guard function that returns if val is not undefined
守卫函数，返回 val 不为 undefined
The value to check. 要检查的值
True if the value is not undefined, false otherwise. 如果值不是undefined则返回true，否则返回false
[1, '', false, undefined].filter(notUndefined) // => [1, '', false] Copy
[1, '', false, undefined].filter(notUndefined) // => [1, '', false]
guard function that returns if val is not undefined
守卫函数，返回 val 不为 undefined