本文翻译并严重删减自five-ways-to-average-with-js-reduce/

如果我们有以下数组：需求是先过滤掉 found 为 false 的 item 再计算求平均值

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几种：

1、for 循环 （可阅读性差，写法并不优雅）

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 分离功能（代码非常干净，实际开发我们更多可能用的是这种方式）

// 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、 使用串联的方式。第二种方式并没有什么问题，只是多了两个中间变量，在可阅读性上我还是更倾向于它。但基于Fluent interface原则(https://en.wikipedia.org/wiki...，我们可以简单改一下函数

// 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、函数编程式。前面三种相信在工作中是很常用到的，这一种方式熟悉 react 的同学肯定不陌生，我们会根据 api 去使用 compose。如果我们给自己设限，要求模仿这种写法去实现呢？强调下在实际开发中可能并没有什么意义，只是说明了 js 的实现不止一种。

// 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、只有一次遍历的情况。上面几种方法实际上都用了三次遍历。如果有一种方法能够只遍历一次呢？需要了解一点数学知识如下图，总之就是经过一系列的公式转换可以实现一次遍历。

// 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);

总结：数学学得好，代码写得少

本文翻译并严重删减自five-ways-to-average-with-js-reduce/ 

翻译来源：https://segmentfault.com/a/1190000019516191

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