几个关于js数组方法reduce的经典片段

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])

关于reduce的用法，这里不再做多述，可以去这里查看

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);

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。

See the Pen reduce anagrams by 糊一笑 (@rynxiao) on CodePen.

片段二：累加器

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

See the Pen reduce sum by 糊一笑 (@rynxiao) on CodePen.

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。

See the Pen reduce count occurrences by 糊一笑 (@rynxiao) on CodePen.

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。深入了解，请参看小火柴的这篇文章http://www.cnblogs.com/xiaohuochai/p/8026074.html

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。

See the Pen reduce curry by 糊一笑 (@rynxiao) on CodePen.

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]]]]);

See the Pen reduce deep flatten array by 糊一笑 (@rynxiao) on CodePen.

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);

See the Pen reduce fibonacci by 糊一笑 (@rynxiao) on CodePen.

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。

See the Pen reduce pipe by 糊一笑 (@rynxiao) on CodePen.

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中经典的compose函数中运用了这种方式，通过对中间件的重重层叠，在真正发起action的时候触发函数执行。

See the Pen reduce middlewares by 糊一笑 (@rynxiao) on CodePen.

片段九：redux-actions对state的加工片段

// 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);
}

See the Pen reduce handleActions by 糊一笑 (@rynxiao) on CodePen.

片段十：数据加工器

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);

See the Pen reduce handleActions by 糊一笑 (@rynxiao) on CodePen.

片段十一：对象空值判断

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

See the Pen reduce handleActions by 糊一笑 (@rynxiao) on CodePen.

片段十二：分组

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');

首先通过map计算出所有的键值，然后再根据建值进行归类

See the Pen reduce group-by by 糊一笑 (@rynxiao) on CodePen.

片段十三：对象过滤

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

See the Pen reduce pick by 糊一笑 (@rynxiao) on CodePen.

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]

See the Pen reduce remove by 糊一笑 (@rynxiao) on CodePen.

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')]);

See the Pen reduce runPromisesInSeries by 糊一笑 (@rynxiao) on CodePen.

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']);

See the Pen reduce orderBy by 糊一笑 (@rynxiao) on CodePen.

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');

See the Pen reduce select by 糊一笑 (@rynxiao) on CodePen.

参考地址

https://zhuanlan.zhihu.com/p/27748589

http://www.jb51.net/article/121572.html

Https://github.com/Chalarangelo/30-seconds-of-code#anagrams-of-string-with-duplicates