'undefined' and 'null' are both values used to represent the absence of a value in JavaScript, but they are used in different contexts.
'undefined' is a primitive value that is automatically assigned to variables that have been declared but have not yet been assigned a value.
It is also returned by functions that do not explicitly return a value.
'null' is a primitive value that represents the intentional absence of any object value.
It is often used to initialize variables that are meant to later hold an object value.
In JavaScript, 'null' is considered an object, while 'undefined' is considered a type.
Overall, 'undefined' is used to represent the absence of a defined value, while 'null' is used to represent the intentional absence of any object value.