不知道大家平常用 Reduce 多不多，反正本瓜用的不多。但实际上，Reduce 能做的，比我们能想到的要多得多，本篇带来 10 个Reduce 常用场景和技巧，一定有你不知道~
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累加/累积
累加我们可能是最熟悉 Reduce 的一种用法，除此之外，还可以用做累积。
// adder
const sum = (...nums) => {
return nums.reduce((sum, num) => sum + num);
};
console.log(sum(1, 2, 3, 4, 10)); // 20
// accumulator
const accumulator = (...nums) => {
return nums.reduce((acc, num) => acc * num);
};
console.log(accumulator(1, 2, 3)); // 6
求最大/最小值
如果你用原生 api 求最大/最小值，无可厚非，Reduce 也能实现同样的效果。
const array = [-1, 10, 6, 5]; const max = Math.max(...array); // 10 const min = Math.min(...array); // -1
const array = [-1, 10, 6, 5]; const max = array.reduce((max, num) => (max > num ? max : num)); const min = array.reduce((min, num) => (min < num ? min : num));
格式化搜索参数
获取 url 上的参数是我们经常面临的需求，用 forEach 遍历可以，用 Reduce 累加更可以，这样可以减少声明 query 对象。
// url https://qianlongo.github.io/vue-demos/dist/index.html?name=fatfish&age=100#/home
// format the search parameters
{
"name": "fatfish",
"age": "100"
}
const parseQuery = () => {
const search = window.location.search;
let query = {};
search
.slice(1)
.split("&")
.forEach((it) => {
const [key, value] = it.split("=");
query[key] = decodeURIComponent(value);
});
return query;
};
const parseQuery = () => {
const search = window.location.search;
return search
.slice(1)
.split("&")
.reduce((query, it) => {
const [key, value] = it.split("=");
query[key] = decodeURIComponent(value);
return query;
}, {});
};
反序列化搜索参数
有了获取 url 参数，就有把参数重新挂在到 url 上面，好用，收藏。
const searchObj = {
name: "fatfish",
age: 100,
// ...
};
const link = `https://medium.com/?name=${searchObj.name}&age=${searchObj.age}`;
// https://medium.com/?name=fatfish&age=100
const stringifySearch = (search = {}) => {
return Object.entries(search)
.reduce(
(t, v) => `${t}${v[0]}=${encodeURIComponent(v[1])}&`,
Object.keys(search).length ? "?" : ""
)
.replace(/&$/, "");
};
const search = stringifySearch({
name: "fatfish",
age: 100,
});
const link = `https://medium.com/${search}`;
console.log(link); // https://medium.com/?name=fatfish&age=100
拉平嵌套数组
我们都会用 .flat(Infinity) 无限拉平所有多维数组成一维数组，只用 reduce 和 flat 也是可以做到这一点的。
const array = [1, [2, [3, [4, [5]]]]]; // expected output [ 1, 2, 3, 4, 5 ] const flatArray = array.flat(Infinity); // [1, 2, 3, 4, 5]
const flat = (array) => {
return array.reduce(
(acc, it) => acc.concat(Array.isArray(it) ? flat(it) : it),
[]
);
};
const array = [1, [2, [3, [4, [5]]]]];
const flatArray = flat(array); // [1, 2, 3, 4, 5]
实现 flat
如果想实现 flat，用 reduce 没错了，又是一个手写原生 api 内部实现，妥妥的刚。
// Expand one layer by default
Array.prototype.flat2 = function (n = 1) {
const len = this.length
let count = 0
let current = this
if (!len || n === 0) {
return current
}
// Confirm whether there are array items in current
const hasArray = () => current.some((it) => Array.isArray(it))
// Expand one layer after each cycle
while (count++ < n && hasArray()) {
current = current.reduce((result, it) => {
result = result.concat(it)
return result
}, [])
}
return current
}
const array = [ 1, [ 2, [ 3, [ 4, [ 5 ] ] ] ] ]
// Expand one layer
console.log(array.flat()) // [ 1, 2, [ 3, [ 4, [ 5 ] ] ] ]
console.log(array.flat2()) // [ 1, 2, [ 3, [ 4, [ 5 ] ] ] ]
// Expand all
console.log(array.flat(Infinity))
console.log(array.flat2(Infinity))
数组去重
数组去重，用 reduce 竟然也可以，写法如下：
const array = [ 1, 2, 1, 2, -1, 10, 11 ] const uniqueArray1 = [ ...new Set(array) ] const uniqueArray2 = array.reduce((acc, it) => acc.includes(it) ? acc : [ ...acc, it ], [])
数组计数
将数组的项进行计数，返回一个 map，分别是每个项重复的次数，reduce 一行代码搞定，收藏！
const count = (array) => {
return array.reduce((acc, it) => (acc.set(it, (acc.get(it) || 0) + 1), acc), new Map())
}
const array = [ 1, 2, 1, 2, -1, 0, '0', 10, '10' ]
console.log(count(array)) // Map(7) {1 => 2, 2 => 2, -1 => 1, 0 => 1, '0' => 1, …}
获取对象多个属性
获取对象的多个属性，然后赋给新的对象，比较笨的做法如下：
// There is an object with many properties
const obj = {
a: 1,
b: 2,
c: 3,
d: 4,
e: 5
// ...
}
// We just want to get some properties above it to create a new object
const newObj = {
a: obj.a,
b: obj.b,
c: obj.c,
d: obj.d
// ...
}
// Do you think this is too inefficient?
用 Reduce 这样解决，就显得明智了许多：
const getObjectKeys = (obj = {}, keys = []) => {
return Object.keys(obj).reduce((acc, key) => (keys.includes(key) && (acc[key] = obj[key]), acc), {});
}
const obj = {
a: 1,
b: 2,
c: 3,
d: 4,
e: 5
// ...
}
const newObj = getObjectKeys(obj, [ 'a', 'b', 'c', 'd' ])
console.log(newObj)
反转字符串
除了 reverse 做数组的翻转，Reduce 也可以，再加上 split，就可以反转字符串啦。
const reverseString = (string) => {
return string.split("").reduceRight((acc, s) => acc + s)
}
const string = 'fatfish'
console.log(reverseString(string)) // hsiftaf
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