Should I use "is" or "are" between two nouns (I think) in this phrase?

Solution 1:

If you are talking about a singular operation (stringify and parse), then you would use "is".

In this function, stringify and parse is necessary.

If you are talking about two operations (stringify and parse), then you would use "are".

In this function, stringify and parse are necessary.

It is likely that you mean the second.