Computed signals for derived values in Angular

Introduction

Computed signals help you automatically update values when other values change. They keep your app data fresh without extra work.

You want to show a total price that updates when item prices change.

You need to display a full name that updates when first or last name changes.

You want to enable or disable a button based on multiple input fields.

You want to calculate a value based on other signals without writing manual update code.

Syntax

Angular

const derivedValue = computed(() => {
  // return a value based on other signals
});

The computed function takes a function that returns a value.

It automatically tracks signals used inside and updates when they change.

Examples

This creates a signal count and a computed signal doubleCount that is always twice the count.

Angular

const count = signal(0);
const doubleCount = computed(() => count() * 2);

This computed signal combines two signals to create a full name that updates automatically.

Angular

const firstName = signal('Jane');
const lastName = signal('Doe');
const fullName = computed(() => `${firstName()} ${lastName()}`);

Sample Program

This Angular component shows a shopping cart with an item price and quantity. The total price is a computed signal that updates automatically when quantity changes. Clicking the button increases quantity and updates total.

Angular

import { Component, signal, computed } from '@angular/core';
import { CommonModule } from '@angular/common';

@Component({
  selector: 'app-root',
  standalone: true,
  imports: [CommonModule],
  template: `
    <h1>Shopping Cart</h1>
    <p>Item price: {{ itemPrice() | currency }}</p>
    <p>Quantity: {{ quantity() }}</p>
    <p><strong>Total: {{ total() | currency }}</strong></p>
    <button (click)="increase()">Add One</button>
  `
})
export class AppComponent {
  itemPrice = signal(10);
  quantity = signal(1);

  total = computed(() => this.itemPrice() * this.quantity());

  increase() {
    this.quantity.update(q => q + 1);
  }
}

Important Notes

Computed signals only recalculate when their dependencies change, making them efficient.

Use computed inside components or services to keep derived data reactive.

Summary

Computed signals automatically update derived values when dependencies change.

They simplify keeping your UI and data in sync without manual updates.

Practice

1. What is the main purpose of a computed signal in Angular's signal system?

easy

A. To replace all signals with a single global state

B. To store static data that never changes

C. To automatically update a value based on other signals when they change

D. To manually trigger UI updates

Solution

1. Step 1: Understand what computed signals do

   Computed signals derive their value from other signals and update automatically when those signals change.

2. Step 2: Compare options with this behavior

   Only To automatically update a value based on other signals when they change describes automatic updates based on dependencies, which matches computed signals.

3. Final Answer:

   To automatically update a value based on other signals when they change -> Option C

4. Quick Check:

   Computed signals = auto-update derived values [OK]

Hint: Computed signals auto-update when dependencies change [OK]

Common Mistakes:

• Thinking computed signals store static data
• Confusing manual updates with automatic updates
• Assuming computed signals replace all signals

2. Which of the following is the correct syntax to create a computed signal in Angular?

easy

A. const total = computed(() => price() + tax());

B. const total = signal(() => price + tax);

C. const total = computed(price + tax);

D. const total = signal(price() + tax());

Solution

1. Step 1: Recall computed signal syntax

   Computed signals use computed(() => ...) with a function returning the derived value.

2. Step 2: Check each option

   const total = computed(() => price() + tax()); correctly uses computed(() => price() + tax()). const total = signal(() => price + tax); wrongly uses signal and no function. const total = computed(price + tax); misses the function wrapper. const total = signal(price() + tax()); uses signal instead of computed.

3. Final Answer:

   const total = computed(() => price() + tax()); -> Option A

4. Quick Check:

   Computed syntax = computed(() => value) [OK]

Hint: Use computed(() => ...) with a function for derived values [OK]

Common Mistakes:

• Using signal() instead of computed() for derived values
• Passing expressions directly without a function
• Not calling dependent signals as functions

3. Given the code below, what will be logged after count.set(5) is called?

const count = signal(0);
const double = computed(() => count() * 2);

console.log(double());
count.set(5);
console.log(double());

medium

A. 0 then 10

B. 0 then 0

C. NaN then 10

D. Error at runtime

Solution

1. Step 1: Evaluate initial values

   Initially, count is 0, so double() returns 0 * 2 = 0.

2. Step 2: After count.set(5)

   Setting count to 5 updates double automatically. Calling double() now returns 5 * 2 = 10.

3. Final Answer:

   0 then 10 -> Option A

4. Quick Check:

   Initial double=0, after update=10 [OK]

Hint: Computed updates automatically after signal changes [OK]

Common Mistakes:

• Assuming computed does not update after set()
• Forgetting to call signals as functions
• Expecting errors due to missing subscriptions

4. Identify the error in the following code snippet using computed signals:

const price = signal(100);
const tax = signal(0.1);
const total = computed(() => price + tax * price);

console.log(total());

medium

A. Computed signals cannot use arithmetic operations

B. Signals must be called as functions inside computed

C. Signals cannot be used inside computed

D. Missing initial value for tax signal

Solution

1. Step 1: Check how signals are accessed

   Signals are functions and must be called with () to get their current value.

2. Step 2: Analyze the computed expression

   The expression uses price and tax directly without calling them, so it uses the signal objects, not their values.

3. Final Answer:

   Signals must be called as functions inside computed -> Option B

4. Quick Check:

   Access signals with () inside computed [OK]

Hint: Always call signals as functions inside computed [OK]

Common Mistakes:

• Using signal variables directly without ()
• Thinking computed disallows arithmetic
• Ignoring signal initial values

5. You want to create a computed signal that returns the full name by combining two signals: firstName and lastName. Which code correctly updates the full name when either signal changes and avoids unnecessary recomputations?

hard

A. const fullName = computed(() => firstName() + lastName());

B. const fullName = signal(`${firstName()} ${lastName()}`);

C. const fullName = computed(() => firstName + ' ' + lastName);

D. const fullName = computed(() => `${firstName()} ${lastName()}`);

Solution

1. Step 1: Understand the goal

   The computed signal should combine firstName and lastName signals and update automatically when either changes.

2. Step 2: Evaluate each option

   const fullName = computed(() => `${firstName()} ${lastName()}`); correctly calls both signals as functions and concatenates with a space. const fullName = signal(`${firstName()} ${lastName()}`); uses signal which won't update automatically. const fullName = computed(() => firstName + ' ' + lastName); uses signal variables directly without calling them. const fullName = computed(() => firstName() + lastName()); concatenates without space.

3. Final Answer:

   const fullName = computed(() => `${firstName()} ${lastName()}`); -> Option D

4. Quick Check:

   Call signals with () and combine with space [OK]

Hint: Use computed with template literals calling signals () [OK]

Common Mistakes:

• Using signal() instead of computed() for derived values
• Not calling signals as functions
• Forgetting space between names