# Property Based Testing

> Use when writing, reviewing, or modifying property-based tests, such as those using `fast-check`, or when asked to add test coverage.

- Skill: `tomeraberbach/property-based-testing` (Agent Skill)
- Install (CLI): `npx skillmds add tomeraberbach/property-based-testing`
- Raw SKILL.md: https://api.skillmd.com/api/skills/tomeraberbach/property-based-testing/raw
- Safety review: pending
- Works with: Claude Code, Claude.ai, OpenAI Codex
- Category: Coding & Dev Tools
- Author: TomerAberbach (https://skillmd.com/u/tomeraberbach)
- Updated: 2026-09-10
- Page: https://skillmd.com/skills/tomeraberbach/property-based-testing

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Before starting, load `/authoring-tests`.

Follow these principles when writing property-based tests:

- Write focused properties with focused arbitraries to explore slices of the
  input space

- Prefer arbitraries that generate the desired input space directly instead of
  generating a larger input space and then filtering it

- Assert only what's straightforward to infer from the input arbitraries.
  Asserting the full value is often infeasible. If no assertion seems feasible,
  then the input space is too broad

- Don't constrain an arbitrary unless the constraint is necessary to satisfy the
  property or to resolve a performance problem

# Common properties

- "Output is always/never X for all inputs". Example: for any number `n`,
  `Math.floor(n)` is an integer

- "When input is X, then output is always/never Y". Example: for any array
  `data` with no duplicates, the result of removing duplicates from `data` is
  `data` itself

- "Complex implementation X is equivalent to simpler implementation Y". Example:
  "`c` is contained inside sorted array `data` for binary search" is equivalent
  to "`c` is contained inside `data` for linear search"

- Totality: function returns rather than throwing for all valid inputs. Example:
  `JSON.parse(JSON.stringify(x))` never throws for serializable `x`

- Deterministic: always returns the same output for the same input. Example: for
  any date `d`, `formatDate(d)` always returns the same string

- Side-effect free: does not mutate the input or non-local state. Example: for
  any array `data`, `sorted(data)` leaves `data` unchanged

- Bounded output: output is always within a known range. Example:
  `clamp(x, low, high)` always returns a value between `low` and `high`

- Structural invariant: output always has a guaranteed shape/structure. Example:
  `partition(predicate, data)` always produces two arrays whose combined length
  equals `data.length`

- Closure under operation: applying `f` to valid inputs always produces a valid
  output of the same type/domain. Example: `add(positiveInt, positiveInt)` is
  always a positive integer

- Identity element: there exists an input that leaves output unchanged. Example:
  `concat(xs, [])` equals `xs`

- Absorption/annihilation: certain inputs collapse the result regardless of the
  other. Example: `and(false, x)` is always `false`

- Idempotent: running twice is the same as running once, either in its effect on
  non-local state or when passing its first output as its second input. Example:
  for any array `data`, `sort(sort(data))` equals `sort(data)`

- Commutative: rearranging argument order doesn't affect output. Example: for
  any numbers `a` and `b`, `add(a, b)` equals `add(b, a)`

- Associative: regrouping arguments for multiple calls doesn't affect output.
  Example: `concat(concat(a, b), c)` equals `concat(a, concat(b, c))`

- Distributive: `f(a ∪ b) === f(a) ∪ f(b)`. Example: `map(f, [...xs, ...ys])`
  equals `[...map(f, xs), ...map(f, ys)]`

- Inverse/symmetry/roundtrips: `f` and `g` are inverses of each other. Example:
  `decode(encode(x))` equals `x`

- Transitivity: if `f(a, b)` and `f(b, c)`, then `f(a, c)`. Example: if
  `isAncestor(a, b)` and `isAncestor(b, c)`, then `isAncestor(a, c)`

- Monotonic: if input increases (or decreases), output always changes in the
  same direction. Example: sorting more elements never produces fewer elements

- Consistent ordering: if `a` comes before `b` in the input, the output keeps
  that order. Example: a stable sort never reorders equal elements relative to
  each other

- Prefix/suffix closure: if `f(x)` holds, it holds for any sub-input, or
  conversely, for any superset. Example: if `isValid(data)` then
  `isValid(data.slice(0, n))` for all `n`

These aren't exhaustive. Reason from first principles when none fits.

# `fast-check` tips

- Use `fc.clone(arb, count)` to produce multiple equal values instead of
  `structuredClone` or `JSON.parse(JSON.stringify(...))`

- Use `fc.uniqueArray(arb, { minLength, maxLength })` to produce unique values
  instead of `fc.tuple(...arbs).filter(...)`