Selectors for derived state in Angular

Introduction

Selectors help you get specific pieces of data from your app's state. They can also create new data by combining or changing existing state values.

You want to show a list filtered by user input without changing the original list.

You need to calculate a total price from items in a shopping cart.

You want to combine user info and settings into one object for display.

You want to avoid repeating code that picks parts of the state in many places.

Syntax

Angular

import { createSelector } from '@ngrx/store';

const selectFeature = (state) => state.feature;

const selectDerived = createSelector(
  selectFeature,
  (feature) => {
    // return derived data here
  }
);

createSelector takes one or more selectors as input and a function that returns new data.

The function runs only when input data changes, improving performance.

Examples

This selector returns the number of items in the list.

Angular

const selectItems = (state) => state.items;

const selectItemCount = createSelector(
  selectItems,
  (items) => items.length
);

This combines user and settings data into one object.

Angular

const selectUser = (state) => state.user;
const selectSettings = (state) => state.settings;

const selectUserProfile = createSelector(
  selectUser,
  selectSettings,
  (user, settings) => ({
    name: user.name,
    theme: settings.theme
  })
);

Sample Program

This Angular component shows a shopping cart list and the total price. The total price is calculated using a selector that sums item prices times quantities.

Angular

import { Component } from '@angular/core';
import { createSelector } from '@ngrx/store';

interface AppState {
  cart: { items: { name: string; price: number; quantity: number }[] };
}

const state: AppState = {
  cart: {
    items: [
      { name: 'Apple', price: 1, quantity: 3 },
      { name: 'Banana', price: 2, quantity: 2 }
    ]
  }
};

const selectCartItems = (state: AppState) => state.cart.items;

const selectTotalPrice = createSelector(
  selectCartItems,
  (items) => items.reduce((total, item) => total + item.price * item.quantity, 0)
);

@Component({
  selector: 'app-cart',
  standalone: true,
  template: `
    <h2>Shopping Cart</h2>
    <ul>
      <li *ngFor="let item of items">{{item.name}} x{{item.quantity}} - ${{item.price}}</li>
    </ul>
    <p><strong>Total: ${{total}}</strong></p>
  `
})
export class CartComponent {
  items = state.cart.items;
  total = selectTotalPrice(state);
}

Important Notes

Selectors help keep your UI code clean by moving data logic out of components.

Using selectors improves performance because they only recalculate when needed.

Summary

Selectors get and create data from your app state.

They help avoid repeating code and improve performance.

Use createSelector with input selectors and a function to make derived data.

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