[Solved]How to take default value of a?

Question

https://code.sololearn.com/WqaOExkzq02X/?ref=app
I want to take default value of a, and willing to input value of b in function.
I don't want to change order.
I tried many times.
I think there should be a way, because I think it is necessary.

Answer 1

Use undefined instead of null.

Answer 2

Thanks!
I had thought for a moment but didn't tried, my bad.