测试是否理解 JavaScript 的核心，闭包和作用域（转）

 英文原文地址：http://dmitry.baranovskiy.com/post/91403200

Dmitry Baranovskiy 的博客中有篇文章（http://dmitry.baranovskiy.com/post/91403200），其中有五段小代码，用来测试是否理解 JavaScript 的核心，闭包和作用域。大家先试着给出 alert 语句的输出结果。之后再建一个测试文件，检测你的答案。

Quick test for real understanding of JavaScript core beyond closures and scopes. Here five small scripts. Try to answer what will be alerted in each case without running them in the console. Then you could create a test file and easily check your answers. Ready?

if (!("a" in window)) {
    var a = 1;
}
alert(a);
var a = 1,
    b = function a(x) {
        x && a(--x);
    };
alert(a);
function a(x) {
    return x * 2;
}
var a;
alert(a);
function b(x, y, a) {
    arguments[2] = 10;
    alert(a);
}
b(1, 2, 3);
function a() {
    alert(this);
}
a.call(null);