优化 JavaScript 既有趣又有利可图

// setup
let _ = 0

// 1. monomorphic
const o1 = { a: 1, b: _, c: _, d: _, e: _ }
const o2 = { a: 1, b: _, c: _, d: _, e: _ }
const o3 = { a: 1, b: _, c: _, d: _, e: _ }
const o4 = { a: 1, b: _, c: _, d: _, e: _ }
const o5 = { a: 1, b: _, c: _, d: _, e: _ } // all shapes are equal

// 2. polymorphic
const o1 = { a: 1, b: _, c: _, d: _, e: _ }
const o2 = { a: 1, b: _, c: _, d: _, e: _ }
const o3 = { a: 1, b: _, c: _, d: _, e: _ }
const o4 = { a: 1, b: _, c: _, d: _, e: _ }
const o5 = { b: _, a: 1, c: _, d: _, e: _ } // this shape is different

// 3. megamorphic
const o1 = { a: 1, b: _, c: _, d: _, e: _ }
const o2 = { b: _, a: 1, c: _, d: _, e: _ }
const o3 = { b: _, c: _, a: 1, d: _, e: _ }
const o4 = { b: _, c: _, d: _, a: 1, e: _ }
const o5 = { b: _, c: _, d: _, e: _, a: 1 } // all shapes are different

// test case
function add(a1, b1) {
  return a1.a + a1.b + a1.c + a1.d + a1.e +
         b1.a + b1.b + b1.c + b1.d + b1.e }
 
let result = 0
for (let i = 0; i < 1000000; i++) {
  result += add(o1, o2)
  result += add(o3, o4)
  result += add(o4, o5)
}

// setup:
const numbers = Array.from({ length: 10_000 }).map(() => Math.random())

// 1. functional
const result =
  numbers
    .map(n => Math.round(n * 10))
    .filter(n => n % 2 === 0)
    .reduce((a, n) => a + n, 0)

// 2. imperative
let result = 0
for (let i = 0; i < numbers.length; i++) {
  let n = Math.round(numbers[i] * 10)
  if (n % 2 !== 0) continue
  result = result + n
}

// 1. proxy access
const point = new Proxy({ x: 10, y: 20 }, { get: (t, k) => t[k] })
 
for (let _ = 0, i = 0; i < 100_000; i++) { _ += point.x }

// 2. direct access
const point = { x: 10, y: 20 }
const x = point.x
 
for (let _ = 0, i = 0; i < 100_000; i++) { _ += x }

// 1. nested access
const a = { state: { center: { point: { x: 10, y: 20 } } } }
const b = { state: { center: { point: { x: 10, y: 20 } } } }
const get = (i) => i % 2 ? a : b
 
let result = 0
for (let i = 0; i < 100_000; i++) {
  result = result + get(i).state.center.point.x }

// 2. direct access
const a = { x: 10, y: 20 }.x
const b = { x: 10, y: 20 }.x
const get = (i) => i % 2 ? a : b
 
let result = 0
for (let i = 0; i < 100_000; i++) {
  result = result + get(i) }

// setup:
const K = 1024
const length = 1 * K * K
 
// Theses points are created one after the other, so they are allocated
// sequentially in memory.
const points = new Array(length)
for (let i = 0; i < points.length; i++) {
  points[i] = { x: 42, y: 0 }
}
 
// This array contains the *same data* as above, but shuffled randomly.
const shuffledPoints = shuffle(points.slice())

// 1. sequential
let _ = 0
for (let i = 0; i < points.length; i++) { _ += points[i].x }

// 2. random
let _ = 0
for (let i = 0; i < shuffledPoints.length; i++) { _ += shuffledPoints[i].x }

// setup:
const KB = 1024
const MB = 1024 * KB
 
// These are approximate sizes to fit in those caches. If you don't get the
// same results on your machine, it might be because your sizes differ.
const L1  = 256 * KB
const L2  =   5 * MB
const L3  =  18 * MB
const RAM =  32 * MB
 
// We'll be accessing the same buffer for all test cases, but we'll
// only be accessing the first 0 to `L1` entries in the first case,
// 0 to `L2` in the second, etc.
const buffer = new Int8Array(RAM)
buffer.fill(42)
 
const random = (max) => Math.floor(Math.random() * max)

// 1. L1
let r = 0; for (let i = 0; i < 100000; i++) { r += buffer[random(L1)] }

// 2. L2
let r = 0; for (let i = 0; i < 100000; i++) { r += buffer[random(L2)] }

// 3. L3
let r = 0; for (let i = 0; i < 100000; i++) { r += buffer[random(L3)] }

// 4. RAM
let r = 0; for (let i = 0; i < 100000; i++) { r += buffer[random(RAM)] }

// setup:
const USERS_LENGTH = 1_000

// setup:
const byId = {}
Array.from({ length: USERS_LENGTH }).forEach((_, id) => {
  byId[id] = { id, name: 'John'}
})
let _ = 0

// 1. [] access
Object.keys(byId).forEach(id => { _ += byId[id].id })

// 2. direct access
Object.values(byId).forEach(user => { _ += user.id })

// setup:
const USERS_LENGTH = 100_000

// setup:
const key = 'requestId'
const values = Array.from({ length: 100_000 }).fill(42)

// 1. without eval
function createMessages(key, values) {
  const messages = []
  for (let i = 0; i < values.length; i++) {
    messages.push({ [key]: values[i] })
  }
  return messages
}
 
createMessages(key, values)

// 2. with eval
function createMessages(key, values) {
  const messages = []
  const createMessage = new Function('value',
    `return { ${JSON.stringify(key)}: value }`
  )
  for (let i = 0; i < values.length; i++) {
    messages.push(createMessage(values[i]))
  }
  return messages
}
 
createMessages(key, values)

// setup:
const classNames = ['primary', 'selected', 'active', 'medium']

// 1. mutation
const result =
  classNames
    .map(c => `button--${c}`)
    .join(' ')

// 2. concatenation
const result =
  classNames
    .map(c => 'button--' + c)
    .reduce((acc, c) => acc + ' ' + c, '')

// setup:
const descriptions = ['apples', 'oranges', 'bananas', 'seven']
const someTags = {
  apples: '::promotion::',
}
const noTags = {}
 
// Turn the products into a string, with their tags if applicable
function productsToString(description, tags) {
  let result = ''
  description.forEach(product => {
    result += product
    if (tags[product]) result += tags[product]
    result += ', '
  })
  return result
}
 
// Specialize it now
function productsToStringSpecialized(description, tags) {
  // We know that `tags` is likely to be empty, so we check
  // once ahead of time, and then we can remove the `if` check
  // from the inner loop
  if (isEmpty(tags)) {
    let result = ''
    description.forEach(product => {
      result += product + ', '
    })
    return result
  } else {
    let result = ''
    description.forEach(product => {
      result += product
      if (tags[product]) result += tags[product]
      result += ', '
    })
    return result
  }
}
function isEmpty(o) { for (let _ in o) { return false } return true }
 

// 1. not specialized
for (let i = 0; i < 100; i++) {
  productsToString(descriptions, someTags)
  productsToString(descriptions, noTags)
  productsToString(descriptions, noTags)
  productsToString(descriptions, noTags)
  productsToString(descriptions, noTags)
}

// 2. specialized
for (let i = 0; i < 100; i++) {
  productsToStringSpecialized(descriptions, someTags)
  productsToStringSpecialized(descriptions, noTags)
  productsToStringSpecialized(descriptions, noTags)
  productsToStringSpecialized(descriptions, noTags)
  productsToStringSpecialized(descriptions, noTags)
}

// setup:
const userIds = Array.from({ length: 1_000 }).map((_, i) => i)
const adminIdsArray = userIds.slice(0, 10)
const adminIdsSet = new Set(adminIdsArray)

// 1. Array
let _ = 0
for (let i = 0; i < userIds.length; i++) {
  if (adminIdsArray.includes(userIds[i])) { _ += 1 }
}

// 2. Set
let _ = 0
for (let i = 0; i < userIds.length; i++) {
  if (adminIdsSet.has(userIds[i])) { _ += 1 }
}