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testland/fast-check-testing

Use whenever the task mentions property-based, generative or invariant tests in JavaScript or TypeScript (fast-check): fc.assert and fc.property, arbitraries and composite inputs, map, chain and filter, shrinking and seeds, with Jest, Vitest or Mocha. Has a worked example to start from.

0.96x

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Impact

94%

0.96x

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fast-check arbitraries, combinators, and composite inputs

How to build the generators passed to fc.property(<arbitraries...>, predicate).

Arbitraries catalog

Per fast-check-overview:

Arbitrary	Generates
`fc.string()`	Strings
`fc.integer()`	Integers
`fc.float()` / `fc.double()`	Floats
`fc.boolean()`	Booleans
`fc.array(item)`	Arrays of `item`
`fc.tuple(a, b, ...)`	Fixed-length tuples
`fc.record({ key: ... })`	Objects with specified properties
`fc.option(item)`	`item` or `null`
`fc.constantFrom(...)`	One of fixed values
`fc.oneof(a, b, ...)`	One of multiple arbitraries
`fc.uuid()` / `fc.ipV4()` / `fc.emailAddress()` / `fc.webUrl()`	Format-specific
`fc.date()`	Dates
`fc.uniqueArray(item)`	Arrays without duplicates
`fc.dictionary(key, value)`	Map / Record types

Combinators (.map / .chain / .filter)

Per fast-check-overview: "Extensible via map() and chain() combinators."

// .map: transform a generated value
const evenInteger = fc.integer().map(n => n * 2);

// .chain: dependent generation (later value depends on earlier)
const stringWithKnownLength = fc.integer({ min: 1, max: 100 })
  .chain(len => fc.string({ minLength: len, maxLength: len }));

// .filter: reject (use sparingly - slow when filter rejects most)
const positiveInteger = fc.integer().filter(n => n > 0);
// Better:
const positiveInteger = fc.integer({ min: 1 });

.filter() discards rejections; .map() transforms. Prefer .map() and constrained arbitraries over .filter() when possible.

Composite arbitraries via `fc.record`

const user = fc.record({
  id: fc.uuid(),
  email: fc.emailAddress(),
  age: fc.integer({ min: 18, max: 100 }),
  tags: fc.uniqueArray(fc.constantFrom('admin', 'beta', 'churn-risk')),
  createdAt: fc.date({ min: new Date('2020-01-01'), max: new Date() }),
});

it('serializes user to JSON and back', () => {
  fc.assert(
    fc.property(user, (u) => {
      expect(JSON.parse(JSON.stringify(u))).toEqual({
        ...u,
        createdAt: u.createdAt.toISOString(),
      });
    })
  );
});

fc.record produces objects with the specified shape; each field is sampled per its arbitrary.