Signal vs observable comparison in Angular - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to create a signal in Angular.

Angular

const count = [1](0);

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

Asignal

Bobservable

CSubject

DBehaviorSubject

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'observable' instead of 'signal' to create a signal.

Confusing Subject with signal.

Signals in Angular are created using the signal function.

2fill in blank

medium

Complete the code to subscribe to an observable in Angular.

Angular

myObservable.[1](() => console.log('Data received'));

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

Anext

Bemit

Csubscribe

Dlisten

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'emit' which is for EventEmitters.

Using 'next' which is a method on Subjects, not on observables directly.

To listen to values from an observable, you use the subscribe method.

3fill in blank

hard

Fix the error in the code to update a signal's value.

Angular

count.[1]((val) => val + 1);

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

Aset

Bemit

Cnext

Dupdate

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'set' which replaces the value directly but does not take a function.

Using 'next' which is for Subjects.

Signals use the update method to change their value based on the current value.

4fill in blank

hard

Fill both blanks to create an observable from a signal and subscribe to it.

Angular

const obs = toObservable([1]);
obs.[2](() => console.log('Signal changed'));

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

Acount

Bsubscribe

Cnext

Dsignal

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'next' instead of 'subscribe' to listen to observable.

Using 'signal' instead of the signal variable name.

You convert a signal to an observable using toObservable and then subscribe to it with subscribe.

5fill in blank

hard

Fill all three blanks to create a signal, convert it to an observable, and subscribe to it.

Angular

const [1] = [2](0);
const obs = toObservable([3]);
obs.subscribe(() => console.log('Value changed'));

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

Acount

Bsignal

Attempts:

3 left

💡 Hint

Common Mistakes

Using different variable names for the signal and observable conversion.

Using 'observable' instead of 'signal' to create the signal.

First, create a signal named count using signal. Then convert count to an observable.

Practice

1. Which statement best describes an Angular signal compared to an observable?

easy

A. A signal requires manual subscription to receive updates.

B. A signal handles multiple asynchronous events over time.

C. A signal holds a single reactive value and updates UI automatically.

D. A signal is used only for HTTP requests.

Solution

1. Step 1: Understand what a signal represents

   Signals hold a single reactive value that updates the UI automatically when changed.

2. Step 2: Compare with observable behavior

   Observables handle streams of data over time and require subscriptions, unlike signals.

3. Final Answer:

   A signal holds a single reactive value and updates UI automatically. -> Option C

4. Quick Check:

   Signal = single reactive value [OK]

Hint: Signals hold one value; observables handle streams [OK]

Common Mistakes:

• Thinking signals handle multiple async events like observables
• Believing signals require subscriptions
• Confusing signals with HTTP request handlers

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

easy

A. const count = new Observable(0);

B. const count = signal(0);

C. const count = subscribe(0);

D. const count = createObservable(0);

Solution

1. Step 1: Recall Angular signal creation syntax

   Signals are created using the signal() function with an initial value.

2. Step 2: Identify incorrect options

   Observable creation uses new Observable(), subscribe is a method, and createObservable() is not valid.

3. Final Answer:

   const count = signal(0); -> Option B

4. Quick Check:

   signal() creates signals [OK]

Hint: Use signal() function to create signals [OK]

Common Mistakes:

• Using new Observable() to create a signal
• Confusing subscribe() with signal creation
• Using non-existent createObservable() function

3. Given the code below, what will be logged to the console?

const count = signal(1);
count.set(5);
console.log(count());

medium

A. 1

B. An error because signals cannot be set

C. undefined

D. 5

Solution

1. Step 1: Understand signal value update

   The signal is created with initial value 1, then updated to 5 using set().

2. Step 2: Check the value returned by calling the signal

   Calling count() returns the current value, which is 5 after set().

3. Final Answer:

   5 -> Option D

4. Quick Check:

   Signal value after set() = 5 [OK]

Hint: Calling signal() returns current value [OK]

Common Mistakes:

• Assuming initial value remains after set()
• Thinking signals cannot be updated
• Confusing signal() call with observable subscription

4. What is wrong with this Angular code using an observable?

const obs = new Observable(subscriber => {
  subscriber.next(1);
});
obs.next(2);

medium

A. Observables do not have a next() method on the instance.

B. Observable must be created with signal() instead.

C. Subscriber function cannot call next().

D. Observable must be subscribed before calling next().

Solution

1. Step 1: Understand Observable instance methods

   Observable instances do not have a next() method; next() is called on the subscriber inside the constructor.

2. Step 2: Identify misuse of next() outside subscriber

   Calling obs.next(2) is invalid and causes an error.

3. Final Answer:

   Observables do not have a next() method on the instance. -> Option A

4. Quick Check:

   next() is on subscriber, not observable instance [OK]

Hint: next() is called inside subscriber, not on observable [OK]

Common Mistakes:

• Trying to call next() on observable instance
• Confusing signal() with observable creation
• Believing subscription is needed before next()

5. You want to manage a simple counter state that updates the UI immediately when changed. Which approach is best and why?

Option A: Use a signal to hold the counter value.
Option B: Use an observable and subscribe to updates.
Option C: Use a Promise to fetch the counter value.
Option D: Use a BehaviorSubject without subscription.

hard

A. Signal is best because it holds a single reactive value and updates UI automatically.

B. Observable is best because it handles multiple async events efficiently.

C. Promise is best because it resolves once with the counter value.

D. BehaviorSubject without subscription updates UI automatically.

Solution

1. Step 1: Identify the requirement for simple immediate UI update

   A simple counter state that updates UI immediately fits the signal use case.

2. Step 2: Compare other options

   Observable requires subscription and is better for streams; Promise resolves once; BehaviorSubject needs subscription to update UI.

3. Final Answer:

   Signal is best because it holds a single reactive value and updates UI automatically. -> Option A

4. Quick Check:

   Simple state + auto UI update = signal [OK]

Hint: Use signals for simple reactive state, observables for streams [OK]

Common Mistakes:

• Choosing observable for simple state without subscription
• Using Promise for reactive UI updates
• Assuming BehaviorSubject updates UI without subscription