计算平均值的不同实现方式

摘要：本文翻译并严重删减自如果我们有以下数组需求是先过滤掉为的再计算求平均值一般想到的方法不外乎是一下几种循环可阅读性差，写法并不优雅使用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使用串联的方式。

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