FormBuilder service in Angular - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to create a form group using FormBuilder.

Angular

this.form = this.formBuilder.[1]({
  name: [''],
  age: ['']
});

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

Aform

Bgroup

Ccontrol

Darray

Attempts:

3 left

💡 Hint

Common Mistakes

Using control() instead of group() to create multiple controls.

Trying to call form() which is not a FormBuilder method.

The group method of FormBuilder creates a new form group.

2fill in blank

medium

Complete the code to inject FormBuilder into the component constructor.

Angular

constructor(private [1]: FormBuilder) {}

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

Abuilder

Bform

CformBuilder

Dfb

Attempts:

3 left

💡 Hint

Common Mistakes

Using a variable name that does not match the injected service.

Forgetting to mark the parameter as private.

By convention, the FormBuilder service is injected with the name formBuilder.

3fill in blank

hard

Fix the error in the code to add a required validator to the 'email' control.

Angular

this.form = this.formBuilder.group({
  email: ['', [1]]
});

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

AValidators.required{}

BValidators.required()

CValidators.required[]

DValidators.required

Attempts:

3 left

💡 Hint

Common Mistakes

Adding parentheses after Validators.required causing a runtime error.

Using invalid syntax like brackets or braces.

The Validators.required is a property, not a function call, so no parentheses are needed.

4fill in blank

hard

Fill both blanks to create a form group with a 'username' control that is required and has a minimum length of 5.

Angular

this.form = this.formBuilder.group({
  username: ['', [[1], [2]]]
});

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

AValidators.required

BValidators.minLength(5)

CValidators.maxLength(5)

DValidators.email

Attempts:

3 left

💡 Hint

Common Mistakes

Using maxLength instead of minLength.

Not wrapping validators in an array.

Use Validators.required and Validators.minLength(5) together to enforce both rules.

5fill in blank

hard

Fill all three blanks to create a form group with 'password' and 'confirmPassword' controls, both required, and add a custom validator named 'passwordMatch'.

Angular

this.form = this.formBuilder.group({
  password: ['', [1]],
  confirmPassword: ['', [2]]
}, { validators: [3] });

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

A[Validators.required]

BValidators.required

CpasswordMatch

DValidators.minLength(8)

Attempts:

3 left

💡 Hint

Common Mistakes

Passing Validators.required without array brackets for controls.

Using 'validator' instead of 'validators' in group options.

Calling the custom validator as a function instead of passing the reference.

Both controls need validators in arrays. The group-level validator is the custom passwordMatch function.

Practice

1. What is the main purpose of Angular's FormBuilder service?

easy

A. To send form data directly to the server

B. To style forms with CSS classes automatically

C. To create reactive forms easily with less code

D. To generate HTML form elements dynamically

Solution

1. Step 1: Understand FormBuilder's role

   FormBuilder is a service in Angular that helps create reactive forms quickly and cleanly.

2. Step 2: Compare options with FormBuilder's purpose

   Only To create reactive forms easily with less code correctly describes that FormBuilder helps build forms easily with less code.

3. Final Answer:

   To create reactive forms easily with less code -> Option C

4. Quick Check:

   FormBuilder = create reactive forms easily [OK]

Hint: FormBuilder builds forms, not styles or sends data [OK]

Common Mistakes:

• Confusing FormBuilder with styling tools
• Thinking FormBuilder sends data to server
• Assuming FormBuilder creates HTML elements

2. Which of the following is the correct way to inject FormBuilder in an Angular component constructor?

easy

A. constructor(private fb = new FormBuilder()) {}

B. constructor(private fb: FormBuilder) {}

C. constructor(FormBuilder fb) {}

D. constructor(fb: FormBuilder) {}

Solution

1. Step 1: Recall Angular dependency injection syntax

   Angular injects services by declaring them as private/public in the constructor with type.

2. Step 2: Identify correct syntax for FormBuilder injection

   constructor(private fb: FormBuilder) {} uses correct TypeScript and Angular DI syntax: private fb: FormBuilder.

3. Final Answer:

   constructor(private fb: FormBuilder) {} -> Option B

4. Quick Check:

   Inject FormBuilder with private and type [OK]

Hint: Use private and type in constructor for DI [OK]

Common Mistakes:

• Using new keyword inside constructor
• Missing access modifier (private/public)
• Incorrect parameter order or syntax

3. Given this code snippet, what will be the initial value of the form control named email?

this.form = this.fb.group({
  email: ['[e-mail]'],
  password: ['']
});

medium

A. '[e-mail]'

B. undefined

C. null

D. An empty string

Solution

1. Step 1: Understand FormBuilder group initialization

   The email control is initialized with the array containing the string '[e-mail]'.

2. Step 2: Identify initial value of the control

   The first element in the array is the initial value, so the email control starts with '[e-mail]'.

3. Final Answer:

   '[e-mail]' -> Option A

4. Quick Check:

   Initial control value = first array element [OK]

Hint: First array item is initial form control value [OK]

Common Mistakes:

• Assuming empty string as default
• Confusing null or undefined as initial value
• Ignoring array syntax for initial value

4. What is wrong with this Angular FormBuilder code?

this.form = this.fb.group({
  username: '',
  age: this.fb.control(0)
});

medium

A. Age control cannot have initial value 0

B. Mixing shorthand and explicit control creation causes error

C. Cannot use this.fb.control inside group

D. Missing array brackets for username control initialization

Solution

1. Step 1: Check FormBuilder group syntax rules

   Each control should be initialized with an array or a FormControl instance.

2. Step 2: Identify issue with username control

   Username is initialized as a string directly, missing array brackets or FormControl wrapper.

3. Final Answer:

   Missing array brackets for username control initialization -> Option D

4. Quick Check:

   Controls need array or FormControl, not plain string [OK]

Hint: Use array or FormControl for each control in group [OK]

Common Mistakes:

• Using plain values without array or FormControl
• Thinking mixed syntax causes error
• Believing zero is invalid initial value

5. You want to create a form with name and email controls using FormBuilder. The email must be required and valid email format. Which code correctly sets up this form with validators?

hard

A. this.form = this.fb.group({ name: ['', Validators.required], email: ['', [Validators.required, Validators.email]] });

B. this.form = this.fb.group({ name: ['', Validators.email], email: ['', Validators.required] });

C. this.form = this.fb.group({ name: ['', [Validators.required, Validators.email]], email: ['', Validators.required] });

D. this.form = this.fb.group({ name: ['', []], email: ['', Validators.email] });

Solution

1. Step 1: Understand validator requirements

   Name must be required; email must be required and valid email format.

2. Step 2: Check each option's validators

   this.form = this.fb.group({ name: ['', Validators.required], email: ['', [Validators.required, Validators.email]] }); correctly applies Validators.required to name and both Validators.required and Validators.email to email.

3. Final Answer:

   this.form = this.fb.group({ name: ['', Validators.required], email: ['', [Validators.required, Validators.email]] }); -> Option A

4. Quick Check:

   Required + email validators on email control [OK]

Hint: Use array for multiple validators on a control [OK]

Common Mistakes:

• Swapping validators between controls
• Not using array for multiple validators
• Missing required validator on email