markdown JS Null，Undefined和Undeclared

Posted 2021-05-05

## Differences between Null, Undefined, and Undeclared

- **undefined** is a variable that has been declared but no value exists and is a type of itself ‘undefined’.
- **null** is a value of a variable and is a type of object.
- **undeclared** variables is a variable that has been declared without ‘var’, 'let, or 'const' keywords.