What is Property-Based Testing?

In traditional example-based testing, a developer writes specific inputs and expected outputs: given input A, expect output B. This is easy to understand but limited by the tester's imagination. Property-based testing inverts this: you define a property that should always hold, and the framework generates hundreds or thousands of input combinations to try to disprove it.

For example, instead of testing that sort([3, 1, 2]) === [1, 2, 3], you define the property: "for any list, the sorted output should have the same length, all original elements present, and each element should be less than or equal to the next." The framework then generates random lists of varying sizes and types, including edge cases like empty lists, lists with duplicates, and lists with negative numbers.

Popular libraries include fast-check (JavaScript/TypeScript), Hypothesis (Python), and QuickCheck (Haskell, the original). These libraries also implement "shrinking," which automatically simplifies a failing input to the smallest example that still reproduces the failure.

Property-based testing is particularly effective for pure functions, data transformations, serialization/deserialization, and mathematical operations.