Thanks to the referential transparency ensured by ADR 6, unit test of Alfa code is usually very easy, simply comparing the actual result with the expected one (often as their serialisation), without need for complex setup, mocks, or other test tricks.

We're therefore implementing a very lightweight wrapper for tests.

import { test } from "@siteimprove/alfa-test";

test("My test", (t) => {
  const actual = …;

  t.deepEqual(actual.toJSON(), { type: …, …})
});

Sometimes, it is convenient to generate random tests with random values. The alfa-test library is offering test controller to handle that.

• The assertion function that is passed to test(name, assertion) receives additional rng and seed parameters. The rng is a function () => number. The seed was used to initialize the Random Number Generator, can be used for better displaying errors and for re-playability.
• The test function itself accepts an optional Controller object which can be used to set the seed for the RNG, or to change the number of iterations to run the test (default to 1 since most tests are not random tests). The Controller object also accepts a wrapper function of type (iteration: number, rng: RNG) => RNG that can be used to turn the random numbers into useful data, or for introspection.

The provided rng function is guaranteed to generate the same sequence of numbers on sequential calls, if the same seed is provided by the controller. If no seed is provided, a random one will be used.

By default, each test is only run once. Use the Controller object to change the number of iterations.

Tests that make use of the RNG are encouraged to print the seed in their error message in order to allow re-playability and investigation by feeding the failing seed back to the test.

For re-playability, use the Controller parameter to select the seed to use (which guarantees the exact same sequence of numbers is produced), and to introspect on fine details by wrapping the RNG, e.g.,

/**
 * Return a random string between "0" and "100" (inclusive).
 * Print the generated number, as well as the iteration number (use for debugging).
 */
function wrapper(rng: RNG<number>, iteration: number): RNG<string> {
  return () => {
    const res = rng();
    console.log(`On iteration ${iteration}, I generated ${res}`);
    return `${res * 100}`;
  };
}

test(
  "Sum computes the sum of two numbers represented as strings",
  (t, rng, seed) => {
    // These use the post-wrapper RNG.
    const a = rng();
    const b = rng();
    // Print the seed in error message to allow introspection.
    const actual = sum(a, b, `Failed with seed ${seed}`);

    t.deepEqual(actual, `${a + b}`);
  },
  {
    wrapper,
    iterations: 100,
    // Set the seed for debugging, if you want to replay the same sequence of numbers.
    seed: 1234,
  },
);