generative-property

Declare a property such as a round-trip or invariant, generate inputs with wide arbitraries like fc.jsonValue(), and assert with fc.assert using fixed numRuns and seed for reproducibility. When a counterexample appears, fast-check shrinks it to a minimal failing input you can pin as a regression test.

Pairwise testing generates a small covering array where every pair of parameter values appears at least once, instead of testing all combinations. Use it when independent parameters like OS, runtime version, and mode make exhaustive combination testing explode; most interaction defects surface at 2-way strength.

Four property types apply: invariants the output always satisfies, round-trip relations like decode(encode(x)) === x, metamorphic relations between transformed inputs, and agreement with a known slow-but-correct oracle. If none of these fit, example-based testing is a better choice.

Wide generators produce edge-case numbers where toBe (Object.is) treats -0 and +0 as different and NaN === NaN is always false. Define equivalence as part of the property itself, choosing ===, tolerance-based comparison, or toStrictEqual deliberately to avoid flaky failures.

Avoid PBT when the only property you can state copies the implementation's own logic, since both can be wrong simultaneously and the oracle is worthless. Also avoid it when no property type fits the target; use example-based equivalence partitioning and boundary value analysis instead.

Write constraints such as 'if os=mac then node≠18' and feed them to a constraint-aware covering array generator like ACTS or PICT. Without constraints, the generator produces many meaningless cases that fail for reasons unrelated to real defects.