Javascript如何声明函数参数？ [英] How does Javascript declare function parameters?

问题描述

function func(x = y, y = 2) { 
    return [x, y];
}

func(); // ReferenceError: y is not defined
func(1); // [1, 2]

如上面的代码所示，函数中有一个隐藏的TDZ参数范围，解释了为什么下面的代码失败：

As the code above implied, there is a hidden TDZ in the function parameters scope, which explains why the code below fails:

function func(arg) {
    let arg = 1; // SyntaxError: Identifier 'arg' has already been declared
}

所以函数参数应该用声明，但让我困惑的是：

So function parameters should be declared with let, but what confused me is :

function func(arg) {
    var arg = 1;
    console.log(arg); // 1
}

此代码工作正常。

为什么你可以使用 var 重新声明变量？ Javascript如何声明函数参数？

Why you can redeclare variables using var? How does Javascript declare function parameters?

编辑：
我确切知道你不能使用let来重新声明一个变量。这里的问题是，如果使用 let 声明函数参数，那么：

edit: I know exactly you can't use let to redeclare a variable. The question here is if function parameters is declared using let, so:

function func(arg) {
    var arg = 1;
}

如下：

let arg; // arg parameter declares here
var arg = 1; // func body 

为什么这可以无异常地运行？

and why this can run without an exception?

推荐答案

函数参数范围内有隐藏的TDZ

there is a hidden TDZ in the function parameters scope

确实如此。有关更多示例，请查看此处。

Yes indeed. Have a look here for some more examples.

Javascript如何声明函数参数？

How does Javascript declare function parameters?

作为参数 - 参见这里逐步解释。它们既不与 let 相同，也不与 var 相同，它们有自己的语义。引入默认初始化程序的ES6给予它们与让相同的TDZ限制，以捕获更多程序员错误。

As parameters - see here for a step-by-step explanation. They're neither the same as let nor the same as var, they have their own semantics. ES6, which introduced default initialisers, gave them the same TDZ limitations as in let to catch more programmer mistakes.

为什么你可以使用 var 重新声明变量？

因为在ES5之前，重新声明变量不是错误条件，并且需要保留此行为以不破坏Web。它只能用于新功能，如让和 const - 或使用默认初始化程序的参数列表，尝试 function x（bar，bar）{} vs function x（bar，bar = 1）{} 。

Because until ES5, redeclaring a variable was not an error condition, and this behaviour needed to be preserved to not break the web. It could only be introduced for new features, like let and const - or argument lists that use default initialisers, try function x(bar, bar) {} vs function x(bar, bar=1){}.

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