JavaScript中Reduce10个常用场景技巧
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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