摘要：本文翻譯并嚴(yán)重刪減自如果我們有以下數(shù)組需求是先過濾掉為的再計(jì)算求平均值一般想到的方法不外乎是一下幾種循環(huán)可閱讀性差，寫法并不優(yōu)雅使用分離功能代碼非常干凈，實(shí)際開發(fā)我們更多可能用的是這種方式使用串聯(lián)的方式。

本文翻譯并嚴(yán)重刪減自five-ways-to-average-with-js-reduce/

如果我們有以下數(shù)組：需求是先過濾掉 found 為 false 的 item 再計(jì)算求平均值

const victorianSlang = [
  {
    term: "doing the bear",
    found: true,
    popularity: 108
  },
  {
    term: "katterzem",
    found: false,
    popularity: null
  },
  {
    term: "bone shaker",
    found: true,
    popularity: 609
  },
  {
    term: "smothering a parrot",
    found: false,
    popularity: null
  }
  //……
];

一般想到的方法不外乎是一下幾種：

1、for 循環(huán) （可閱讀性差，寫法并不優(yōu)雅）

let popularitySum = 0;
let itemsFound = 0;
const len = victorianSlang.length;
let item = null;
for (let i = 0; i < len; i++) {
  item = victorianSlang[i];
  if (item.found) {
    popularitySum = item.popularity + popularitySum;
    itemsFound = itemsFound + 1;
  }
}
const averagePopularity = popularitySum / itemsFound;
console.log("Average popularity:", averagePopularity);

2、 使用 filter/map/reduce 分離功能（代碼非常干凈，實(shí)際開發(fā)我們更多可能用的是這種方式）

// Helper functions
// ----------------------------------------------------------------------------
function isFound(item) {
  return item.found;
}

function getPopularity(item) {
  return item.popularity;
}

function addScores(runningTotal, popularity) {
  return runningTotal + popularity;
}

// Calculations
// ----------------------------------------------------------------------------

// Filter out terms that weren"t found in books.
const foundSlangTerms = victorianSlang.filter(isFound);

// Extract the popularity scores so we just have an array of numbers.
const popularityScores = foundSlangTerms.map(getPopularity);

// Add up all the scores total. Note that the second parameter tells reduce
// to start the total at zero.
const scoresTotal = popularityScores.reduce(addScores, 0);

// Calculate the average and display.
const averagePopularity = scoresTotal / popularityScores.length;
console.log("Average popularity:", averagePopularity);

3、 使用串聯(lián)的方式。第二種方式并沒有什么問題，只是多了兩個(gè)中間變量，在可閱讀性上我還是更傾向于它。但基于Fluent interface原則(https://en.wikipedia.org/wiki...，我們可以簡單改一下函數(shù)

// Helper functions
// ---------------------------------------------------------------------------------
function isFound(item) {
  return item.found;
}

function getPopularity(item) {
  return item.popularity;
}

// We use an object to keep track of multiple values in a single return value.
function addScores({ totalPopularity, itemCount }, popularity) {
  return {
    totalPopularity: totalPopularity + popularity,
    itemCount: itemCount + 1
  };
}

// Calculations
// ---------------------------------------------------------------------------------
const initialInfo = { totalPopularity: 0, itemCount: 0 };
const popularityInfo = victorianSlang
  .filter(isFound)
  .map(getPopularity)
  .reduce(addScores, initialInfo);
const { totalPopularity, itemCount } = popularityInfo;
const averagePopularity = totalPopularity / itemCount;
console.log("Average popularity:", averagePopularity);

4、函數(shù)編程式。前面三種相信在工作中是很常用到的，這一種方式熟悉 react 的同學(xué)肯定不陌生，我們會(huì)根據(jù) api 去使用 compose。如果我們給自己設(shè)限，要求模仿這種寫法去實(shí)現(xiàn)呢？強(qiáng)調(diào)下在實(shí)際開發(fā)中可能并沒有什么意義，只是說明了 js 的實(shí)現(xiàn)不止一種。

// Helpers
// ----------------------------------------------------------------------------
const filter = p => a => a.filter(p);
const map = f => a => a.map(f);
const prop = k => x => x[k];
const reduce = r => i => a => a.reduce(r, i);
const compose = (...fns) => arg => fns.reduceRight((arg, fn) => fn(arg), arg);

// The blackbird combinator.
// See: https://jrsinclair.com/articles/2019/compose-js-functions-multiple-parameters/
const B1 = f => g => h => x => f(g(x))(h(x));

// Calculations
// ----------------------------------------------------------------------------

// We"ll create a sum function that adds all the items of an array together.
const sum = reduce((a, i) => a + i)(0);

// A function to get the length of an array.
const length = a => a.length;

// A function to divide one number by another.
const div = a => b => a / b;

// We use compose() to piece our function together using the small helpers.
// With compose() you read from the bottom up.
const calcPopularity = compose(
  B1(div)(sum)(length),
  map(prop("popularity")),
  filter(prop("found"))
);

const averagePopularity = calcPopularity(victorianSlang);
console.log("Average popularity:", averagePopularity);

5、只有一次遍歷的情況。上面幾種方法實(shí)際上都用了三次遍歷。如果有一種方法能夠只遍歷一次呢？需要了解一點(diǎn)數(shù)學(xué)知識(shí)如下圖，總之就是經(jīng)過一系列的公式轉(zhuǎn)換可以實(shí)現(xiàn)一次遍歷。

// Average function
// ----------------------------------------------------------------------------

function averageScores({ avg, n }, slangTermInfo) {
  if (!slangTermInfo.found) {
    return { avg, n };
  }
  return {
    avg: (slangTermInfo.popularity + n * avg) / (n + 1),
    n: n + 1
  };
}

// Calculations
// ----------------------------------------------------------------------------

// Calculate the average and display.
const initialVals = { avg: 0, n: 0 };
const averagePopularity = victorianSlang.reduce(averageScores, initialVals).avg;
console.log("Average popularity:", averagePopularity);

總結(jié)：數(shù)學(xué)學(xué)得好，代碼寫得少