Example of Property-based Testing using the fast-check library.

sort.spec.js

import * as fc from 'fast-check';
import { sort } from '../src/sort';

test('should contain the same items', () => {
  const count = (tab, element) => tab.filter(v => v === element).length;
  fc.assert(
    fc.property(fc.array(fc.integer()), data => {
      const sorted = sort(data);
      assert(sorted.length === data.length);
      for (const item of data) {
        assert(count(sorted, item) ==== count(data, item));
      }
    })
  );
});

test('should produce ordered array', () => {
  fc.assert(
    fc.property(fc.array(fc.integer()), data => {
      const sorted = sort(data);
      for (let idx = 1; idx < sorted.length; ++idx) {
        assert(sorted[idx - 1] <= sorted[idx]);
      }
    })
  );
});