javascript - 有人可以澄清我这个 try-catch block ( undefined variable )有什么问题吗？ (选择性捕捉，JS)

Question

我正在阅读 Bugs and Error Handling 中的第 8 章“Eloquent JavaScript” 。我遇到了一个关于错误代码的例子，但我仍然没有明白作者给出的关于导致失败的原因的解释。让我们给出上下文:在“Selective Catching”部分，作者说:

Invalid uses of the language, such as referencing a nonexistent variable, looking up a property on null, or calling something that’s not a function, will also result in exceptions being raised. Such exceptions can be caught just like your own exceptions.

When a catch body is entered, all we know is that something in our try body caused an exception. But we don’t know what, or which exception it caused.

JavaScript (in a rather glaring omission) doesn’t provide direct support for selectively catching exceptions: either you catch them all or you don’t catch any. This makes it very easy to assume that the exception you get is the one you were thinking about when you wrote the catch block.

But it might not be. Some other assumption might be violated, or you might have introduced a bug somewhere that is causing an exception. Here is an example, which attempts to keep on calling promptDirection until it gets a valid answer:

for (;;) {
  try {
    var dir = promtDirection("Where?"); // ← typo!
    console.log("You chose ", dir);
    break;
  } catch (e) {
    console.log("Not a valid direction. Try again.");
  }
}

The for (;;) construct is a way to intentionally create a loop that doesn’t terminate on its own. We break out of the loop only when a valid direction is given. But we misspelled promptDirection, which will result in an “undefined variable” error. Because the catch block completely ignores its exception value (e), assuming it knows what the problem is, it wrongly treats the variable error as indicating bad input. Not only does this cause an infinite loop, but it also “buries” the useful error message about the misspelled variable.

我想我没有得到解释，因为英语不是我的母语。我不明白的是大胆的句子。我不明白为什么 catch block 确实忽略变量 e 以及有关错误输入的事情。 catch block 是否假定 e 是错误的输入？

请有人向我解释一下这一点。提前致谢。

Answer 1

作者并不是说 JavaScript 总是忽略 e；他正在谈论这个具体的例子。这个示例代码写得很糟糕(故意):

catch (e) {
    console.log("Not a valid direction. Try again.");
}

我们捕获一个异常(称为e)，但我们不会尝试找出异常的种类。我们总是，总是说“不是一个有效的方向。再试一次。”

此代码假设只有一件事可能出错:输入无效。