JavaScript 對象迭代方法與性能比較

原文地址：Iterating Over JavaScript Object Entries and their Performance - 5 Techniques

本文地址：https://ssshooter.com/2020-06...

Object.entries

返回對象全部可枚舉的鍵值對，不會追尋原型鏈上的 key

let obj = {
  key1: 'value1',
  key2: 'value2',
  key3: 'value3',
}

Object.entries(obj).forEach(entry => {
  let key = entry[0]
  let value = entry[1]
  // entry 會是這樣 ["key1", "value1"]
})

Object.keys

返回對象全部可枚舉的鍵

let obj = {
  key1: 'value1',
  key2: 'value2',
  key3: 'value3',
}

Object.keys(obj).forEach(key => {
  let value = obj[key]
})

Object.values

返回對象全部可枚舉的值

let obj = {
  key1: 'value1',
  key2: 'value2',
  key3: 'value3',
}

Object.values(obj).forEach(value => {
  // 只能使用 value
})

for…in loop

迭代可枚舉屬性，會順着原型鏈找下去

let obj = {
  key1: 'value1',
  key2: 'value2',
  key3: 'value3',
}

for (const key in obj) {
  let value = obj[key]

  if (obj.hasOwnProperty(key)) {
    // 自己的
  } else {
    // 來自原型鏈的
  }
}

Object.getOwnPropertyNames

返回對象全部（包括不可枚舉）的鍵（原文說會找原型鏈是錯的）

let obj = {
  key1: 'value1',
  key2: 'value2',
  key3: 'value3',
}

Object.getOwnPropertyNames(obj).forEach(key => {
  let value = obj[key]
})

性能比較

下面的代碼用上面的幾種方法遍歷有 1000000 個屬性的對象，循環 10 次

const { PerformanceObserver, performance } = require('perf_hooks')

let objectSize = 1000000
let iterations = 10

console.log(
  'Starting performance test with %d object size and %d iterations',
  objectSize,
  iterations
)

let values = {
  ENTRIES: 0,
  KEYS: 0,
  VALUES: 0,
  FORIN: 0,
  GETOWP: 0,
}

const obs = new PerformanceObserver(items => {
  let entry = items.getEntries()[0]
  console.log(entry.name, entry.duration)
  values[entry.name] += entry.duration
  performance.clearMarks()
})
obs.observe({ entryTypes: ['measure'] })

function generateObject() {
  let obj = {}
  for (let i = 0; i < objectSize; i++) {
    obj['key' + Math.random()] = 'val' + Math.random()
  }
  return obj
}

for (let i = 0; i < iterations; i++) {
  let obj = generateObject()

  //Object.entries
  performance.mark('A')
  Object.entries(obj).forEach(entry => {
    let key = entry[0]
    let value = entry[1]
  })
  performance.mark('B')
  performance.measure('ENTRIES', 'A', 'B')

  //Object.Keys
  performance.mark('A')
  Object.keys(obj).forEach(key => {
    let value = obj[key]
  })
  performance.mark('B')
  performance.measure('KEYS', 'A', 'B')

  //Object.Values
  performance.mark('A')
  Object.values(obj).forEach(value => {})
  performance.mark('B')
  performance.measure('VALUES', 'A', 'B')

  //For In
  performance.mark('A')
  for (const key in obj) {
    let value = obj[key]
  }
  performance.mark('B')
  performance.measure('FORIN', 'A', 'B')

  //Object.getOwnPropertyNames
  performance.mark('A')
  Object.getOwnPropertyNames(obj).forEach(key => {
    let value = obj[key]
  })
  performance.mark('B')
  performance.measure('GETOWP', 'A', 'B')
}

console.log(
  Object.entries(values).sort((a, b) => {
    return a[1] - b[1]
  })
)

下面的結果是我本身跑的，順序的是指賦值的時候直接用 index，隨機則是鍵值對都插入隨機數，獲得的性能排序是和做者同樣的，也由於 node.js 和 chrome 都是 V8，因此這個應該也是表明在瀏覽器上的性能排序。

// 順序
;[
  ['FORIN', 4677.321499],
  ['KEYS', 4812.776572],
  ['GETOWP', 8610.906197],
  ['VALUES', 9914.674390999999],
  ['ENTRIES', 19338.083694],
]

// 隨機
;[
  ['KEYS', 4502.579589],
  ['FORIN', 4678.013548000001],
  ['GETOWP', 8880.325031999999],
  ['VALUES', 10104.106962],
  ['ENTRIES', 17089.637588999998],
]

以前據說引擎會猜想下一個值讓運行速度更快，看數據彷佛沒有太大影響。

也算是一點乾貨，快點來原文給做者鼓鼓掌吧 👏