ECMAScript 2015:for 循环中的 const

2022-01-23 00:00:00 for-loop scope constants javascript ecmascript-6

以下两个(或两者都不/两者)代码片段中的哪一个应该在完整的 ECMAScript 2015 实现中工作:

Which of the two (or neither/ both) code fragments below should be working in a complete ECMAScript 2015 implementation:

for (const e of a)

for (const i = 0; i

据我了解，第一个示例应该可以工作，因为每次迭代都会初始化 e.第二个版本的i不应该也是这样吗?

From my understanding, the first example should work because e is initialized for each iteration. Shouldn't this also be the case for i in the second version?

我很困惑，因为现有的实现(Babel、IE、Firefox、Chrome、ESLint)似乎不一致，并且具有 const 的完整实现，两个循环变体的各种行为;我也无法在标准中找到具体点，因此将不胜感激.

I'm confused because existing implementations (Babel, IE, Firefox, Chrome, ESLint) do not seem to be consistent and have a complete implementation of const, with various behaviours of the two loop variants; I'm also not able to find a concrete point in the standard, so that would be much appreciated.

推荐答案

以下 for-of 循​​环有效:

The following for-of loop works:

for (const e of a)

ES6 规范将其描述为:

The ES6 specification describes this as:

ForDeclaration : LetOrConst ForBinding

ForDeclaration : LetOrConst ForBinding

http://www.ecma-international.org/ecma-262/6.0/index.html#sec-for-in-and-for-of-statements-static-semantics-boundnames

命令式 for 循环将不起作用:

The imperative for loop will not work:

for (const i = 0; i < a.length; i += 1)

这是因为声明只在循环体执行前被评估一次.

This is because the declaration is only evaluated once before the loop body is executed.

http://www.ecma-international.org/ecma-262/6.0/index.html#sec-for-statement-runtime-semantics-labelledevaluation

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