以下是个人在工作中收藏总结的一些关于javascript数组方法reduce的相关代码片段，后续遇到其他使用这个函数的场景，将会陆续添加，这里作为备忘。

javascript数组那么多方法，为什么我要单挑reduce方法，一个原因是我对这个方法掌握不够，不能够用到随心所欲。另一个方面，我也感觉到了这个方法的庞大魅力，在许多的场景中发挥着神奇的作用。

理解reduce函数

reduce() 方法接收一个函数作为累加器（accumulator），数组中的每个值（从左到右）开始缩减，最终为一个值。

arr.reduce([callback, initialValue])

看如下例子：

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

片段一：字母游戏

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？

reduce负责筛选出每一次执行的首字母，递归负责对剩下字母的排列组合。

片段二：累加器

const sum = arr => arr.reduce((acc, val) => acc + val, 0);
sum([1, 2, 3]);

片段三：计数器

const countOccurrences = (arr, value) => arr.reduce((a, v) => v === value ? a + 1 : a + 0, 0);
countOccurrences([1, 2, 3, 2, 2, 5, 1], 1);

循环数组，每遇到一个值与给定值相等，即加1，同时将加上之后的结果作为下次的初始值。

片段四：函数柯里化

函数柯里化的目的就是为了储存数据，然后在最后一步执行。

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);

通过判断函数的参数取得当前函数的length(当然也可以自己指定)，如果所传的参数比当前参数少，则继续递归下面，同时储存上一次传递的参数。

片段五：数组扁平化

const deepFlatten = arr => 
 arr.reduce((a, v) => a.concat(Array.isArray(v) ? deepFlatten(v) : v), []);
deepFlatten([1, [2, [3, 4, [5, 6]]]]);

片段六：生成菲波列契数组

const fibonacci = n => Array(n).fill(0).reduce((acc, val, i) => acc.concat(i > 1 ? acc[i - 1] + acc[i - 2] : i), []);
fibonacci(5);

片段七：管道加工器

const pipe = (...funcs) => arg => funcs.reduce((acc, func) => func(acc), arg);
pipe(btoa, x => x.toUpperCase())("Test");

通过对传递的参数进行函数加工，之后将加工之后的数据作为下一个函数的参数，这样层层传递下去。

片段八：中间件

const dispatch = action => {
 console.log('action', action);
 return action;
}
const middleware1 = dispatch => {
 return action => {
 console.log("middleware1");
 const result = dispatch(action);
 console.log("after middleware1");
 return result;
 }
}
const middleware2 = dispatch => {
 return action => {
 console.log("middleware2");
 const result = dispatch(action);
 console.log("after middleware2");
 return result;
 }
}
const middleware3 = dispatch => {
 return action => {
 console.log("middleware3");
 const result = dispatch(action);
 console.log("after middleware3");
 return result;
 }
}
const compose = middlewares => middlewares.reduce((a, b) => args => a(b(args)))

const middlewares = [middleware1, middleware2, middleware3];
const afterDispatch = compose(middlewares)(dispatch);

const testAction = arg => {
 return { type: "TEST_ACTION", params: arg };
};
afterDispatch(testAction("1111"));

// 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 * 0.7424392;
 },
 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), {});
pick({ a: 1, b: '2', c: 3 }, ['a', 'c']);

根据给出的键值来遍历，比较对象中是否存在相同键值的的值，然后通过逗号表达式把赋值后的对象赋给下一个的初始值

片段十四：数组中删除指定位置的值

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);

首先根据filter函数过滤出数组中符合条件的值，然后使用reduce在原数组中删除符合条件的值，可以得出最后arr的值变成了[1, 3]

片段十五：promise按照顺序执行

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')]);

片段十六：排序

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']);

片段十七：选择

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');