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js数组方法reduce经典用法代码分享

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 "").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 "color: #ff0000">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 "color: #ff0000">片段五:数组扁平化

const deepFlatten = arr => 
 arr.reduce((a, v) => a.concat(Array.isArray(v) "color: #ff0000">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 "color: #ff0000">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
// 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 "sourceCode">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);
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] "sourceCode">片段十二:分组

const groupBy = (arr, func) =>
arr.map(typeof func === 'function' "sourceCode">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), {});

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)
 "codepen" data-pen-title="reduce pick" data-embed-version="2" data-user="rynxiao" data-default-tab="js" data-slug-hash="QaqONY" data-theme-id="0" data-height="265"> 

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' "codepen" data-pen-title="reduce runPromisesInSeries" data-embed-version="2" data-user="rynxiao" data-default-tab="js" data-slug-hash="QaqOMR" data-theme-id="0" data-height="265">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');

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