How to understand undefined in js

How to understand undefined in js, many novices are not very clear about this, in order to help you solve this problem, the following editor will explain in detail for you, people with this need can come to learn, I hope you can gain something.

1. Like null, it is a special data type that indicates that it is not assigned. When a variable is not defined, its value is undefined, indicating that it is not assigned.

2. In js, undefined is a basic data type with a unique undefined value.

Example

Let an a = = undefined; / / truea = = window.undefined; / / true / / window.undefined attribute is unmodifiable window.undefined = 11 windows. Undefined; / / does undefined help you after reading the above? If you want to know more about the relevant knowledge or read more related articles, please follow the industry information channel, thank you for your support.