JavaScript对象迭代方法和性能的比较

返回对象所有可枚举的键值对，不会追寻原型链上的 key

let obj = {
  key1: &＃39;value1&＃39;,
  key2: &＃39;value2&＃39;,
  key3: &＃39;value3&＃39;,
}
Object.entries(obj).forEach(entry => {
  let key = entry[0]
  let value = entry[1]
  // entry 会是这样 ["key1", "value1"]
})

Object.keys

返回对象所有可枚举的键

let obj = {
  key1: &＃39;value1&＃39;,
  key2: &＃39;value2&＃39;,
  key3: &＃39;value3&＃39;,
}
Object.keys(obj).forEach(key => {
  let value = obj[key]
})

Object.values

返回对象所有可枚举的值

let obj = {
  key1: &＃39;value1&＃39;,
  key2: &＃39;value2&＃39;,
  key3: &＃39;value3&＃39;,
}
Object.values(obj).forEach(value => {
  // 只能使用 value
})

for…in loop

迭代可枚举属性，会顺着原型链找下去

let obj = {
  key1: &＃39;value1&＃39;,
  key2: &＃39;value2&＃39;,
  key3: &＃39;value3&＃39;,
}
for (const key in obj) {
  let value = obj[key]
  if (obj.hasOwnProperty(key)) {
    // 本身的
  } else {
    // 来自原型链的
  }
}

Object.getOwnPropertyNames

返回对象所有（包括不可枚举）的键（原文说会找原型链是错的）

let obj = {
  key1: &＃39;value1&＃39;,
  key2: &＃39;value2&＃39;,
  key3: &＃39;value3&＃39;,
}
Object.getOwnPropertyNames(obj).forEach(key => {
  let value = obj[key]
})

性能比较

下面的代码用上面的几种方法遍历有 1000000 个属性的对象，循环 10 次

const { PerformanceObserver, performance } = require(&＃39;perf_hooks&＃39;)

let objectSize = 1000000
let iteratiOns= 10

console.log(
  &＃39;Starting performance test with %d object size and %d iterations&＃39;,
  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: [&＃39;measure&＃39;] })

function generateObject() {
  let obj = {}
  for (let i = 0; i  {
    let key = entry[0]
    let value = entry[1]
  })
  performance.mark(&＃39;B&＃39;)
  performance.measure(&＃39;ENTRIES&＃39;, &＃39;A&＃39;, &＃39;B&＃39;)

  //Object.Keys
  performance.mark(&＃39;A&＃39;)
  Object.keys(obj).forEach(key => {
    let value = obj[key]
  })
  performance.mark(&＃39;B&＃39;)
  performance.measure(&＃39;KEYS&＃39;, &＃39;A&＃39;, &＃39;B&＃39;)

  //Object.Values
  performance.mark(&＃39;A&＃39;)
  Object.values(obj).forEach(value => {})
  performance.mark(&＃39;B&＃39;)
  performance.measure(&＃39;VALUES&＃39;, &＃39;A&＃39;, &＃39;B&＃39;)

  //For In
  performance.mark(&＃39;A&＃39;)
  for (const key in obj) {
    let value = obj[key]
  }
  performance.mark(&＃39;B&＃39;)
  performance.measure(&＃39;FORIN&＃39;, &＃39;A&＃39;, &＃39;B&＃39;)

  //Object.getOwnPropertyNames
  performance.mark(&＃39;A&＃39;)
  Object.getOwnPropertyNames(obj).forEach(key => {
    let value = obj[key]
  })
  performance.mark(&＃39;B&＃39;)
  performance.measure(&＃39;GETOWP&＃39;, &＃39;A&＃39;, &＃39;B&＃39;)
}

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

下面的结果是我自己跑的，顺序的是指赋值的时候直接用 index，随机则是键值对都插入随机数，得到的性能排序是和作者一样的，也因为 node.js 和 chrome 都是 V8，所以这个应该也是代表在浏览器上的性能排序。

// 顺序
;[
  [&＃39;FORIN&＃39;, 4677.321499],
  [&＃39;KEYS&＃39;, 4812.776572],
  [&＃39;GETOWP&＃39;, 8610.906197],
  [&＃39;VALUES&＃39;, 9914.674390999999],
  [&＃39;ENTRIES&＃39;, 19338.083694],
]

// 随机
;[
  [&＃39;KEYS&＃39;, 4502.579589],
  [&＃39;FORIN&＃39;, 4678.013548000001],
  [&＃39;GETOWP&＃39;, 8880.325031999999],
  [&＃39;VALUES&＃39;, 10104.106962],
  [&＃39;ENTRIES&＃39;, 17089.637588999998],
]

之前听说引擎会猜测下一个值让运行速度更快，看数据似乎没有太大影响。

也算是一点干货，快点来原文给作者鼓鼓掌吧

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