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js数组高阶方法reduce经典用法代码分享

时间:2019-12-14 12:10:36      阅读:84      评论:0      收藏:0      [点我收藏+]

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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 ? [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 ? 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

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: , actor: 班主任 },
{ name: 王小妞, age: 23, sex: , actor: 英语老师 }
]
},
{
name: 明星班,
teachers: [
{ name: 欧阳娜娜, age: 29, sex: , actor: 班主任 },
{ name: 李易峰, age: 28, sex: , actor: 体育老师 },
{ name: 杨幂, age: 111, sex: , 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

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), {});
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);

 

 

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js数组高阶方法reduce经典用法代码分享

标签:mbed   场景   filter   namespace   col   ted   tst   逗号   ini   

原文地址:https://www.cnblogs.com/GenuisZ/p/12038201.html

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