当您使用点表示法时， key 表示对象中的实际属性，不在那里。因此，返回 undefined ，它不等于 true 。

When you use dot notation, key means the actual property in the object, which will not be there. So, undefined is returned which is not equal to true.

当您使用 [] 表示法时，您正在访问对象中的属性，其名称在变量 key 中。所以，这将有效。

When you use [] notation you are accessing the property in the object with the name in the variable key. So, that will work.

例如，

var myObj = {
    myVar : 1
};

for (var key in myObj) {
    console.log(key);
    console.log(myObj.key);
    console.log(myObj[key]);
}

这将打印，

myVar
undefined
1

因为， myObj 没有名为 key 的成员（ myObj.key 尝试获取名为 key 的成员，在下一种情况下， myObj 有一个名为的成员 myVar （ myObj [key] 尝试使用键中的值获取成员）。

Because, myObj has no member named key (myObj.key tries to get the member with the name key) and in the next case, myObj has a member named myVar (myObj[key] tries to get the member with the value in key).

点符号

jslint更喜欢点符号。

[]符号

这提供了灵活性。您可以使用变量动态访问成员。

This offers flexibility. You can dynamically access the members with a variable.