javascript-testing-expert

it('should...', () => {
  // Arrange
  code;

  // Act
  code;

  // Assert
  code;
});

const user: User = {
  name: 'Paul', // unused
  birthday: '2010-02-03',
};
const age = computeAge(user);
//...

import { describe } from 'vitest';
import { it, fc } from '@fast-check/vitest';

describe('computeAge', () => {
  it('should compute a positive age', ({ g }) => {
    // Arrange
    const user: User = {
      name: g(fc.string), // unused
      birthday: '2010-02-03',
    };

    // Act
    const age = computeAge(user);

    // Assert
    expect(age).toBeGreaterThan(0);
  });
});

// Arrange
vi.setSystemTime(g(fc.date, { min: new Date('2010-02-04'), noInvalidDate: true }));
const user: User = {
  name: g(fc.string), // unused
  birthday: '2010-02-03',
};

// for all a, b, c strings
// b is a substring of a + b + c
it.prop([fc.string(), fc.string(), fc.string()])('should detect the substring', (a, b, c) => {
  // Arrange
  const text = a + b + c;
  const pattern = b;

  // Act
  const result = isSubstring(text, pattern);

  // Assert
  expect(result).toBe(true);
});

it('should resolve in call order', async () => {
  // Arrange
  const seenAnswers = [];
  const call = vi.fn().mockImplementation((v) => Promise.resolve(v));

  // Act
  const queued = queue(call);
  await Promise.all([queued(1).then((v) => seenAnswers.push(v)), queued(2).then((v) => seenAnswers.push(v))]);

  // Assert
  expect(seenAnswers).toEqual([1, 2]);
});

it('should resolve in call order', async () => {
  await fc.assert(
    fc.asyncProperty(fc.scheduler(), async (s) => {
      // Arrange
      const seenAnswers = [];
      const call = vi.fn().mockImplementation((v) => Promise.resolve(v));

      // Act
      const queued = queue(s.scheduleFunction(call));
      await s.waitFor(
        Promise.all([queued(1).then((v) => seenAnswers.push(v)), queued(2).then((v) => seenAnswers.push(v))]),
      );

      // Assert
      expect(seenAnswers).toEqual([1, 2]);
    }),
  );
});

// Source: https://fast-check.dev/blog/2024/07/18/integrating-faker-with-fast-check/
import { Faker, Randomizer, base } from '@faker-js/faker';
import fc from 'fast-check';

class FakerBuilder<TValue> extends fc.Arbitrary<TValue> {
  constructor(private readonly generator: (faker: Faker) => TValue) {
    super();
  }
  generate(mrng: fc.Random, biasFactor: number | undefined): fc.Value<TValue> {
    const randomizer: Randomizer = {
      next: (): number => mrng.nextDouble(),
      seed: () => {}, // no-op, no support for updates of the seed, could even throw
    };
    const customFaker = new Faker({ locale: base, randomizer });
    return new fc.Value(this.generator(customFaker), undefined);
  }
  canShrinkWithoutContext(value: unknown): value is TValue {
    return false;
  }
  shrink(value: TValue, context: unknown): fc.Stream<fc.Value<TValue>> {
    return fc.Stream.nil();
  }
}

function fakerToArb<TValue>(generator: (faker: Faker) => TValue): fc.Arbitrary<TValue> {
  return new FakerBuilder(generator);
}

fc.assert(
  fc.property(
    fakerToArb((faker) => faker.person.firstName),
    fakerToArb((faker) => faker.person.lastName),
    (firstName, lastName) => {
      // code
    },
  ),
);

// with @fast-check/vitest
import { it, fc } from '@fast-check/vitest';
it('...', ({ g }) => {
  //...
});

// with fast-check
import { it } from 'vitest';
import fc from 'fast-check';
it('...', () => {
  fc.assert(
    fc.property(fc.gen(), (g) => {
      //...
    }),
  );
});

// with @fast-check/vitest
import { it, fc } from '@fast-check/vitest';
it.prop([...arbitraries])('...', (...values) => {
  //...
});

// with fast-check
import { it } from 'vitest';
import fc from 'fast-check';
it('...', () => {
  fc.assert(
    fc.property(...arbitraries, (...values) => {
      //...
    }),
  );
});

// with @fast-check/vitest
import { it, fc } from '@fast-check/vitest';
it.prop([...arbitraries])('...', async (...values) => {
  //...
});

// with fast-check
import { it } from 'vitest';
import fc from 'fast-check';
it('...', async () => {
  await fc.assert(
    fc.asyncProperty(...arbitraries, async (...values) => {
      //...
    }),
  );
});