js數組高階方法reduce經典用法代碼分享

如下是我的在工做中收藏總結的一些關於javascript數組方法reduce的相關代碼片斷，後續遇到其餘使用這個函數的場景，將會陸續添加，這裏做爲備忘。

javascript數組那麼多方法，爲何我要單挑reduce方法，一個緣由是我對這個方法掌握不夠，不可以用到爲所欲爲。另外一個方面，我也感受到了這個方法的龐大魅力，在許多的場景中發揮着神奇的做用。

理解reduce函數

reduce() 方法接收一個函數做爲累加器（accumulator），數組中的每一個值（從左到右）開始縮減，最終爲一個值。

arr.reduce([callback, initialValue])

看以下例子：

let arr = [1, 2, 3, 4, 5];

// 10表明初始值，p表明每一次的累加值，在第一次爲10
// 若是不存在初始值，那麼p第一次值爲1
// 此時累加的結果爲15
let sum = arr.reduce((p, c) => p + c, 10); // 25
// 轉成es5的寫法即爲：
var sum = arr.reduce(function(p, c) {
 console.log(p);
 return p + c;
}, 10);

片斷一：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"); // 結果會是什麼呢？

reduce負責篩選出每一次執行的首字母，遞歸負責對剩下字母的排列組合。

片斷二：累加器

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

片斷三：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，同時將加上以後的結果做爲下次的初始值。

片斷四：函數柯里化

函數柯里化的目的就是爲了儲存數據，而後在最後一步執行。

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);

經過判斷函數的參數取得當前函數的length(固然也能夠本身指定)，若是所傳的參數比當前參數少，則繼續遞歸下面，同時儲存上一次傳遞的參數。

片斷五：數組扁平化

const deepFlatten = arr => 
 arr.reduce((a, v) => a.concat(Array.isArray(v) ? deepFlatten(v) : v), []);
deepFlatten([1, [2, [3, 4, [5, 6]]]]);

片斷六：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);

片斷七：管道加工器

const pipe = (...funcs) => arg => funcs.reduce((acc, func) => func(acc), arg);
pipe(btoa, x => x.toUpperCase())("Test");

經過對傳遞的參數進行函數加工，以後將加工以後的數據做爲下一個函數的參數，這樣層層傳遞下去。

片斷八：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，經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。

片斷九：redux-actions對state的加工片斷

// 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);
 // 每一種ACTION下對應的reducer處理方式
 const reducers = Reflect.ownkeys(flattenedReducerMap).map(type => handleAction(
  type,
  flattenedReducerMap[type],
  defaultState
 ));
 // 狀態的加工器，用於對reducer的執行
 const reducer = reduceReducers(...reducers);
 // reducer觸發
 return (state = defaultState, action) => reducer(state, action);
}

片斷十：數據加工器

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);

片斷十一：對象空值判斷

let school = {
 name: 'Hope middle school',
 created: '2001',
 classes: [
  {
   name: '三年二班',
   teachers: [
    { name: '張二蛋', age: 26, sex: '男', actor: '班主任' },
    { name: '王小妞', age: 23, sex: '女', actor: '英語老師' }
   ]
  },
  {
   name: '明星班',
   teachers: [
    { name: '歐陽娜娜', age: 29, sex: '女', actor: '班主任' },
    { name: '李易峯', age: 28, sex: '男', actor: '體育老師' },
    { name: '楊冪', age: 111, sex: '女', actor: '藝術老師' }
   ]
  }
 ]
};
// 常規作法
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); // 張二蛋

片斷十二：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'); 

首先經過 map計算出全部的鍵值，而後再根據建值進行歸類

片斷十三：對象過濾

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']);

根據給出的鍵值來遍歷，比較對象中是否存在相同鍵值的的值，而後經過逗號表達式把賦值後的對象賦給下一個的初始值

片斷十四：數組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);

 

首先根據filter函數過濾出數組中符合條件的值，而後使用reduce在原數組中刪除符合條件的值，能夠得出最後arr的值變成了[1, 3]

片斷十五：promise按照順序執行

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')]);

片斷十六：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']);

片斷十七：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');