Consider the following excerpt from ECMA-262 v5.1 (which I recently saw in this question):
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.
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;
}
Does anybody know why? Also, why does a catch
clause need its own lexical environment?
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'sLexicalEnvironment
is switched to the new one, while theVariableEnvironment
("whose environment record holds bindings created byVariableStatements
andFunctionDeclarations
") stays unchanged.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 currentVariableEnvironment
so it will not be able to access the exception:(Update: this is no longer true in ES6, where block-level function declarations are valid and close over the block scope)