为什么 catch 子句有自己的词法环境? [英] Why do catch clauses have their own lexical environment?

问题描述

考虑以下摘录 来自 ECMA-262 v5.1(我最近看到在这个问题中:

Consider the following excerpt from ECMA-262 v5.1 (which I recently saw in this question):

词法环境是一种规范类型，用于根据 ECMAScript 代码的词法嵌套结构定义标识符与特定变量和函数的关联.词法环境由环境记录和对外部词法环境的可能为空的引用组成.通常，词法环境与 ECMAScript 代码的某些特定句法结构相关联，例如 FunctionDeclaration、WithStatement 或 TryStatement 的 Catch 子句，并且每次评估此类代码时都会创建一个新的词法环境.

A Lexical Environment is a specification type used to define the association of Identifiers to specific variables and functions based upon the lexical nesting structure of ECMAScript code. A Lexical Environment consists of an Environment Record and a possibly null reference to an outer Lexical Environment. Usually a Lexical Environment is associated with some specific syntactic structure of ECMAScript code such as a FunctionDeclaration, a WithStatement, or a Catch clause of a TryStatement and a new Lexical Environment is created each time such code is evaluated.

我认为这意味着 catch 子句的主体会像函数一样提升自己的变量，但显然 事实并非如此:

I thought that meant the body of catch clauses would hoist its own variables like functions do, but apparently that's not the case:

var a = 1;
try {
    console.log(x); // ReferenceError
} catch(ex) {
    console.log(a); // 1, not undefined
    var a = 3;
}

有人知道为什么吗?另外，为什么 catch 子句需要自己的词法环境?

Does anybody know why? Also, why does a catch clause need its own lexical environment?

推荐答案

是的，catch 子句确实有自己的词法环境.查看在评估时发生了什么:它创建一个新的(从当前的派生而来)并将异常标识符绑定到它.执行catch块时，当前执行上下文的 LexicalEnvironment 切换到新的，而 VariableEnvironment("其环境记录保存了由 VariableStatements 和 FunctionDeclarations") 保持不变.

Yes, catch clauses indeed have their own Lexical Environments. Check out what happens when it is evaluated: It creates a new one (deriving from the current one) and binds the exception-identifier to it. When executing the catch block, the current Execution Context's LexicalEnvironment is switched to the new one, while the VariableEnvironment("whose environment record holds bindings created by VariableStatements and FunctionDeclarations") stays unchanged.

console.log(a); // undefined - declared from within the catch,
                // but in the current VariableEnvironment
a = 1;
console.log(typeof ex); // undefined - no binding
try {
    console.log(ex); // a ReferenceError in this LexicalEnvironment
} catch (ex) { // introducing the new LexicalEnvironment
    console.log(ex); // …and it works here!
    var a = 3; // variable declaration
}

<小时>

有趣的事实:如果您尝试在 catch 子句中声明一个函数(尽管在块中语法无效，但通常接受函数声明语句")，它的范围将成为当前的VariableEnvironment，因此它将无法访问异常:

---

Fun fact: If you try to declare a function inside a catch clause (though syntactically invalid in a block, "function declaration statements" are often accepted), its scope will become the current VariableEnvironment so it will not be able to access the exception:

try {throw "some"} catch(x) { function y(){console.log(x, typeof x);} y(); }
                    // throws a ReferenceError for x   ^

_{(更新:这在 ES6 中不再适用，其中块级函数声明有效并且在块范围内关闭)}

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