Computed signals for derived values in Angular - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What is the output of this Angular computed signal example?

Consider this Angular standalone component using signals and computed signals. What will be displayed inside the paragraph?

Angular

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

@Component({
  selector: 'app-root',
  standalone: true,
  template: `<p>{{ fullName() }}</p>`
})
export class AppComponent {
  firstName = signal('Jane');
  lastName = signal('Doe');
  fullName = computed(() => `${this.firstName()} ${this.lastName()}`);
}

A Error: fullName is not a function

B Jane Doe

C Doe

D Jane

💡 Hint

Remember that computed signals are functions that return the derived value.

The computed signal fullName combines firstName and lastName signals. Calling fullName() returns the concatenated string 'Jane Doe'.

❓ state_output

intermediate

What is the value of 'fullName' after updating signals?

Given this Angular component, what is the value returned by fullName() after the lastName signal is updated?

Angular

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

@Component({
  selector: 'app-root',
  standalone: true,
  template: `{{ fullName() }}`
})
export class AppComponent {
  firstName = signal('John');
  lastName = signal('Smith');
  fullName = computed(() => `${this.firstName()} ${this.lastName()}`);

  constructor() {
    this.lastName.set('Brown');
  }
}

A John Brown

B John Smith

C Brown

D John

💡 Hint

Signals update reactively, so computed signals reflect the latest values.

The lastName signal is updated to 'Brown' in the constructor. The computed fullName reflects this change and returns 'John Brown'.

📝 Syntax

advanced

Which option correctly defines a computed signal for the sum of two signals?

You want to create a computed signal that returns the sum of two number signals: countA and countB. Which code snippet is correct?

A sum = computed(() => countA() + countB);

B sum = computed(() => countA + countB);

C sum = computed(() => this.countA() + this.countB());

D sum = computed(() => this.countA + this.countB());

💡 Hint

Remember to call signals as functions to get their current values.

Signals are functions. To get their values inside computed, you must call them with (). Option C correctly calls both signals.

🔧 Debug

advanced

Why does this computed signal not update when signals change?

Look at this Angular component code. Why does fullName not update when firstName or lastName change?

Angular

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

@Component({
  selector: 'app-root',
  standalone: true,
  template: `{{ fullName() }}`
})
export class AppComponent {
  firstName = signal('Alice');
  lastName = signal('Wonderland');
  fullName = computed(() => this.firstName + ' ' + this.lastName);

  constructor() {
    this.firstName.set('Bob');
  }
}

A Because fullName is missing a return statement.

B Because signals cannot be used inside computed signals.

C Because the constructor updates firstName before fullName is defined.

D Because signals are not called as functions inside computed, so dependencies are not tracked.

💡 Hint

Check how signals are accessed inside the computed function.

Signals must be called as functions to track dependencies. Using this.firstName without () returns the signal object, not its value, so computed does not update.

🧠 Conceptual

expert

What happens if a computed signal depends on another computed signal that throws an error?

In Angular signals, if computed signal A depends on computed signal B, and B throws an error during evaluation, what is the behavior of A when accessed?

A Accessing A will also throw the same error because it depends on B.

B A will return undefined silently without throwing an error.

C A will cache the last successful value and not throw an error.

D A will ignore B's error and return a default value.

💡 Hint

Think about how errors propagate in reactive computed chains.

If a computed signal throws an error, any computed signal depending on it will also throw that error when accessed, propagating the failure.

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