js數組方法reduce經典用法

javascript數組那麼多方法，為什麼我要單挑reduce方法，一個原因是我對這個方法掌握不夠，不能夠用到隨心所欲。另一個方面，我也感覺到了這個方法的龐大魅力，在許多的場景中發揮著神奇的作用。本文主要給大家整理了許多關於js數組方法reduce的經典程式碼片段，能夠讓大家更好的理解reduce的實例用法，一起學習下吧。希望能幫助大家。

以下是個人在工作中收藏總結的一些關於javascript數組方法reduce的相關程式碼片段，後續遇到其他使用這個函數的場景，將會陸續添加，這裡作為備忘。

理解reduce函數

reduce() 方法接收一個函數作為累加器（accumulator），數組中的每個值（從左到右）開始縮減，最終為一個值。

arr.reduce([callback, initialValue])

看如下範例：

#

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：字母遊戲

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負責篩選出每一次執行的首字母，遞歸負責剩下字母的排列組合。

片段二：累加器

#

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

## 片段三：計數器

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

 

。循環數組，每遇到一個值與給定值相等，即加1，同時將加上之後的結果作為下次的初始值。

片段四：函數柯里化

函數柯里化的目的就是為了儲存數據，然後在最後一步執行。

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);

透過判斷函數的參數來取得目前函數的

length(當然也可以自己指定)，如果所傳的參數比目前參數少，則繼續遞歸下面，同時儲存上一次傳遞的參數。

片段五：陣列扁平化

#

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：生成菲波列契數組

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");

透過對傳遞的參數進行函數加工，之後將加工之後的資料作為下一個函數的參數，這樣層層傳遞下去。

片段八：中間件

#

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中經典的compose函數中運用了這種方式，透過對中間件的重重層疊，在真正發起action的時候觸發函數執行。

片段九：redux-actions對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：資料加工器

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：物件空值判斷

##

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：分組

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');

先透過map

計算出所有的鍵值，然後再根據建值進行歸類

#片段十三：物件過濾

const pick = (obj, arr) =>
arr.reduce((acc, curr) => (curr in obj && (acc[curr] = obj[curr]), acc), {});

根據給定的鍵值來遍歷，比較物件中是否存在相同鍵值的的值，然後透過逗號表達式把賦值後的物件賦給下一個的初始值

片段十四：在陣列中刪除指定位置的值

#

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);

 

先根據filter函數過濾出數組中符合條件的值，然後使用reduce

在原始數組中刪除符合條件的值，可以得出最後arr的值變成了[1 , 3]

片段十五：promise依照順序執行

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: &#39;fred&#39;, age: 48 }, { name: &#39;barney&#39;, age: 36 }, { name: &#39;fly&#39;, age: 26 }];
orderBy(users, [&#39;name&#39;, &#39;age&#39;], [&#39;asc&#39;, &#39;desc&#39;]); 
orderBy(users, [&#39;name&#39;, &#39;age&#39;]);

##########片段十七：選擇#################### #######

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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JavaScript中數組Reduce()函數的用法和使用技巧

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