Answer 1

Sony Hossain null is a primitive value it is used for missing object. To represent intentionally missing object null is used.

Answer 2

You can pass a "context" to the apply method, this is the first argument, and referring to the context with the "this" keyword, the function itself can use values and variables that are defined elsewhere.
null means that you are not giving any special context to this function.

https://stackoverflow.com/questions/33640079/why-would-you-pass-null-to-apply-or-call