deleteTodo.js

function deleteTodo(text) {
  return {
    type: DELETE_TODO,
    payload: {
      text
    }
  };
}

const initialState = []

function todoReducer(state = initialState, action) {
  switch(action.type) {
  case(DELETE_TODO):
    return todos.filter((todo) => todo.text = action.payload.text);
  }
}

describe('deleting a todo', () => {
  describe('when there are no todos', () => {
    const state = [];

    it('returns the state unchanged', () => {
      assertEqual(todoReducer(state, deleteTodo('foo')), state)
    });
  });

  describe('when the todo exists alone', () => {
    const theTodo = { text: 'foo' };
    const state = [theTodo];

    it('removes the todo', () => {
      const newState = todoReducer(state, deleteTodo('foo'));

      assert(!newState.includes(theTodo));
    });
  });

  describe('when the todo exists with other todos', () => {
    const firstTodo = { text: 'foo' };
    const secondTodo = { text: 'bar' };
    const state = [firstTodo, secondTodo];

    it('removes the target todo', () => {
      const newState = todoReducer(state, deleteTodo('foo'));

      assert(!newState.includes(firstTodo));
    });

    it('leaves the other todos', () => {
      const newState = todoReducer(state, deleteTodo('foo'));

      assert(newState.includes(secondTodo));
    });
  });
});

deleteTodoActionGenerator = jsv.record({
  type: jsv.constant(DELETE_TODO),
  payload: jsv.string
});

todoGenerator = jsv.record({
  text: jsv.string
});

initialStateGenerator = jsv.array(todoGenerator);

initialStateWithTodo = jsv.pair(deleteTodoGenerator, initialStateGenerator).smap(
  (todo, initialState) => [todo, initialState.concat(todo)],
  (todo, initialState) => [todo, initialState.slice(0, -1)]
);

initialStateWithoutTodo = jsv.pair(deleteTodoGenerator, initialStateGenerator).smap(
  (todo, initialState) => [todo, initialState.filter((t) => t.text === todo.text)],
  (todo, initialState) => [todo, initialState.concat(todo)]
);

describe('deleting a todo', () => {
  describe('when deleting a todo that exists', () => {
    jsc.property('it removes the todo',
      initialStateWithTodo,
      ([todo, initialState]) => {
        const newState = todoReducer(initialState, deleteTodo(todo.text));

        assert(!newState.include(todo));
      }
    );
    jsc.property('it only removes a single todo', () => {
      initialStateWithTodo,
      ([todo, initialState]) => {
        const newState = todoReducer(initialState, deleteTodo(todo.text));

        assert(newState.length === initialState.length - 1);
      }
    });
  });

  describe('when deleting a todo that does not exist', () => {
    jsc.property('it does not modify the state',
      initialStateWithoutTodo,
      ([todo, initialState]) => {
        const newState = todoReducer(initialState, deleteTodo(todo.text));

        assert(_.equals(initialState, newState));
      }
    );
  });