函数参数 [英] function arguments

问题描述

function Foo(f) {
   var f = f;    
}

这里面的函数，变量 f 是 Foo 的本地（它有一个函数范围），但为什么变量 f in参数列表没有冲突？也许是因为它绑定在 Foo.arguments 对象中？

Here inside the function, variable f is local to the Foo (it has a function scope), but why is the variable f in the argument list not in conflict? Maybe because it is bound inside the Foo.arguments object?

在其他语言中我们不能声明一个参数变量与本地变量同名。

In other languages we cannot declare an argument variable with the same name as a local variable.

这个名称歧义是如何解决的？或者，如何在方法中稍后引用两个不同的 f 变量中的每一个？

How is this name ambiguity resolved? Or, How do you reference each of the two distinct f variables later in the method?

推荐答案

JavaScript做了一些显然不直观的事情 - 你感兴趣的事情被称为提升 - JS将var声明移动到函数的顶部，它们的唯一目的是将此变量名称保留为函数范围中的局部变量。有时，这会导致很多奇怪。如果变量名已经被保留为局部变量（例如它是一个参数），var声明将完全被删除。

JavaScript does a couple of things that aren't obviously intuitive - the one you're interested in is called "hoisting" - JS moves var declarations to the top of a function, where they serve the sole purpose of reserving this variable name as a local variable in the function's scope. Sometimes, this leads to lots of weirdness. If the variable name is already reserved as a local variable (e.g. it's an argument) the var declaration gets dropped entirely.

JS的另一个不直观的部分是它如何处理参数变量和参数对象（有点特殊，正如Hippo所示）。但这并不一定是你感兴趣的 - 对你的例子来说重要的是参数还声明变量名称是函数的本地名称。

Another unintuitive part of JS is how it deals with argument variables and the arguments object (which are a bit special, as Hippo showed). That's not necessarily what you're interested in, though - what's important for your example is that arguments also declare that variable name as local to the function.

所有结果这是当你有一个 var f 以及一个参数名 f 时，`var f'会被删除，你的例子相当于：

The result of all this is that when you have a var f as well as an argument name f, the `var f' gets dropped, and your example is equivalent to:

function Foo(f) {
   f = f;
}

你可以在Hippo的例子中看到这个，因为：

You can see this in Hippo's example, because:

function foo(f) {
    console.log(f); // --> 11
    console.log(arguments); // --> array [11]
    var f = 10;
    console.log(f); // --> 10
    console.log(arguments); // --> [10] (!!!)
}

相当于：

function foo(f) {
    var f;
    console.log(f); // --> 11
    console.log(arguments); // --> array [11]
    f = 10;
    console.log(f); // --> 10
    console.log(arguments); // --> [10] (!!!)
}

相当于：

function foo(f) {
    console.log(f); // --> 11
    console.log(arguments); // --> array [11]
    f = 10;
    console.log(f); // --> 10
    console.log(arguments); // --> [10] (!!!)
}

有关详细信息，请阅读 10.1.3 - 变量实例化（第37页底部） ECMA-262 ，JS规范。

For more details, read up section 10.1.3 - Variable Instantiation (bottom of p.37) in ECMA-262, the JS specification.

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