使用var关键字定义变量是一个好习惯

You can declare a variable in two ways:

• With the keyword var. For example, var x = 42. This syntax can be used to declare bothlocal and global variables.
• By simply assigning it a value. For example, x = 42. This always declares aglobal variable and generates a strict JavaScript warning. You shouldn't use this variant.

其实我觉得应该加强语法规则，要么用var关键字要么用global关键字，这样就不会出岔子了。