testCounters.js

const testAddCounter = () => {
  const listBefore = [];
  const listAfter = [0];

  deepFreeze(listBefore);

  expect(
    addCounter(listBefore)
  ).toEqual(listAfter);
};

const testRemoveCounter = () => {
  const listBefore = [0, 10, 20];
  const listAfter = [0, 20];

  deepFreeze(listBefore);

  expect (
    removeCounter(listBefore, 1)
  ).toEqual(listAfter);
};

const testIncrementCounter = () => {
  const listBefore = [0, 10, 20];
  const listAfter = [0, 11, 20];

  deepFreeze(listBefore);

  expect(
    incrementCounter(listBefore, 1)
  ).toEqual(listAfter);
};

testAddCounter();
testRemoveCounter();
testIncrementCounter();
console.log('All tests passed')