ECMAScript 2015：const in for循环 [英] ECMAScript 2015: const in for loops

问题描述

下面的两个（或两者/两者）代码片段中的哪一个应该在完整的ECMAScript 2015实现中工作：

for（const （a）a）（i = a; i = 1; i = a; a，a，a，a，b， ）

根据我的理解，第一个例子应该工作，因为 e 被初始化为每次迭代。在第二个版本中，对于 i 也不是这样吗？

我感到困惑，因为现有的实现（Babel，IE，Firefox，Chrome，ESLint）似乎并不一致，并且具有完全实现 const ，具有两种循环变体的各种行为;我也不能在标准中找到具体的一点，所以这将是非常感激的。

解决方案

-of循环工程：

  for（const e of a）
  

ES6规范描述为：

ForDeclaration：LetOrConst ForBinding

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

命令式循环不起作用：

  for（const i = 0; i   

这是因为在执行循环体之前，声明只进行一次评估。

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

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 < a.length; i += 1)

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?

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.

解决方案

The following for-of loop works:

for (const e of a)

The ES6 specification describes this as:

ForDeclaration : LetOrConst ForBinding

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

The imperative for loop will not work:

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

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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