JavaScript：使用严格模式在try / catch中定义一个常量 [英] JavaScript: define a constant inside try / catch with strict mode

问题描述

今天我遇到了一个奇怪的JS错误，在try / catch块中使用 const ，我想更好地理解导致它的原因。

Today I run into a weird JS bug, working with const inside a try/catch block, and I'd like to better understand what is causing it.

让我们看一个代码示例，值得超过千言万语：

Let's look at a code example, that is worth more than a thousand words:

try {
  const FOO = 'bar';
  console.log('inside:', FOO);
} catch (e) {}
console.log('outside:', FOO);

这将记录：

inside: bar
outside: bar

如果我们切换到严格模式虽然：

If we switch to "strict mode" though:

'use strict';
try {
  const FOO = 'bar';
  console.log('inside:', FOO);
} catch (e) {}
console.log('outside:', FOO);

现在相同的代码会产生错误：

Now the same code produces an error:

ReferenceError: FOO is not defined

如果我们更改 const 但 var 但是：

'use strict';
try {
  var foo = 'bar';
  console.log('inside:', foo);
} catch (e) {}
console.log('outside:', foo);

然后一切正常，即使在严格模式下：

Then it all works fine again, even in "strict mode":

inside: bar
outside: bar

任何人都可以帮助我理解为什么 const 赋值在严格模式下的try / catch块内不起作用？

Can anyone please help me understand why the const assignment is not working inside a try/catch block in "strict mode"?

谢谢！

推荐答案

const ，如ECMAScript 6所定义，是一个块级变量声明。你得到 ReferenceError ，因为它不在之外的范围内尝试。

const, as defined by ECMAScript 6, is a block-level variable declaration. You get a ReferenceError because it's not in scope outside of the try.

然而， const / const #Firefox-specific_notesrel =nofollow noreferrer>一些引擎早在ES6之前，作为 var 的不可变对应物，具有功能级范围行为（没有TDZ）。如果您处于草率模式（您不应该这样），您可能仍会在浏览器的遗留支持中体验这一点。

However, const was introduced in some engines long before ES6, as a immutable counterpart to var, with function-level scope behaviour (and without TDZ). If you're in sloppy mode (which you should not be), you might still experience this as part of your browser's legacy support.

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