- 累加
const sum = [1,2,3,4].reduce((accu, cur) => accu + cur, 10); // 20- 数组去重
// 惯例数组const uniqueArr = [1,2,3,4,2,3,2,3,5,1].reduce((accu, cur) => !accu.includes(cur) ? accu.concat(cur) : accu, []);// [1,2,3,4,5]// 等价于：[...new Set([1,2,3,4,2,3,2,3,5,1])];Array.from(new Set(1,2,3,4,2,3,2,3,5,1));// 对象数组(按属性对object去重)const arr = [ {team: 'The clippers', player:'Paul' }, {team: 'The cavaliers', player:'James' }, {team: 'The nets', player:'Durant' }, {team: 'The lakers', player:'James' }];const uniqueArr = arr.reduce((accu, cur) => { if (!accu.find((item) => item.player === cur.player)) { accu.concat(cur); } return accu;}, []); // [// {team: 'The clippers', player:'Paul'},// {team: 'The cavaliers', player:'James'},// {team: 'The nets', player:'Durant'}// ]- 递归实现数组降维
const flattenArray = (arr) => { arr.reduce((accu, cur) => Array.isArray(curr) ? accu.concat(func(curr)) : accu.concat(curr), []);};flattenArray([8, [5, 4], 1, 3, [7, 5, 10, [3, 6, 2]], 4, 3, 2, 4]); // // [ 8, 5, 4, 1, 3, 7, 5, 10, 3, 6, 2, 4, 3, 2, 4 ]- 按属性分组
var people = [ {team: 'The clippers', player:'Paul'}, {team: 'The cavaliers', player:'James'}, {team: 'The nets', player:'Durant'}, {team: 'The lakers', player:'James'}];function groupBy(objectArray, property) { return objectArray.reduce(function (acc, obj) { var key = obj[property]; if (!acc[key]) { acc[key] = []; } acc[key].push(obj); return acc; }, {});}var groupedPlayer = groupBy(people, 'player');// groupedPlayer is:// {// James: [// {team: 'The lakers', player:'James'},// {team: 'The cavaliers', player:'James'},// ],// Paul: [{team: 'The clippers', player:'Paul'}],// Durant: [team: 'The nets', player:'Durant']// }- 频率统计
const names = ['Alice', 'Bob', 'Tiff', 'Bruce', 'Alice'];const countedNames = names.reduce(function (allNames, name) { name in allNames ? allNames[name]++ : allNames[name] = 1; return allNames;}, {});// countedNames is:// { 'Alice': 2, 'Bob': 1, 'Tiff': 1, 'Bruce': 1 }实现map, filter, forEach等的性能
- reduce实现map
Array.prototype.reduceMap = function(callback) { return this.reduce((accu, cur, index) => accu.concat(callback(cur, index)); }, []);};[1,2,3,4].reduceMap((item, index) => item + index); // [1,3,5,7]- reduce实现forEach
Array.prototype.reduceForEach = function(callback) { this.reduce((accu, cur, index, array) => { callback(cur, index, array); }, []);};[1, 2, 3, 4].reduceForEach((item, index, array) => { console.log(item, index, array)});// 1 0 [1,2,3,4]// 2 1 [1,2,3,4]// 3 2 [1,2,3,4]// 4 3 [1,2,3,4]- reduce实现filter
Array.prototype.reduceFilter = function (callback) { return this.reduce((accu, cur, index, array) => callback(cur, index, array)) ? accu.concat(cur) : accu, []);};[1, 2, 3, 4].reduceFilter(item => item % 2 === 0); // [2, 4]- reduce实现find
const obj = [{ a: 1 }, { a: 2 }, { a: 3 }, { a: 4 }]Array.prototype.reduceFind = function (callback) { return this.reduce((accu, cur, index, array) => { if (callback(cur, index, array)) { if (accu instanceof Array && accu.length === 0) { accu = cur; } } if ((index === array.length - 1) && accu instanceof Array && accu.length === 0) { accu = undefined; } return accu; }, []);};obj.reduceFind(item => item.a % 2 == 0); // {a: 2}obj.reduceFind(item => item.a % 9 == 0); // undefined