reduce的高级用法

01:代替map和filter
const arr = [0, 1, 2, 3];
// 代替map：[0, 2, 4, 6]
const arrA = arr.map(item => item * 2);
const arrB = arr.reduce((items, item) => {
    return [...items, item * 2]
}, []);
 
// 代替filter：[2, 3]
const A = arr.filter(item => {
    return item > 1
});
const B = arr.reduce((items, item) => {
    return item > 1 ? [...items, item] : t
}, []);
 
// 代替map和filter：[4, 6]
const C = arr.map(item => {
    return item * 2
}).filter(item => {
    return item > 2
});
const D = arr.reduce((items, item) => {
    return item * 2 > 2 ? [...items, item * 2] : items
}, []);

02:数组分割
function Chunk(arr = [], size = 1) {
    return arr.length ? arr.reduce((items, item) => {
          return items[items.length - 1].length === size 
          ? items.push([item]) : items[items.length - 1].push(item)
          , items
         }, [[]]) 
      : [];
}
 
 
const arr = [1, 2, 3, 4, 5];
Chunk(arr, 2); 
// [[1, 2], [3, 4], [5]]
 
03:数组过滤
function Difference(arr = [], oarr = []) {
    return arr.reduce((items, item) => {
        return (!oarr.includes(item) && items.push(item), items)
    }, []);
}
 
const arr1 = [1, 2, 3, 4, 5];
const arr2 = [2, 3, 6]
Difference(arr1, arr2); 
// [1, 4, 5]
04:数组填充
function Fill(arr=[],val="",start=0,end=arr.length) {
  if (start < 0 || start >= end || end > arr.length){
      return arr
  };
  return [
      ...arr.slice(0, start),
      ...arr.slice(start, end).reduce((items, item) => {
          return (items.push(val || item), items)
      }, []),
      ...arr.slice(end, arr.length)
  ];
}
 
const arr = [0, 1, 2, 3, 4, 5, 6];
Fill(arr, "aaa", 2, 5); 
// [0, 1, "aaa", "aaa", "aaa", 5, 6]
 
 
05:数组扁平
function Flat(arr = []) {
    return arr.reduce((items, item) => {
        return items.concat(
           Array.isArray(item) ? Flat(item) : item
          )
    }, [])
}
 
 
 
06:数组去重
function Uniq(arr = []) {
    return arr.reduce((items, item) => {
        return items.includes(item) ? items : [...items, item]
        ,[]
    });
}
 
const arr = [2, 1, 0, 3, 2, 1, 2];
Uniq(arr); // [2, 1, 0, 3]
 
 
08:数组最大最小值
function Max(arr = []) {
    return arr.reduce((items, item) => {
        return items > item ? items : item
    });
}
 
function Min(arr = []) {
    return arr.reduce((items, item) => {
        return items < item ? items : item
    });
}
 
 
const arr = [12, 45, 21, 65, 38, 76, 108, 43];
Max(arr); // 108
Min(arr); // 12
 
09:数组成员独立拆解
function Unzip(arr = []) {
    return arr.reduce(
        (items, item) => (item.forEach((w, i) => items[i].push(w)), items),
        Array.from({ length: Math.max(...arr.map(item => item.length)) }).map(v => [])
    );
}
 
const arr = [["a", 1, true], ["b", 2, false]];
Unzip(arr); 
// [["a", "b"], [1, 2], [true, false]]
 
10:对数组成员个数进行统计
此方法是字符统计和单词统计的原理，入参时把字符串处理成数组即可

function Count(arr = []) {
    return arr.reduce((items, item) => {
        return items[item] = (items[item] || 0) + 1, items
    }, {});
}
 
const arr = [0, 1, 1, 2, 2, 2];
Count(arr); 
// { 0: 1, 1: 2, 2: 3 }
 
 
11:对数组成员特性进行分组
function Group(arr = [], key) {
    return key ? arr.reduce((t, v) => (
        !t[v[key]] && (t[v[key]] = []), t[v[key]].push(v), t
    ), {}) : {};
}
 
const arr = [
    { area: "GZ", name: "YZW", age: 27 },
    { area: "GZ", name: "TYJ", age: 25 },
    { area: "SZ", name: "AAA", age: 23 },
    { area: "FS", name: "BBB", age: 21 },
    { area: "SZ", name: "CCC", age: 19 }
]; 
// 以地区area作为分组依据
Group(arr, "area"); 
//{ GZ: Array(2), SZ: Array(2), FS: Array(1) }
 
12:对数组成员包含的关键字进行统计
function Keyword(arr = [], keys = []) {
    return keys.reduce((t, v) => (
        arr.some(w => w.includes(v)) && t.push(v), t),
    []);
}
 
const text = [
    "今天天气真好，我想出去钓鱼",
    "我一边看电视，一边写作业",
    "小明喜欢同桌的小红，又喜欢后桌的小君，真TM花心",
    "最近上班喜欢摸鱼的人实在太多了，代码不好好写，在想入非非"
];
const keyword = ["偷懒", "喜欢", "睡觉", "摸鱼", "真好", "一边", "明天"];
Keyword(text, keyword); 
// ["喜欢", "摸鱼", "真好", "一边"]
 
13:字符串翻转
function ReverseStr(str = "") {
    return str.split("").reduceRight((items, item) => items + item);
}
 
const str = "reduce最牛逼";
ReverseStr(str); 
// "逼牛最ecuder"
 
 
14:累加累乘
function Accumulation(...vals) {
    return vals.reduce((t, v) => t + v, 0);
}
 
function Multiplication(...vals) {
    return vals.reduce((t, v) => t * v, 1);
}
 
Accumulation(1, 2, 3, 4, 5); // 15
Multiplication(1, 2, 3, 4, 5); // 120

  15：权重求和

const scores = [
    { score: 90, subject: "chinese", weight: 0.5 },
    { score: 95, subject: "math", weight: 0.3 },
    { score: 85, subject: "english", weight: 0.2 }
];
const result = scores.reduce((t, v) => t + v.score * v.weight, 0); // 90.5

16:返回对象指定的键值 function GetKeys(obj = {}, keys = []) { return Object.keys(obj).reduce((items, item) => ( keys.includes(item) && (items[item] = obj[item]), items ), {}); } const target = { a: 1, b: 2, c: 3, d: 4 }; const keyword = ["a", "d"]; GetKeys(target, keyword); // { a: 1, d: 4 }