js数组方法reduce经典用法代码分享
网络 2018-01-19 1454
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。
1 | arr.reduce([callback, initialValue]) |
看如下例子：
1 2 3 4 5 6 7 8 9 10 11 | 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
1 2 3 4 5 6 7 8 9 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？ |
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
1 2 | const sum = arr => arr.reduce((acc, val) => acc + val, 0); sum([1, 2, 3]); |
片段三：计数器
1 2 | 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。
1 2 3 4 | 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
1 2 3 | 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
1 2 | 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
1 2 | 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
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 | 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。
片段九：redux-actions对state的加工片段
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 | // 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); } |
片段十：数据加工器
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 | 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); |
片段十一：对象空值判断
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 | 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 |
片段十二：分组
1 2 3 4 5 6 7 | 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
片段十三：对象过滤
const pick = (obj, arr) =>
arr.reduce((acc, curr) => (curr in obj && (acc[curr] = obj[curr]), acc), {});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
1 2 3 4 5 6 7 8 | 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
1 2 3 4 | 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
1 2 3 4 5 6 7 8 9 10 11 12 13 | 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
1 2 3 4 | 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'); |
