Selectors for derived state in Angular - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to create a selector that gets the list of users from the state.

Angular

export const selectUsers = createSelector([1], (state) => state.users);

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

AselectAppState

BselectUserState

CselectAllUsers

DselectFeature

Attempts:

3 left

💡 Hint

Common Mistakes

Using a selector that points to the whole app state instead of the user slice.

Confusing selector names that do not exist.

The selector needs to start from the user state slice, which is accessed by selectUserState.

2fill in blank

medium

Complete the code to create a selector that counts the number of completed tasks.

Angular

export const selectCompletedCount = createSelector(selectTasks, (tasks) => tasks.filter(task => task.completed [1] true).length);

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

A===

B!==

C>

D<

Attempts:

3 left

💡 Hint

Common Mistakes

Using inequality operator which filters out completed tasks.

Using greater or less than operators which do not apply to booleans.

The filter should check if task.completed is exactly true to count completed tasks.

3fill in blank

hard

Fix the error in the selector that tries to get the active user by ID.

Angular

export const selectActiveUser = createSelector(selectUsers, (users, [1]) => users.find(user => user.id === props.activeUserId));

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

Aprops

Bstate

Cparams

Dcontext

Attempts:

3 left

💡 Hint

Common Mistakes

Using state or params which are not valid parameters here.

Not passing any parameter and causing runtime errors.

The second argument to the projector function is props, which contains parameters like activeUserId.

4fill in blank

hard

Fill both blanks to create a selector that returns tasks filtered by a dynamic status.

Angular

export const selectTasksByStatus = createSelector(selectTasks, (tasks, [1]) => tasks.filter(task => task.status [2] props.status));

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

Aprops

Bstate

C===

D!==

Attempts:

3 left

💡 Hint

Common Mistakes

Using state instead of props.

Using inequality operator which filters out matching tasks.

The selector uses props to get the status, and compares task.status with === to filter correctly.

5fill in blank

hard

Fill all three blanks to create a selector that returns the names of users older than a given age.

Angular

export const selectUserNamesOlderThan = createSelector(selectUsers, (users, [1]) => users.filter(user => user.age [2] props.minAge).map(user => user.[3]));

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

Aprops

B>

Cname

Dstate

Attempts:

3 left

💡 Hint

Common Mistakes

Using state instead of props.

Using wrong comparison operator like <.

Mapping to a wrong property instead of name.

The selector uses props to get minAge, filters users older than minAge using >, and maps to their name.

Practice

1. What is the main purpose of using createSelector in Angular state management?

easy

A. To dispatch actions to the store

B. To directly modify the state values

C. To compute derived data from the state efficiently

D. To subscribe to HTTP requests

Solution

1. Step 1: Understand what createSelector does

   createSelector is used to create selectors that compute derived data from the state without modifying it.

2. Step 2: Differentiate from other store operations

   Modifying state or dispatching actions are done by reducers and actions, not selectors.

3. Final Answer:

   To compute derived data from the state efficiently -> Option C

4. Quick Check:

   Derived data = createSelector [OK]

Hint: Remember: selectors read and compute, not modify state [OK]

Common Mistakes:

• Confusing selectors with actions or reducers
• Thinking selectors modify state
• Assuming selectors handle side effects

2. Which of the following is the correct syntax to create a selector that derives the total count from a list in the state using createSelector?

easy

A. const selectTotal = createSelector(list => list.length, selectList);

B. const selectTotal = createSelector(selectList, list => list.length);

C. const selectTotal = createSelector(selectList, list => list.count);

D. const selectTotal = createSelector(selectList, list => list.size());

Solution

1. Step 1: Check the order of arguments in createSelector

   The first argument(s) are input selectors, followed by a projector function.

2. Step 2: Verify the projector function logic

   The projector function receives the selected data and returns the derived value. Using list.length correctly gets the count.

3. Final Answer:

   const selectTotal = createSelector(selectList, list => list.length); -> Option B

4. Quick Check:

   Input selectors first, then projector function [OK]

Hint: Input selectors first, projector function last in createSelector [OK]

Common Mistakes:

• Swapping input selectors and projector function order
• Using incorrect property like count or size()
• Calling methods instead of accessing properties

3. Given the following selectors:

const selectItems = (state) => state.items;
const selectCompletedItems = createSelector(selectItems, items => items.filter(item => item.done));

What will selectCompletedItems return if state.items is [{done: true}, {done: false}, {done: true}]?

medium

A. [{done: true}, {done: true}]

B. []

C. [{done: false}]

D. [{done: true}, {done: false}]

Solution

1. Step 1: Understand the input selector

   selectItems returns the full list of items from state.

2. Step 2: Apply the projector function filter

   The projector filters items where item.done is true, so it keeps only those objects.

3. Final Answer:

   [{done: true}, {done: true}] -> Option A

4. Quick Check:

   Filter done=true items = [{done:true}, {done:true}] [OK]

Hint: Filter returns only items matching condition [OK]

Common Mistakes:

• Including false done items in result
• Returning original list without filtering
• Confusing filter condition logic

4. Identify the error in this selector code:

const selectUsers = (state) => state.users;
const selectActiveUsers = createSelector(selectUsers, users => users.active);

medium

A. The projector function incorrectly accesses users.active instead of filtering

B. The input selector should return a property, not the whole state

C. createSelector requires at least two input selectors

D. The selector should be an async function

Solution

1. Step 1: Check the input selector

   selectUsers correctly returns state.users, which is valid.

2. Step 2: Analyze the projector function

   The projector accesses users.active, but users is likely an array, so it should filter or map, not access a property.

3. Final Answer:

   The projector function incorrectly accesses users.active instead of filtering -> Option A

4. Quick Check:

   Projector must handle data type correctly [OK]

Hint: Check if projector matches data type (array vs object) [OK]

Common Mistakes:

• Assuming users is an object, not an array
• Trying to access properties on arrays directly
• Misunderstanding projector function role

5. You want to create a selector that returns the number of completed tasks and the number of pending tasks from state.
Given:

const selectTasks = (state) => state.tasks;

Which of the following createSelector implementations correctly derives this data?

hard

A. const selectTaskCounts = createSelector(selectTasks, tasks => tasks.doneCount)

B. const selectTaskCounts = createSelector(selectTasks, tasks => { completed: tasks.filter(t => t.done).length, pending: tasks.filter(t => !t.done).length });

C. const selectTaskCounts = createSelector(selectTasks, tasks => [tasks.filter(t => t.done).length, tasks.filter(t => !t.done).length]);

D. const selectTaskCounts = createSelector(selectTasks, tasks => ({ completed: tasks.filter(t => t.done).length, pending: tasks.filter(t => !t.done).length }));

Solution

1. Step 1: Check the syntax of the projector function

   const selectTaskCounts = createSelector(selectTasks, tasks => ({ completed: tasks.filter(t => t.done).length, pending: tasks.filter(t => !t.done).length })); returns an object with properties completed and pending correctly using parentheses to return the object literal.

2. Step 2: Identify errors in other options

   const selectTaskCounts = createSelector(selectTasks, tasks => { completed: tasks.filter(t => t.done).length, pending: tasks.filter(t => !t.done).length }); lacks parentheses around the object, causing a syntax error. const selectTaskCounts = createSelector(selectTasks, tasks => [tasks.filter(t => t.done).length, tasks.filter(t => !t.done).length]); returns an array, not an object. const selectTaskCounts = createSelector(selectTasks, tasks => tasks.doneCount) accesses a non-existent property.

3. Final Answer:

   const selectTaskCounts = createSelector(selectTasks, tasks => ({ completed: tasks.filter(t => t.done).length, pending: tasks.filter(t => !t.done).length })); -> Option D

4. Quick Check:

   Use parentheses to return object literals in arrow functions [OK]

Hint: Wrap object in parentheses to return from arrow function [OK]

Common Mistakes:

• Missing parentheses around object literal
• Returning array instead of object
• Accessing undefined properties