The Magic of Generative Testing: Fast-Check in JavaScript

(Video at https://youtu.be/a2J_FSkxWKo)

Generative testing (aka property testing), popularized by the Haskell library QuickCheck, is a technique of:

- specifying invariant laws your code expects to exhibit
- generating random inputs to verify the laws
- simplifying failures to find error boundaries
- providing replay mechanisms for easy debugging

In this BrooklynJS talk, I show the motivations for and basics of property testing in JavaScript, via the library fast-check.

1. The Magic of Generative Testing BrooklynJS · 2019-06-20
2. glebec glebec glebec glebec g_lebec Gabriel Lebec
3. describe('`sort`', () => { it('transforms `[2, 1, 3]` to `[1, 2, 3]`', () => { expect(sort([2, 1, 3])).toEqual([1, 2, 3]) }) })
4. describe('`sort`', () => { it('transforms `[2, 1, 3]` to `[1, 2, 3]`', () => { expect(sort([2, 1, 3])).toEqual([1, 2, 3]) }) }) ✅ Passes!
5. describe('`sort`', () => { it('transforms `[2, 1, 3]` to `[1, 2, 3]`', () => { expect(sort([2, 1, 3])).toEqual([1, 2, 3]) }) })
6. describe('`sort`', () => { it('transforms `[2, 1, 3]` to `[1, 2, 3]`', () => { expect(sort([2, 1, 3])).toEqual([1, 2, 3]) }) }) • nums => [1, 2, 3] • nums => [nums[1], nums[0], nums[2]] • nums => realSort([ ...new Set(nums)]) • nums => realSort([nums[0], ...nums.slice(1)]) • nums => realSort(nums.map(n => Math.round(n)) • nums => nums.sort() All these pass…
7. describe('`sort`', () => { it('transforms `[2, 1, 3]` to `[1, 2, 3]`', () => { expect(sort([2, 1, 3])).toEqual([1, 2, 3]) }) }) So what do we do?
8. describe('`sort`', () => { it('transforms `[2, 1, 3]` to `[1, 2, 3]`', () => { expect(sort([2, 1, 3])).toEqual([1, 2, 3]) }) it('transforms `[2, 3, 1]` to `[1, 2, 3]`', () => { expect(sort([2, 3, 1])).toEqual([1, 2, 3]) }) })
9. describe('`sort`', () => { it('transforms `[2, 1, 3]` to `[1, 2, 3]`', () => { expect(sort([2, 1, 3])).toEqual([1, 2, 3]) }) it('transforms `[2, 3, 1]` to `[1, 2, 3]`', () => { expect(sort([2, 3, 1])).toEqual([1, 2, 3]) }) it('transforms `[1.5, -4]` to `[-4, 1.5]`', () => { expect(sort([1.5, -4])).toEqual([-4, 1.5]) }) })
10. describe('`sort`', () => { it('transforms `[2, 1, 3]` to `[1, 2, 3]`', () => { expect(sort([2, 1, 3])).toEqual([1, 2, 3]) }) it('transforms `[2, 3, 1]` to `[1, 2, 3]`', () => { expect(sort([2, 3, 1])).toEqual([1, 2, 3]) }) it('transforms `[1.5, -4]` to `[-4, 1.5]`', () => { expect(sort([1.5, -4])).toEqual([-4, 1.5]) }) it('transforms `[]` to `[]`', () => { expect(sort([])).toEqual([]) }) })
11. describe('`sort`', () => { it('transforms `[2, 1, 3]` to `[1, 2, 3]`', () => { expect(sort([2, 1, 3])).toEqual([1, 2, 3]) }) it('transforms `[2, 3, 1]` to `[1, 2, 3]`', () => { expect(sort([2, 3, 1])).toEqual([1, 2, 3]) }) it('transforms `[1.5, -4]` to `[-4, 1.5]`', () => { expect(sort([1.5, -4])).toEqual([-4, 1.5]) }) it('transforms `[]` to `[]`', () => { expect(sort([])).toEqual([]) }) it('transforms `[1, 1, 1, 1, 1]` to `[1, 1, 1, 1, 1]`', () => { expect(sort([1, 1, 1, 1, 1])).toEqual([1, 1, 1, 1, 1]) }) it('transforms `[999, 500, 2]` to `[999. 500, 2]`', () => { expect(sort([999, 500, 2])).toEqual([2, 500, 999]) }) it('transforms `[0x821, 1.3e5]` to `[0x821, 1.3e5]`', () => { expect(sort([0x821, 1.3e5])).toEqual([0x821, 1.3e5]) }) it('transforms `[0]` to `[0]`', () => { expect(sort([0])).toEqual([0]) }) it('transforms `[-0]` to `[-0]`', () => { expect(sort([-0])).toEqual([-0]) }) it('transforms `[1, 2, 1, 2]` to `[1, 1, 2, 2]`', () => { expect(sort([1, 2, 1, 2])).toEqual([1, 1, 2, 2]) }) it('transforms `[10, 11, 12, 9001, -1]` to `[-1, 10, 11, 12, 9001]`', () => { expect(sort([10, 11, 12, 9001, -1])).toEqual([-1, 10, 11, 12, 9001]) }) it('transforms `[0.9999999, 1, 0.9999999]` to `[0.9999999, 0.9999999, 1]`', () => { expect(sort([0.9999999, 1, 0.9999999])).toEqual([0.9999999, 0.9999999, 1])
12. It's a bit of a pain…
13. But are we confident?
14. ERROR: cannot read
 'name' of undefined 100% coverage Build succeeded Tests passed As humans, we focus
 on the "happy path"
15. Surely there is
 a better way…
16. What are we saying in this test? What is the *point*? describe('`sort`', () => { it('transforms `[2, 1, 3]` to `[1, 2, 3]`', () => { expect(sort([2, 1, 3])).toEqual([1, 2, 3]) }) })
17. describe('`sort`', () => { it('transforms `[2, 1, 3]` to `[1, 2, 3]`', () => { expect(sort([2, 1, 3])).toEqual([1, 2, 3]) }) }) Is it this particular array we care about?
18. describe('`sort`', () => { it('results in an ordered array', () => { expect(sort([2, 1, 3])).toEqual([1, 2, 3]) }) }) …or is it this particular *law*? aka "property" (of sorting)? aka "invariant" (behavior, regardless of input)
19. • Sorting a list of nums results in an ordered list • Sorting a sorted list is a no-op (idempotence) • Sorting neither adds, removes, nor changes elements ⚖
20. ⚖ // :: [Number] -> Bool function sortOrders (arr) { const sorted = sort(arr) for (let i = 0; i < arr.length - 1; i ++) { if (arr[i] > arr[i + 1]) return false } return true } a predicate function (returns Bool) aka property check we've explicitly named our law
21. What if we generate
 the tests?
22. // :: (Number) -> Number function randomInt (limit = 1) { return Math.floor(Math.random() * limit) } // :: () -> [Number] function randomArr () { return Array.from( { length: randomInt(LIMIT_LEN) }, () => randomInt(LIMIT_NUM) ) } ad-hoc data generators
23. describe('`sort`', () => { it('outputs ordered arrays', () => { for (let t = 0; t < LIMIT_TESTS; t ++) { expect(isOrdered(sort(randomArr()))).toBe(true) } }) }) generate and verify many random cases ✅ Passes!
24. describe('`sort`', () => { it('outputs ordered arrays', () => { for (let t = 0; t < LIMIT_TESTS; t ++) { expect(isOrdered(sort(randomArr()))).toBe(true) } }) }) …but what happens if it fails?
25. FAIL src/01-sort.spec.ts • `sort` › outputs ordered arrays expect(received).toBe(expected) // Object.is equality Expected: true Received: false
26. FAIL src/01-sort.spec.ts • `sort` › outputs ordered arrays expect(received).toBe(expected) // Object.is equality Expected: true Received: false which input array failed? what was the output from sort? …let's just add a console.log…
27. …let's just add a console.log… ✅ Passes! Oops! Nondeterminism strikes again!
28. …re-ran, failed again… but why does THIS array fail? FAIL `sort` › outputs ordered arrays Input: [93, 2, 0, 1, 90101, 29, 350, 301, 620] Output: [0, 1, 2, 29, 301, 250, 620, 90101, 93] Expected: true Received: false
29. Trickier than we thought
30. laborious ad-hoc error-prone generators (did you notice we never generated negative ints?) random array generator maybe repeated empty arr non-instructive failure (input? output?) irreproducible failing cases may be unnecessarily large
31. Surely there is
 a better way (again)…
32. Use a
 Property Testing Library
33. QuickCheck Claessen & Hughes
34. JSVerify Oleg Grenrus Fast-Check Nicolas Dubien
35. ⚖ specify law (predicate function) ⏳ support for async predicates generates progressively bigger random inputs . common edge cases baked-in failing cases progressively "shrunk" (simplified) ⚠ finds error boundary deterministically pseudorandom can specify seed
36. import * as fc from 'fast-check' describe('`sort`', () => { it('outputs ordered arrays', () => { fc.assert( fc.property( fc.array(fc.integer()), (nums) => isOrdered(sort(nums)) ) ) }) })
37. import * as fc from 'fast-check' describe('`sort`', () => { it('outputs ordered arrays', () => { fc.assert( fc.property( fc.array(fc.integer()), (nums) => isOrdered(sort(nums)) ) ) }) }) a fast-check "property" instance, which…
38. import * as fc from 'fast-check' describe('`sort`', () => { it('outputs ordered arrays', () => { fc.assert( fc.property( fc.array(fc.integer()), (nums) => isOrdered(sort(nums)) ) ) }) }) generates arrays of ints, and…
39. import * as fc from 'fast-check' describe('`sort`', () => { it('outputs ordered arrays', () => { fc.assert( fc.property( fc.array(fc.integer()), (nums) => isOrdered(sort(nums)) ) ) }) }) feeds them to this predicate
40. import * as fc from 'fast-check' describe('`sort`', () => { it('outputs ordered arrays', () => { fc.assert( fc.property( fc.array(fc.integer()), (nums) => isOrdered(sort(nums)) ) ) }) }) run many times, and throw upon failure…
41. PASS src/01-sort.spec.ts `sort` ✓ outputs ordered arrays (11ms) maybe it passes…
42. FAIL src/01-sort.spec.ts `sort` ✕ outputs ordered arrays (9ms) • `sort` › outputs ordered arrays Property failed after 4 tests { seed: 1213330267, path: "3:1:3:2:3:2:3:2", endOnFailure: true } Counterexample: [[-1,-2]] Shrunk 7 time(s) but if it fails, it fails elegantly
43. FAIL src/01-sort.spec.ts `sort` ✕ outputs ordered arrays (9ms) • `sort` › outputs ordered arrays Property failed after 4 tests { seed: 1213330267, path: "3:1:3:2:3:2:3:2", endOnFailure: true } Counterexample: [[-1,-2]] Shrunk 7 time(s) ran until failure (or configured limit)
44. FAIL src/01-sort.spec.ts `sort` ✕ outputs ordered arrays (9ms) • `sort` › outputs ordered arrays Property failed after 4 tests { seed: 1213330267, path: "3:1:3:2:3:2:3:2", endOnFailure: true } Counterexample: [[-1,-2]] Shrunk 7 time(s) failure was simplified 7 times
45. FAIL src/01-sort.spec.ts `sort` ✕ outputs ordered arrays (9ms) • `sort` › outputs ordered arrays Property failed after 4 tests { seed: 1213330267, path: "3:1:3:2:3:2:3:2", endOnFailure: true } Counterexample: [[-1,-2]] Shrunk 7 time(s) minimal edge case found
46. FAIL src/01-sort.spec.ts `sort` ✕ outputs ordered arrays (9ms) • `sort` › outputs ordered arrays Property failed after 4 tests { seed: 1213330267, path: "3:1:3:2:3:2:3:2", endOnFailure: true } Counterexample: [[-1,-2]] Shrunk 7 time(s) can reproduce at will
47. test assertion takes an optional config import * as fc from 'fast-check' describe('`sort`', () => { it('outputs ordered arrays', () => { fc.assert( fc.property( fc.array(fc.integer()), (nums) => isOrdered(sort(nums)) ), ) }) })
48. import * as fc from 'fast-check' describe('`sort`', () => { it('outputs ordered arrays', () => { fc.assert( fc.property( fc.array(fc.integer()), (nums) => isOrdered(sort(nums)) ), { seed: 1213330267, verbose: true } ) }) }) can specify PRNG seed
49. • `sort` › outputs ordered arrays Property failed after 4 tests { seed: 1213330267, path: "3:1:3:2:3:2:3:2", endOnFailure: true } Counterexample: [[-1,-2]] Shrunk 7 time(s) Encountered failures were: - [[-10,-1823440029,-3,-8,109548166,22,236538465]] - [[-1823440029,-3,-8,22]] - [[-1823440029,-2,-8,22]] - [[-1,-1823440029,-8,22]] - [[-1,-8,22]] - [[-1,-8]] - [[-1,-4]] - [[-1,-2]] debug same code path, and/or see how minimal case was found
50. "Very cute, but I need more complex inputs."
51. describe("(one of) De Morgans Laws", () => { test('not (P and Q) <=> (not P) or (not Q)', () => { fc.assert(fc.property( fc.boolean(), fc.boolean(), (p, q) => { return !(p && q) === !p || !q } )) }) }) can easily request multiple arbitraries (generator + shrinker = "arbitrary")
52. describe('`Array#filter`', () => { it('always results in a same-or-shorter-length arr', () => { fc.assert(fc.property( fc.array(fc.anything()), fc.func(fc.boolean()), (arr, pred) => { return arr.filter(pred).length <= arr.length } )) }) }) built-in arbitraries compose beautifully ("combinator pattern") array of any vals function returning bools
53. describe('`isOdd`', () => { it('correctly identifies odd integers', () => { fc.assert(fc.property( fc.integer().map(n => n * 2 + 1), isOdd )) }) }) function isOdd (num) { return num % 2 === 1 } a new arbitrary which generates odd integers can transform arbitraries via functional techniques like `map` and `filter` (fails on -1!)
54. tons more! size minima / maxima ✍ special strings (ascii, unicode, ipv4, json) custom generators from scratch
55. Monads and even… ‼ ‼ ‼
56. Summary
57. Complementary to other strats Unit tests for specific important cases Fuzzers for robustness against garbage input Prop tests for confidence that API contract holds <- property-based testing image credit: Nicolas Dubien
58. Plain JS -> zero-effort integration No special plugins, matchers, or bindings needed Drop into Mocha, AVA, Jest, Tape, etc. Insert assertions from Chai, Sinon, Supertest, etc.
59. THANKS! - https: //speakerdeck.com/glebec/the-magic-of-generative-testing - https: //dev.to/glebec/property-testing-with-jsverify-part-i-3one - https: // www.cs.tufts.edu/~nr/cs257/archive/john-hughes/quick.pdf - YouTube: "Don't Write Tests"! (John Hughes) - https: //github.com/dubzzz/fast-check