, reducer, defaultState) => {

 const types = type.toString();

 const [nextReducer, throwReducer] = [reducer, reducer];

 return (state = defaultState, action) => {

  const { type: actionType } = action;

  if (!actionType || types.indexOf(actionType.toString()) === -1) {

   return state;

  }

  return (action.error === true ? throwReducer : nextReducer)(state, action);

 }

}

// reduce-reducers/src/index.js

const reduceReducer = (...reducers) => {

 return (previous, current) => {

  reducers.reduce((p, r) => r(p, current), previous);

 }

}

// redux-actions/src/handleActions.js

const handleActions = (handlers, defaultState, { namespace } = {}) => {

 // reducers的扁平化

 const flattenedReducerMap = flattenReducerMap(handles, namespace);

 // 每一种ACTION下对应的reducer处理方式

 const reducers = Reflect.ownkeys(flattenedReducerMap).map(type => handleAction(

  type,

  flattenedReducerMap[type],

  defaultState

 ));

 // 状态的加工器，用于对reducer的执行

 const reducer = reduceReducers(...reducers);

 // reducer触发

 return (state = defaultState, action) => reducer(state, action);

}

const reducers = {

 totalInEuros: (state, item) => {

  return state.euros += item.price * 0.897424392;

 },

 totalInYen: (state, item) => {

  return state.yens += item.price * 113.852;

 }

};

const manageReducers = reducers => {

 return (state, item) => {

  return Object.keys(reducers).reduce((nextState, key) => {

   reducers[key](state, item);

   return state;

  }, {})

 }

}

const bigTotalPriceReducer = manageReducers(reducers);

const initialState = { euros: 0, yens: 0 };

const items = [{ price: 10 }, { price: 120 }, { price: 1000 }];

const totals = items.reduce(bigTotalPriceReducer, initialState);

let school = {

 name: 'Hope middle school',

 created: '2001',

 classes: [

  {

   name: '三年二班',

   teachers: [

    { name: '张二蛋', age: 26, sex: '男', actor: '班主任' },

    { name: '王小妞', age: 23, sex: '女', actor: '英语老师' }

   ]

  },

  {

   name: '明星班',

   teachers: [

    { name: '欧阳娜娜', age: 29, sex: '女', actor: '班主任' },

    { name: '李易峰', age: 28, sex: '男', actor: '体育老师' },

    { name: '杨幂', age: 111, sex: '女', actor: '艺术老师' }

   ]

  }

 ]

};

// 常规做法

school.classes &&

school.classes[0] &&

school.classes[0].teachers &&

school.classes[0].teachers[0] &&

school.classes[0].teachers[0].name

// reduce方法

const get = (p, o) => p.reduce((xs, x) => (xs && xs[x] ? xs[x] : null), o);

get(['classes', 0, 'teachers', 0, 'name'], school); // 张二蛋

const groupBy = (arr, func) =>

arr.map(typeof func === 'function' ? func : val => val[func]).reduce((acc, val, i) => {

 acc[val] = (acc[val] || []).concat(arr[i]);

 return acc;

}, {});

groupBy([6.1, 4.2, 6.3], Math.floor);

groupBy(['one', 'two', 'three'], 'length');

const pick = (obj, arr) =>

arr.reduce((acc, curr) => (curr in obj && (acc[curr] = obj[curr]), acc), {});

const remove = (arr, func) =>

 Array.isArray(arr)

 ? arr.filter(func).reduce((acc, val) => {

   arr.splice(arr.indexOf(val), 1);

   return acc.concat(val);

 }, []) : [];

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

remove(arr, n => n % 2 == 0);

const runPromisesInSeries = ps => ps.reduce((p, next) => p.then(next), Promise.resolve());

const delay = d => new Promise(r => setTimeout(r, d));

const print = args => new Promise(r => r(args));

runPromisesInSeries([() => delay(1000), () => delay(2000), () => print('hello')]);

const orderBy = (arr, props, orders) =>

 [...arr].sort((a, b) =>

  props.reduce((acc, prop, i) => {

   if (acc === 0) {

    const [p1, p2] = orders && orders[i] === 'desc' ? [b[prop], a[prop]] : [a[prop], b[prop]];

    acc = p1 > p2 ? 1 : p1 < p2 ? -1 : 0;

   }

   return acc;

  }, 0)

 );

const users = [{ name: 'fred', age: 48 }, { name: 'barney', age: 36 }, { name: 'fly', age: 26 }];

orderBy(users, ['name', 'age'], ['asc', 'desc']);

orderBy(users, ['name', 'age']);

const select = (from, selector) =>

 selector.split('.').reduce((prev, cur) => prev && prev[cur], from);

const obj = { selector: { to: { val: 'val to select' } } };

select(obj, 'selector.to.val');