Learn how two common array functions - map() and filter() - are syntactic sugar for reduce operations. Learn how to use them, how to compose them, and how using reduce can give you a big performance boost over composing filters and maps over a large…

Take away: Always check you ruturn the accumulator Always pass in the inital value var data = ["vote1", "vote2", "vote1", "vote2"]; var reducer = function(acc, value){ if(acc[value]){ acc[value] = acc[value] + 1; }e…

原文  http://aotu.io/notes/2016/04/15/2016-04-14-js-reduce/   JavaScript中reduce()方法不完全指南 reduce() 方法接收一个函数作为累加器,数组中的每个值(从左到右)开始缩减,最终为一个值,是ES5中新增的又一个数组逐项处理方法,那reduce方法跟foreach.map等数组方法又有啥区别呢. arr.reduce(callback[, initialValue]) - More From MDN callback…

Learn a few advanced reduction patterns: flatten allows you to merge a set of arrays into a single array, the dreaded flatmap allows you to convert an array of objects into an array of arrays which then get flattened, and reduceRight allows you to in…

Sometimes we need to turn arrays into new values in ways that can't be done purely by passing an accumulator along with no knowledge about its context. Learn how to reduce an array of numbers into its mathematical mean in a single reduce step by usin…

Learn how to use array reduction to create functional pipelines by composing arrays of functions. const increase = (input) => { return input + 1; } const decrease = (input) => { return input - 1; } const double = (input) => { return input * 2; }…

以前看到reduce方法,总是看得我头皮发麻,今天无意间又遇到他了,于是学习了下,接触之后,觉得这个方法还挺好用的,在很多地方都可以派上用场,比如,数组中元素求和.数组去重.求数组中的最大值或最小值等等都可以用到它. reduce() 方法接收一个函数作为累加器,数组中的每个值(从左到右)开始缩减,最终计算为一个值. 语法:array.reduce(function(total, currentValue, currentIndex, arr), initialValue) 可以看出它接收一个回…

23333333333333 map var s = []; for(let i=0;i<10;i++){ s.push(i); } function pow(x){ return x*x; } console.log(s.map(pow)); console.log(s.map(String)); reduce()方法接收一个函数作为累加器(accumulator),数组中的每个值(从左到右)开始缩减,最终为一个值. var arr = [10,20,30,40,50]; var reduce…

需求一 假设有一个数组,需要对其中的元素进行求和. const numbers = [1, -1, 2, 3]; 传统写法,使用for循环求和 const numbers = [1, -1, 2, 3]; let sum = 0; for(let n of numbers) sum += n; console.log(sum); // 5 使用reduce求和 reduce()函数的第一个参数是一个callback function,这个function中有2个参数,accumulator相当于…

过去有很长一段时间,我一直很难理解 reduce() 这个方法的具体用法,平时也很少用到它.事实上,如果你能真正了解它的话,其实在很多地方我们都可以用得上,那么今天我们就来简单聊聊 JS 中 reduce () 的用法吧.   案例一:   下面开始我们提供一个包含对象的原始数组: 下面我们用reduce()写一个去重的方法: 首先用一个空对象做准备 然后用一个新值去接受,其中x代表的是结尾的空数组,而这里的y代表的是原数组的所有数据. 空对象里拿数据的id做比较,用三目运算符如果结果为true…