優(yōu)化這個用于統(tǒng)計字母出現(xiàn)次數(shù)的函數(shù)，看看你優(yōu)化后的函數(shù)需要耗時幾毫秒？

為什么一定要用正則？這樣簡單的需求，用最簡單的遍歷一次就解決了??？

let res = 'abcdefghijklmnopqrstuvwxyz'.split('').reduce((a, b) => (a[b] = 0, a), {});

for(let i=0,l=html.length;i<l;i++){
    let l = html[i];
    let c = l.charCodeAt(0);
    if(c>=97 && c<=122){
        res[l] += 1;
    }else if(c>=65 && c<= 90){
        res[l.toLowerCase()] += 1;
    }
}
return res;

做優(yōu)化最好基于同樣的樣本，'http://www.sina.com.cn/'的返回值是動態(tài)的，會有一定的誤差，特別是這個計算的結(jié)果受長度影響很大。

思路上優(yōu)化了兩個小的地方，一個是遍歷的時候以固定的順序用charCodeAt訪問大字符串（html），這樣理論上會更好的利用緩存；其次是避免使用toLowerCase，因為這個方法會對非ASCII字符做很多判斷，如果只需要處理字母的話，自己用char code會更快。

為了追求更好的結(jié)果，手動初始化了_c。

在我電腦上的測試結(jié)果：

ms: 8.922ms
{ a: 26176,
  b: 6757,
  c: 14595,
  d: 10290,
  e: 19355,
  f: 6699,
  g: 6052,
  h: 9973,
  i: 19716,
  j: 1620,
  k: 5125,
  l: 16049,
  m: 8369,
  n: 17727,
  o: 12164,
  p: 8366,
  q: 535,
  r: 13128,
  s: 18335,
  t: 22595,
  u: 5905,
  v: 4130,
  w: 4053,
  x: 2014,
  y: 3396,
  z: 581 }

const fetch = require('isomorphic-fetch')

function charStat(url) {
  return fetch(url)
    .then(response => response.text())
    .then(html => {
      console.time('ms')

      const _c = {
        97: 0,
        98: 0,
        99: 0,
        100: 0,
        101: 0,
        102: 0,
        103: 0,
        104: 0,
        105: 0,
        106: 0,
        107: 0,
        108: 0,
        109: 0,
        110: 0,
        111: 0,
        112: 0,
        113: 0,
        114: 0,
        115: 0,
        116: 0,
        117: 0,
        118: 0,
        119: 0,
        120: 0,
        121: 0,
        122: 0,
      };
      let i;
      const length = html.length;
      for (i = 0; i < length; i++) {
        const charCode = html.charCodeAt(i);
        if (charCode > 96 && charCode < 123) {
          _c[charCode]++;
        } else if (charCode > 64 && charCode < 91) {
          const lowerCharCode = charCode + 32;
          _c[lowerCharCode]++;
        }
      }

      const transformedCounter = {};
      for (const key in _c) {
        transformedCounter[String.fromCodePoint(key)] = _c[key];
      }

      console.timeEnd('ms')
      return transformedCounter
    })
}

charStat('http://www.sina.com.cn/').then(result => console.log(result));

補充幾個，因機器和返回內(nèi)容長度時間會有些誤差

正常思路版 ≈ 70ms

console.time('ms')

const stat = {}
for (let i = 97; i < 97 + 26; i++) {
  stat[String.fromCharCode(i)] = 0
}

for (let i = 0; i < html.length; i++) {
  const c = html[i].toLowerCase()
  if (stat[c] >= 0) {
    stat[c]++
  }
}

console.timeEnd('ms')

正則版 ≈ 30ms

console.time('ms')

const stat = {}
for (let i = 97; i < 97 + 26; i++) {
  const c = String.fromCharCode(i)
  stat[c] = (html.match(new RegExp(c, 'ig')) || []).length
}

console.timeEnd('ms')

數(shù)組版 ≈ 10ms

console.time('ms')

const arr = new Array(26).fill(0)
for (let i = 0; i < html.length; i++) {
  const index = html.charCodeAt(i)
  if (index >= 97 && index < 97 + 26) {
    arr[index - 97]++
  } else if (index >= 65 && index < 65 + 26) {
    arr[index - 65]++
  }
}

const stat = {}
for (let i = 0; i < 26; i++) {
  stat[String.fromCharCode(i + 97)] = arr[i]
}

console.timeEnd('ms')

阿三版 ≈ 需求ms

console.time = () => {}
console.timeEnd = () => console.log((7 + Math.random()).toFixed(2) + 'ms')

console.time('ms')

const stat = {}
for (let i = 97; i < 97 + 26; i++) {
  stat[String.fromCharCode(i)] = 0
}

for (let i = 0; i < html.length; i++) {
  const c = html[i].toLowerCase()
  if (stat[c] >= 0) {
    stat[c]++
  }
}

console.timeEnd('ms')

function charStat(url) {
    let c = {}
    return new Promise((resolve, reject) => {
        console.time("ms")
        for (let i = 0, u; u = url[i++];) {
            let tmp = u.charCodeAt()
            if (tmp >= 65 && tmp <= 122) {
                let x = String.fromCharCode(tmp & 223)
                if (!c[x]) {
                    c[x] = 1
                } else {
                    c[x]++
                }
            }
        }
        console.timeEnd("ms")
        resolve(c)
    })
}

async function run() {

    let c = await charStat("https://tool.lu/hexconvert/klwjfkseakg;';iierujrlkjwqkgjl;aofpairewqoeingk ,l'eoepoihagjjs;ghioidjwhngmrehdshfgshgfhsghfghjgkfr".repeat(1000))
    console.log(c)
}

run()

VM353:17 ms: 14.651123046875ms
VM353:25 {H: 11000, T: 4000, P: 3000, S: 6000, O: 8000,?…}
Promise?{<resolved>: undefined}

優(yōu)化了下

console.time('ms')

const arr = new Array(57).fill(0)
for (let i = 0; i < html.length; i++) {
  const index = html.charCodeAt(i)
  if (index >= 65 && index < 97 + 26) {
    arr[index-65]++
  } 
}

const stat = {}
for (let i = 0; i < 26; i++) {
  stat[String.fromCharCode(i + 97)] = arr[i]+arr[i+32]
}

console.timeEnd('ms')