• 声明变量不带var，JS会在全局对象中创建一个同名属性，坑爹

• JS在函数作用域上倒是和Python有些相似，不过Python是直接抛出错误，而JS是打印出undefined.

.. code:: python

>>> def foo():
...   print(a)
...   a = 1
...
>>> foo()
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
    File "<stdin>", line 3, in foo
UnboundLocalError: local variable 'a' referenced before assignment

.. code:: javascript

> function foo() {
    ...   console.log(x);
    ...   var x = 1;
    ...   console.log(x);
    ...
}
>
> foo()
undefined
1

• JS的in操作符，"x" in {x: 1} 表现还比较正常，但是对数组操作的时候，就不是正规军了:

.. code:: javascript

> 1 in [1]
false
> 1 in [1,2]
true
> "0" in [1]
true
> "0" in [1, 2]

原因在于，JS把前面转成索引。。。

• delete 只是删除引用