In JS I often want to test a parameter value does what is expected in the positive case: foo("A"), but also in the negative case, i.e foo([not "A"]). Where [not "A"] some is some arbitrary string(s) representing the negative condition like say "ZZ". Can the js property based testing library help me generate an arbitrary not "A" elegantly?
PBT libraries usually have some support for filtering ("if you generate a value x such that P(x), discard it and try again"), but it's frowned upon. You should only use it when you're relatively certain it's not going to happen very often. But, it's an option.
You could try to somehow be clever about how you generate the inputs: ("A" + nonempty string | "B-Z" ++ possibly empty string)
Or you could generate a string, check it for "A"-ness, and if true, fix it before running the actual test by appending some random non-empty string after it.
The three approaches might look like (in pseudocode)
1. Filtering
gen.string().filter(s => s !== "A")
2. "Smart" construction
gen.oneOf([
"A" + gen.string().nonempty(),
gen.charBetween("B","Z") + gen.string()
])
3. Naive generation + fix
gen.string().andThen(s => (s === "A") ? s + gen.string().nonempty() : s)
It slows down the testing. All the invalid inputs are generated and then discarded, so itβs much more efficient to come up with a construct that succeeds all the time or at least most of the time.
If you ask for an integer but only want odds (as an example), 50% of the generated values will be tossed out with filtering. If you instead take every integer and make them odd (double and add 1, for instance) then all the generated inputs will be usable.
