for,for...of, for...in, array iteration 运行速度测试

for,for…of, for…in, array iteration 运行速度测试

  • 由于不同浏览器,不同版本性能不一,且控制台本质是是套用了一大堆eval,沙盒化程度高,所以需使用node环境测试来提高准确性
    1
    2
    3
    4
    5
    6
    7
    8
    9
    10
    11
    12
    13
    14
    15
    16
    17
    18
    19

    // 准备待测数组
    const NUM = 1e7;
    let arr = new Array(NUM).fill(1);
    // for 测试
    let arr1 =[];
    console.time('for');
    for (let i = 0; i < arr.length; i++) {
    arr1.push(arr[i])
    }

    console.timeEnd('for');


    // Chrome/75.0.3770.100 Safari/537.36 环境
    //VM1324:10 for: 576.733154296875ms

    // node v10.11.0 环境
    // for: 412.087ms

for 几种写法

  • 常规写法

    1
    2
    3
    4
    5
    6
    let arr1 = []
    console.time('one')
    for (let i = 0; i < arr.length; i++ ){
    arr1.push(arr[i])
    }
    console.timeEnd('one')

    数组长度是会动态变化,每次循环会重新计算length长度,可能会出现死循环

  • cache arr.length

    1
    2
    3
    for (let i = 0, len = arr.length; i < len; i++ ){
    arr1.push(arr[i])
    }

    缓存length 值,无需重新计算length

  • 倒序

    1
    2
    3
    for (let i = arr.length-1; i >= 0; i--){
    arr1.push(arr[i])
    }

    比第二种方法更简洁

  • 倒叙简洁版

    1
    2
    3
    for (let i = arr.length-1; i--;){
    arr1.push(arr[i])
    }

    两个分号之间的表达式为 true 会一直执行直到 判断为 false (i = 0)

  • 正序简洁版

    1
    2
    3
    for (let i = 0, len;len = arr[i++]; ){
    arr1.push(arr[i])
    }

    当 i 大于等于数组长度或arr[i++]值为false时 将停指循环,同时由于arr.length动态变化时可能会造成死循环

for…of

1
2
3
for (let value of arr){
arr1.push(value)
}

es6推出的迭代器,最简洁,可以是用 break,continue和return 终止循环

for…in

1
2
3
for (let key in arr){
arr1.push(arr[key])
}

for…in 一般用于遍历对象,他会将本身属性和原型链上的属性(除系统内置属性)全部遍历出来即便是不可枚举属性(enumerable:false), 可以通过 items.hasOwnProperty来遍历本身属性,由于查询到自己的原型链上,所以性能方面比较差

forEach

1
2
3
arr.forEach(function(value){
arr1.push(value)
})

数组的迭代方法,没有返回值

map

1
2
3
arr.map(function(value){
return arr1.push(value)
})

浅拷贝原数组,并且返回一个新数组

性能测试

测试次数 常规for cache for 倒序 for 倒叙简版 for 正序简版 for for..of for..in forEach map
1 542.121ms 573.618ms 764.181ms 755.961ms 571.464ms 945.199ms 4077.020ms 625.859ms 3573.946ms
2 430.008ms 541.933ms 524.474ms 668.276ms 553.475ms 897.442ms 4402.246ms 605.271ms 2732.859ms
3 409.531ms 661.765ms 534.167ms 655.481ms 600.939ms 1141.093ms 3806.704ms 584.712ms 2779.192ms
4 412.972ms 643.868ms 536.026ms 674.081ms 725.149ms 930.655ms 3201.387ms 599.780ms 3152.499ms
5 417.034ms 624.323ms 520.674ms 799.568ms 574.713ms 943.449ms 3261.512ms 587.182ms 2954.195ms
6 525.771ms 955.737ms 526.208ms 771.443ms 531.962ms 954.199ms 4351.009ms 608.264ms 2888.752ms
7 498.039ms 602.703ms 555.588ms 531.464ms 541.599ms 916.678ms 3264.334ms 596.168ms 2834.663ms
8 431.694ms 523.381ms 544.974ms 527.472ms 517.833ms 1049.283ms 3744.972ms 600.286ms 3467.499ms
9 417.521ms 518.093ms 547.404ms 611.024ms 594.503ms 767.059ms 4979.348ms 601.420ms 3638.023ms
10 424.806ms 557.961ms 535.541ms 837.561ms 541.882ms 772.686ms 3284.424ms 602.443ms 3599.642ms
11 409.402ms 521.131ms 534.265ms 517.709ms 551.397ms 752.101ms 3228.123ms 629.625ms 3535.545ms
12 425.362ms 532.882ms 406.637ms 522.287ms 570.259ms 914.135ms 3449.256ms 800.857ms 3429.123ms
平均值 439.2738ms 578.3565ms 553.1468ms 651.7057ms 507.8718ms 909.0785ms 3,686.96ms 605.6298ms 3,221.5056ms
堆值差 357245536 Byte 357245808 Byte 357245624 Byte 357245872 Byte 357246824 Byte 199268080 Byte 757187208 Byte 357244456 Byte 437247640 Byte

测试总结

  1. 运行效率: 常规for > 正序简版 for> 倒序 for > cache for > 倒叙简版 for> forEach > for..of > map > for..in
  2. 几种普通 for 循环占用内存相差不大, 而 for..of 占用运行内存最小
  3. for..in 性能最差,内存占用高,速度很慢
--------------------- Thank you for reading ---------------------