JavaScript 的 for-of 循环实际上很快（V8）

Published on 01/01/2026
Updated on 14/01/2026

发布于 01/01/2026
更新于 14/01/2026

• #ecmascript
• #ecmascript
• #javascript
• #javascript
• #benchmark
• #benchmark

When it comes to iterating over arrays, for...of loops are believed to be significantly slower than traditional indexed loops. This has some grounding, since for-of loops are based on the iteration protocol, where the iterator has next() method returning an object like this: { value: /* element value */, done: /* a boolean, indicating the end of iteration */ }. Obviously, returning such object for every element will add an overhead.

在遍历数组时，for...of 循环被认为比传统的索引循环慢得多。这是有根据的，因为 for-of 循环基于 iteration protocol，其中迭代器有一个 next() 方法，返回像这样的对象：{ value: /* element value */, done: /* a boolean, indicating the end of iteration */ }。显然，为每个元素返回这样的对象会增加开销。

However, I decided to test this since JS engines are constantly improving and sometimes can do amazing optimizations.

然而，我决定测试一下，因为 JS 引擎在不断改进，有时能进行惊人的优化。

Table of contents

目录

1. The benchmark
   1. The core idea of the benchmark
   2. The setup
   3. The results
2. 基准测试
   1. 基准测试的核心理念
   2. 设置
   3. 结果
3. Conclusion
4. 结论

The benchmark

基准测试

I decided to do the benchmarks with jsbenchmark.com on Chrome 143, Windows 11 and AMD Ryzen 5000U. All the benchmark tests are run sequentially.

我决定使用 jsbenchmark.com 在 Chrome 143、Windows 11 和 AMD Ryzen 5000U 上进行基准测试。所有基准测试都是顺序运行的。

The core idea of the benchmark

基准测试的核心思想

The idea is to create 5 types of arrays: integers, floats, strings, objects and mixed values.

这个想法是创建 5 种类型的数组：整数、浮点数、字符串、对象和混合值。

• The benchmarks will be done with 3 array sizes: 5000, 50000, and 500000.
• 基准测试将使用 3 种数组大小进行：5000、50000 和 500000。
• There will be 6 types of loops:

  • Classic for with i++:

    for (let i = 0; i < arr.length; i++) { DATA.doSomethingWithValue(arr[i]); }

  • Classic for with i++ and cached array length:

    const length = arr.length; for (let i = 0; i < length; i++) { DATA.doSomethingWithValue(arr[i]); }

  • Classic for with i-- reversed:

    for (let i = arr.length - 1; i >= 0; i--) { DATA.doSomethingWithValue(arr[i]); }

  • for-of:

    for (const x of arr) { DATA.doSomething...