undefined

• Also notice that the undefined symbol
  同时要注意，未定义的符号
• Will be referencing undefined variables
  会引用未定义的变量。
• Specifies that the certificate has an undefined name constant
  指定证书具有一个未定义的名称常数。
• The loword is undefined and reserved
  Loword做为保留未定义。
• However , the output of the following example code is undefined
  而下面的代码示例的输出却是未定义的：
• The value of the property is undefined
  该属性的值未定义。
• If it is undefined , we return
  如果此变量没有被定义，将返回
• Nullable types allow a variable to contain a value that is undefined
  可空类型允许变量包含未定义的值。
• The behavior for such a conversion is undefined and unreliable
  。这种转换的行为是不明确的和不可靠的。
• A null value is useful to indicate an undefined or unknown value
  空值对于指示未定义的或未知的值很有用。