The value to return.
The value the function was called with.
This function is useful when you want to explicitly pass a function as a callback that always returns the value it is called with.
It is functionally equivalent to writing an anonymous function that returns it's first argument, but can sometimes make your code more readable.
For example both of these expressions are functionally equivalent:
[1, 2, 3].map(v => v);
[1, 2, 3].map(identity);
Returns the value it is called with.