问题描述
以下代码始终打印传递给参数a的参数，而不管是否存在相同名称的变量.
The following code always prints the argument passed in to parameter a, regardless of the presence of a variable with the same name.
大概是因为参数标识符分别绑定到作用域中的变量.他们的位置在哪里?他们在词汇环境中吗?
Presumably because parameter identifiers are bound separately to variables in scope. Where are they positioned? Are they in the lexical environment?
function foo(a, b = () => a) {
var a = 1
console.log(b())
}
foo() // undefined
foo(2) // 2 var声明是否以特殊的 VariableEnvironment 结尾，而参数放置在LexicalEnvironment中?并且let和const通过将重新定义作为早期错误来避免冲突?
Is it that var declarations end up in the special VariableEnvironment, while parameters are positioned in the LexicalEnvironment? And let and const avoid conflict by making redefinition an early error?
也相关:
- 8.3.2 ResolveBinding(name [, env])
- 8.1.1 Environment Records
推荐答案
如果任何默认值值存在，将为参数创建一个单独的环境记录.
In the event that any default values are present, a separate environment record is created for parameters.
在此位置声明的函数的语义使该环境记录定义其局部范围.规范中的注释(请参见第28条)说:
The semantics of functions declared in this position are such that this environment record defines their local scope. A note in the spec (see clause 28) says:
规范的更多信息:
因此，在没有默认参数的情况下，我推断出一个预先存在的词汇环境(VariableEnvironment或LexicalEnvironment)用于参数绑定.也许.
In the absence of default arguments, therefore, I deduce that one of the pre-existing lexical environments (VariableEnvironment or LexicalEnvironment) is used for parameter bindings. Maybe.
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