• Working in pairs
• Fork and clone repo
• Run npm install to install jest
• Create a test folder where your tests will live
• Implement tests for each function. Provide at least 2 test cases for each function
• Rewrite functions to become pure functions. Use tests to verify the inputs to each function remain unchanged by the execution of each function
• Replace instance of var with constor let as appropriate
• Replace for loops etc with relevant array methods
• Submit pull request with completed code

When testing that inputs have not changed, it's best to produce similar objects for input and expectedInput. This is because, if we run the following code

test('Ensure purity', function() {
  const input = {a: 10};
  const expectedOutput = {a: 20};

  const output = myFunction(input);

  expect(output).toEqual(expectedOutput);
  expect(input).toEqual(input); // will always pass
});

even if input get mutated, the second assertion will always pass because we are comparing input to itself. Instead, provide an expectedInput variable that has same value as input.

test('Ensure purity', function() {
  const input = {a: 10};
  const expectedInput = {a: 10};

  const expectedOutput = {a: 20};

  const output = myFunction(input);

  expect(output).toEqual(expectedOutput);
  expect(input).toEqual(expectedInput); // will check that input was not mutated
});