Input signals and model signals in Angular - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to create a signal with an initial value of 0.

Angular

const count = signal([1]);

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

A0

Bnull

Cundefined

D''

Attempts:

3 left

💡 Hint

Common Mistakes

Using null or undefined as initial value causes unexpected behavior.

Leaving the initial value empty causes errors.

The signal function creates a reactive signal. Here, we start with 0 as the initial value.

2fill in blank

medium

Complete the code to update the signal value by adding 1.

Angular

count.set(count.[1]() + 1);

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

Aget

Bcurrent

Cvalue

Dread

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'value' or 'current' which are not valid methods.

Trying to access the signal value directly without a method.

Use get() to read the current value of a signal before updating it.

3fill in blank

hard

Complete the code to create a signal with initial value 'user'.

Angular

const userModel = [1]('user');

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

AinputSignal

BmodelSignal

CcreateSignal

Dsignal

Attempts:

3 left

💡 Hint

Common Mistakes

Using non-existent functions like 'modelSignal' or 'inputSignal'.

Confusing Angular signals with other frameworks.

The correct function to create a signal in Angular is signal. Other options are invalid.

4fill in blank

hard

Fill both blanks to create a signal and update it with a new value.

Angular

const name = [1]('');
name.[2]('Alice');

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

Asignal

Bset

Cupdate

DcreateSignal

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'update' instead of 'set' to change the value.

Using 'createSignal' which is not an Angular function.

Use signal to create the signal and set to update its value.

5fill in blank

hard

Fill all three blanks to create a signal, read its value, and update it conditionally.

Angular

const age = [1](20);
const currentAge = age.[2]();
if (currentAge < 30) {
  age.[3](currentAge + 1);
}

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

Asignal

Bget

Cset

Dupdate

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'update' instead of 'set' to change the value.

Trying to read the signal value without calling a method.

Create the signal with signal, read its value with get(), and update it with set().

Practice

1. What is the main purpose of Input signals in Angular components?

easy

A. To receive reactive data from parent components

B. To send events to parent components

C. To style the component dynamically

D. To handle user input events

Solution

1. Step 1: Understand Input signals role

   Input signals allow a component to get reactive data from its parent, keeping the data flow reactive and automatic.

2. Step 2: Differentiate from other options

   Sending events to parents is done by outputs, styling is unrelated, and user input events are handled differently.

3. Final Answer:

   To receive reactive data from parent components -> Option A

4. Quick Check:

   Input signals = receive reactive data [OK]

Hint: Input signals bring data in from parents [OK]

Common Mistakes:

• Confusing input signals with output events
• Thinking input signals handle styling
• Assuming input signals manage user events

2. Which of the following is the correct way to declare an input signal in an Angular standalone component?

easy

A. @Input() inputSignal = signal();

B. const inputSignal = signal();

C. const inputSignal = @Input(signal());

D. signal @Input() inputSignal;

Solution

1. Step 1: Recall Angular input signal syntax

   Input signals are declared with the @Input() decorator followed by a signal initialization.

2. Step 2: Check each option

   @Input() inputSignal = signal(); correctly uses @Input() decorator before the signal. const inputSignal = signal(); misses the decorator, C and D have invalid syntax.

3. Final Answer:

   @Input() inputSignal = signal(); -> Option A

4. Quick Check:

   Input signals need @Input() decorator [OK]

Hint: Use @Input() before signal declaration [OK]

Common Mistakes:

• Forgetting the @Input() decorator
• Placing @Input() incorrectly
• Using invalid syntax with signals

3. Given this Angular component code snippet:

export class MyComponent {
  @Input() count = signal(0);

  increment() {
    this.count.update(c => c + 1);
  }
}

What will be the value of count() after calling increment() twice if the initial value is 0?

medium

A. 0

B. 1

C. undefined

D. 2

Solution

1. Step 1: Understand initial signal value

   The signal count starts at 0.

2. Step 2: Apply increment method twice

   Each call to increment() updates the signal by adding 1, so after two calls: 0 + 1 + 1 = 2.

3. Final Answer:

   2 -> Option D

4. Quick Check:

   0 + 2 increments = 2 [OK]

Hint: Each update adds 1 to the signal value [OK]

Common Mistakes:

• Thinking signals don't update automatically
• Confusing method calls with direct assignment
• Assuming initial value changes unexpectedly

4. Identify the error in this Angular component code using input signals:

export class SampleComponent {
  @Input() data = signal();

  ngOnInit() {
    this.data.set('Hello');
  }
}

medium

A. Input signals must be readonly

B. Missing initial value for signal()

C. ngOnInit() is not allowed in standalone components

D. Cannot call set() on input signals

Solution

1. Step 1: Check signal initialization

   The signal data is declared without an initial value, which is invalid.

2. Step 2: Understand signal requirements

   Signals must have an initial value when created, so signal() without arguments causes an error.

3. Final Answer:

   Missing initial value for signal() -> Option B

4. Quick Check:

   Signals need initial values [OK]

Hint: Always initialize signals with a value [OK]

Common Mistakes:

• Leaving signals uninitialized
• Thinking set() is disallowed on inputs
• Confusing lifecycle hooks usage

5. You want to create a component that receives a reactive input signal userName and also maintains a local model signal greeting that updates automatically when userName changes. Which approach correctly implements this behavior?

hard

A. Use @Input() userName: string; and create greeting = signal(''); updated by a setter

B. Use @Input() userName = signal(''); and update greeting manually in ngOnChanges

C. Use @Input() userName = signal(''); and inside the component create greeting = computed(() => `Hello, ${this.userName()}`);

D. Use @Input() userName = signal(''); and assign greeting = signal('Hello'); once in constructor

Solution

1. Step 1: Understand reactive input and model signals

   The input signal userName provides reactive data from parent. The local greeting should react automatically to changes.

2. Step 2: Use computed signal for automatic updates

   Using computed creates a signal that updates whenever userName() changes, keeping greeting in sync.

3. Final Answer:

   Use @Input() userName = signal(''); and greeting = computed(() => `Hello, ${this.userName()}`); -> Option C

4. Quick Check:

   Computed signals auto-update from input signals [OK]

Hint: Use computed() for dependent reactive signals [OK]

Common Mistakes:

• Manually updating signals instead of computed
• Using plain string input instead of signal
• Assigning greeting only once without reactivity