Redux selectors in React Native - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to select the user name from the Redux state.

React Native

const userName = useSelector(state => state.user[1]name);

Drag options to blanks, or click blank then click option'

A.

B->

C:

D,

Attempts:

3 left

💡 Hint

Common Mistakes

Using arrow or colon instead of dot causes syntax errors.

Forgetting to access the nested property.

The dot . operator accesses the name property of the user object in the Redux state.

2fill in blank

medium

Complete the code to create a selector function that returns the list of todos from state.

React Native

const selectTodos = state => state.[1];

Drag options to blanks, or click blank then click option'

Atasks

BtodoList

Citems

Dtodos

Attempts:

3 left

💡 Hint

Common Mistakes

Using incorrect property names like tasks or items.

Confusing the selector with an action creator.

The selector accesses todos property directly from the state object.

3fill in blank

hard

Fix the error in the selector to get completed todos only.

React Native

const selectCompletedTodos = state => state.todos.filter(todo => todo.[1] === true);

Drag options to blanks, or click blank then click option'

AisComplete

Bfinished

Ccompleted

Ddone

Attempts:

3 left

💡 Hint

Common Mistakes

Using property names that don't exist in todo objects.

Forgetting to compare to true.

The property completed is the standard boolean flag for completed todos.

4fill in blank

hard

Fill both blanks to create a memoized selector using reselect.

React Native

import { createSelector } from 'reselect';

const selectTodos = state => state.todos;

const selectCompletedTodos = createSelector(
  selectTodos,
  todos => todos.filter(todo => todo.[1] === [2])
);

Drag options to blanks, or click blank then click option'

Acompleted

Btrue

Cfalse

Ddone

Attempts:

3 left

💡 Hint

Common Mistakes

Using wrong property names or boolean values.

Not using createSelector correctly.

The selector filters todos where completed is true using createSelector for memoization.

5fill in blank

hard

Fill both blanks to create a selector that returns the count of active todos.

React Native

const selectActiveTodoCount = createSelector(
  state => state.todos,
  todos => todos.filter(todo => !todo.[1]).[2]
);

Drag options to blanks, or click blank then click option'

Acompleted

Blength

C+

D-

Attempts:

3 left

💡 Hint

Common Mistakes

Using wrong property names or operators.

Forgetting to negate the completed property.

The selector filters todos where completed is false, then gets the length of that filtered array.

Practice

1. What is the main purpose of a Redux selector in a React Native app?

easy

A. To create UI components

B. To extract specific data from the Redux store state

C. To define the initial state of the store

D. To dispatch actions to update the store

Solution

1. Step 1: Understand Redux store role

   The Redux store holds the entire app state in one place.

2. Step 2: Role of selectors

   Selectors are functions that pick specific parts of that state for use in components.

3. Final Answer:

   To extract specific data from the Redux store state -> Option B

4. Quick Check:

   Selector = extract data [OK]

Hint: Selectors pick data from state, not change it [OK]

Common Mistakes:

• Confusing selectors with action creators
• Thinking selectors update state
• Using selectors to create UI components

2. Which of the following is the correct syntax to define a simple Redux selector function named selectUser that returns state.user?

easy

A. const selectUser = () => state.user;

B. const selectUser = (state) => { state.user }

C. function selectUser() { return user; }

D. const selectUser = state => state.user;

Solution

1. Step 1: Check function parameter

   A selector receives the whole state as an argument, so it must have state parameter.

2. Step 2: Check return statement

   The selector returns state.user directly. Arrow function with implicit return is correct.

3. Final Answer:

   const selectUser = state => state.user; -> Option D

4. Quick Check:

   Arrow function with state param and return [OK]

Hint: Selector must take state and return part of it [OK]

Common Mistakes:

• Omitting the state parameter
• Not returning the value properly
• Using braces without return statement

3. Given the selector const selectCompletedTodos = state => state.todos.filter(todo => todo.completed); and the state { todos: [{id:1, completed:true}, {id:2, completed:false}, {id:3, completed:true}] }, what will selectCompletedTodos(state) return?

medium

A. [{id:1, completed:true}, {id:3, completed:true}]

B. [{id:2, completed:false}]

C. []

D. undefined

Solution

1. Step 1: Understand selector logic

   The selector filters todos where completed is true.

2. Step 2: Apply filter to given state

   Todos with id 1 and 3 have completed:true, so they are included.

3. Final Answer:

   [{id:1, completed:true}, {id:3, completed:true}] -> Option A

4. Quick Check:

   Filter completed todos = correct list [OK]

Hint: Filter returns only items matching condition [OK]

Common Mistakes:

• Selecting incomplete todos instead
• Returning empty array by mistake
• Confusing filter with map

4. What is wrong with this selector code?

const selectUserName = state => { state.user.name }

medium

A. It does not return any value explicitly

B. It uses wrong parameter name

C. It should dispatch an action instead

D. It should be a class method

Solution

1. Step 1: Check function body

   The function uses braces but no return statement, so it returns undefined.

2. Step 2: Fix by adding return

   Either remove braces or add return to return state.user.name.

3. Final Answer:

   It does not return any value explicitly -> Option A

4. Quick Check:

   Missing return in arrow function with braces [OK]

Hint: Arrow with braces needs explicit return [OK]

Common Mistakes:

• Forgetting return inside braces
• Confusing selectors with actions
• Using wrong parameter names

5. You want to create a memoized selector using reselect to get the count of completed todos from state. Which code correctly implements this?

hard

A. import { createSelector } from 'redux';
const selectCompletedCount = createSelector(state => state.todos, todos => todos.length);

B. const selectCompletedCount = state => state.todos.filter(todo => todo.completed).length;

C. import { createSelector } from 'reselect';
const selectTodos = state => state.todos;
const selectCompletedCount = createSelector(
selectTodos,
todos => todos.filter(todo => todo.completed).length
);

D. const selectCompletedCount = createSelector(state => state.todos, todos => todos.completed.length);

Solution

1. Step 1: Use correct import and createSelector

   The createSelector function is imported from 'reselect' and used properly.

2. Step 2: Define input selector and output function

   selectTodos gets todos array; output function filters completed and counts them.

3. Step 3: Check other options for errors

   The plain selector const selectCompletedCount = state => state.todos.filter(todo => todo.completed).length is not memoized; importing createSelector from 'redux' is wrong; todos => todos.completed.length accesses property incorrectly.

4. Final Answer:

   import { createSelector } from 'reselect'; const selectTodos = state => state.todos; const selectCompletedCount = createSelector(selectTodos, todos => todos.filter(todo => todo.completed).length); -> Option C

5. Quick Check:

   Use reselect createSelector with input and output functions [OK]

Hint: Use createSelector from 'reselect' with input and output functions [OK]

Common Mistakes:

• Importing createSelector from wrong package
• Not memoizing selector
• Incorrectly accessing properties in output function