How React Testing Library Can Improve Your Mental Health [Part 2]

You can find the first part here: Why testing?

Let’s continue writing our Redux tests. The file we are testing is:

const initialState = {
   todos: [
       { id: 1, text: 'Do boring stuff', completed: false, color: 'purple' },
       { id: 2, text: 'Write tests!', completed: false, color: 'blue' }
   ],
   filters: {
       status: 'All',
       colors: []
   }
}
function nextTodoId(todos) {
const maxId = todos.reduce((maxId, todo) => Math.max(todo.id, maxId), -1)
return maxId + 1
}
export default function appReducer(state = initialState, action) {
switch (action.type) {
case 'todos/todoAdded':
           {
return {
                   ...state,
                   todos: [
                       ...state.todos,
                       {
                           id: nextTodoId(state.todos),
                           text: action.payload,
                           completed: false
                       }
                   ]
               }
           }
case 'todos/todoToggled':
           {
return {
                   ...state,
                   todos: state.todos.map(todo => {
if (todo.id !== action.payload) {
return todo
                       }
return {
                           ...todo,
                           completed: !todo.completed
                       }
                   })
               }
           }
case 'filters/statusFilterChanged':
           {
return {
// Copy the whole state
                   ...state,
// Overwrite the filters value
                   filters: {
// copy the other filter fields
                       ...state.filters,
// And replace the status field with the new value
                       status: action.payload
                   }
               }
           }
default:
return state
   }
}