问题描述
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
这篇关于ECMAScript 2015:for 循环中的 const的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持跟版网！