JavaScript函数参数和范围 [英] JavaScript function parameter and scope

问题描述

我已经使用下面列出的代码进行了一些测试:

I have done some tests with codes listed below:

function foo(x) {
    alert(y);
}
var y = 'I am defined outside foo definition';
foo();

上面的代码给了我一个警报我在foo定义之外定义".

The above code gives me an alert 'I am defined outside foo definition'.

然后进行另一项测试:

function bar(x) {
    alert(x);
}
var x = 'I am defined outside foo definition';
bar();

function bar(x) {
    alert(x);
}
x = 'I am defined outside bar definition';
bar();

以上两种代码都给我一个警报未定义".

Both the above codes give me an alert 'undefined'.

那为什么呢?

推荐答案

这是因为在声明参数x时

This is because when you declare the parameter x

function bar(x) {...}

您正在隐藏"全局变量x.您只能访问参数x

You are "hiding" the global variable x. You only have access to the parameter x

尝试传递全局x

function bar(x) {alert(x);}
var x = 'I am defined outside bar definition';
bar(x); // here the x is the global x that we pass in
        // the function gets a reference to the global x

如果省略该参数，则全局x将再次变为可见状态

If we omit the parameter then the global x would become visible again

function baz() {alert(x); }
var x = 'global var';
baz(); // 'global var'

请注意，良好的js应用程序中应限制全局变量.

Note that global variables should be limited in good js applications.

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