Signals as modern state primitive in Angular - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

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

Angular

const count = [1](0);

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

Asignal

BuseState

Cref

Dreactive

Attempts:

3 left

💡 Hint

Common Mistakes

Using React's useState instead of Angular's signal

Trying to use ref which is from Vue

Using reactive which is for Angular forms, not signals

In Angular signals, signal() creates a reactive state primitive.

2fill in blank

medium

Complete the code to read the current value of the signal.

Angular

const currentValue = count[1];

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

A()

B.get()

C.value

D[]

Attempts:

3 left

💡 Hint

Common Mistakes

Using .value property which signals do not have

Using .get() which is not part of Angular signals

Using array brackets which is invalid

To get the current value of a signal, call it as a function: count().

3fill in blank

hard

Fix the error in updating the signal value.

Angular

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

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

A[]

B()

C.set

D.value

Attempts:

3 left

💡 Hint

Common Mistakes

Trying to assign directly like count() = ...

Using .value = which is invalid

Using array brackets which is invalid

To update a signal, use the .set(newValue) method.

4fill in blank

hard

Fill both blanks to create a computed signal that doubles the count.

Angular

const doubleCount = computed(() => count[1] * [2]);

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

A.value

B2

Ccount

D()

Attempts:

3 left

💡 Hint

Common Mistakes

Using the signal name without calling it inside computed

Using .value instead of ()

Multiplying by count itself instead of a number

Computed signals read source signals by calling them count() and multiply by 2.

5fill in blank

hard

Fill all three blanks to create a signal, update it, and create a computed signal that adds 10.

Angular

const score = [1](5);
score[2](score() + 3);
const bonusScore = computed(() => score() [3] 10);

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

Asignal

B.set

C+

D-

Attempts:

3 left

💡 Hint

Common Mistakes

Using .value = instead of .set() to update

Using - instead of + in computed

Not creating the signal with signal()

Create signal with signal(5), update with .set(score() + 3), compute score() + 10.

Practice

1. What is the main purpose of using signal() in Angular?

easy

A. To style elements dynamically

B. To create a reactive state value that updates the UI automatically

C. To handle HTTP requests

D. To define a new component

Solution

1. Step 1: Understand the role of signal()

   signal() creates a reactive value that Angular tracks for changes.

2. Step 2: Connect signal() to UI updates

   When the signal value changes, Angular automatically updates the UI where it is used.

3. Final Answer:

   To create a reactive state value that updates the UI automatically -> Option B

4. Quick Check:

   signal() creates reactive state [OK]

Hint: Signals hold reactive values that auto-update UI [OK]

Common Mistakes:

• Confusing signals with components
• Thinking signals handle HTTP
• Assuming signals are for styling

2. Which of the following is the correct way to create a signal with initial value 10 in Angular?

easy

A. const count = signal(10);

B. const count = new Signal(10);

C. const count = createSignal(10);

D. const count = signal.set(10);

Solution

1. Step 1: Recall the syntax for creating signals

   Angular uses signal(initialValue) to create a signal with a starting value.

2. Step 2: Check each option's syntax

   Only const count = signal(10); matches the correct syntax. Others use incorrect constructors or methods.

3. Final Answer:

   const count = signal(10); -> Option A

4. Quick Check:

   signal() creates with initial value [OK]

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

Common Mistakes:

• Using new keyword with signal
• Calling non-existent createSignal function
• Trying to set value during creation

3. Given the code:

const count = signal(0);
count.set(5);
console.log(count());

What will be printed in the console?

medium

A. 5

B. 0

C. undefined

D. Error: count is not a function

Solution

1. Step 1: Understand signal creation and update

   The signal count starts at 0, then set(5) changes its value to 5.

2. Step 2: Reading the signal value

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

3. Final Answer:

   5 -> Option A

4. Quick Check:

   count.set(5) updates value, count() reads it [OK]

Hint: Use count() to read updated signal value [OK]

Common Mistakes:

• Thinking count() returns initial value
• Confusing set() with reading value
• Assuming count is not callable

4. Identify the error in this Angular signal code:

const name = signal('Alice');
name.update(name + ' Smith');

medium

A. Incorrect initial value type for signal

B. Missing parentheses when calling signal()

C. Using update() with a string instead of a function

D. Using set() instead of update()

Solution

1. Step 1: Understand update() usage

   update() expects a function that receives the current value and returns the new value.

2. Step 2: Analyze the code's argument to update()

   The code passes name + ' Smith' which is a string, not a function, causing an error.

3. Final Answer:

   Using update() with a string instead of a function -> Option C

4. Quick Check:

   update() needs a function argument [OK]

Hint: Pass a function to update(), not a direct value [OK]

Common Mistakes:

• Passing value directly to update()
• Confusing set() and update() usage
• Ignoring function argument requirement

5. You want to create a signal that holds a list of numbers and add a new number to it reactively. Which code correctly updates the signal to add number 7?

hard

A. numbers.push(7);

B. numbers.set(numbers() + 7);

C. numbers = signal([...numbers(), 7]);

D. numbers.update(list => [...list, 7]);

Solution

1. Step 1: Understand how to update array signals

   Signals hold values immutably; to add an item, create a new array with the old items plus the new one.

2. Step 2: Check each option's correctness

   numbers.update(list => [...list, 7]); uses update() with a function that returns a new array including 7, which is correct.
   numbers.set(numbers() + 7); tries to add 7 to an array directly, causing a type error.
   numbers = signal([...numbers(), 7]); tries to reassign the signal variable, which is incorrect.
   numbers.push(7); calls push() on the signal itself, which is not valid.

3. Final Answer:

   numbers.update(list => [...list, 7]); -> Option D

4. Quick Check:

   Use update() with function returning new array [OK]

Hint: Use update(fn) to immutably add items to array signals [OK]

Common Mistakes:

• Trying to mutate signal value directly
• Reassigning signal variable instead of updating
• Using set() with wrong value type