FormArray for dynamic fields in Angular - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to import the FormArray class from Angular forms.

Angular

import { FormGroup, FormControl, [1] } from '@angular/forms';

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

AFormBuilder

BFormArray

CValidators

DNgForm

Attempts:

3 left

💡 Hint

Common Mistakes

Importing FormBuilder instead of FormArray

Forgetting to import FormArray

Importing from wrong Angular package

The FormArray class is imported from @angular/forms to create dynamic form controls.

2fill in blank

medium

Complete the code to create a FormArray inside a FormGroup named 'skills'.

Angular

this.form = new FormGroup({ skills: new [1]([]) });

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

AFormControl

BFormGroup

CFormBuilder

DFormArray

Attempts:

3 left

💡 Hint

Common Mistakes

Using FormControl instead of FormArray

Using FormGroup instead of FormArray

Passing an object instead of an array

Use FormArray to hold a dynamic list of form controls inside the form group.

3fill in blank

hard

Fix the error in adding a new FormControl to the FormArray named 'skills'.

Angular

this.form.get('skills').[1](new FormControl(''));

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

Apush

BaddControl

Cappend

DpushControl

Attempts:

3 left

💡 Hint

Common Mistakes

Using addControl which is for FormGroup

Using append which does not exist

Using pushControl which is invalid

The FormArray uses the push method to add new controls dynamically.

4fill in blank

hard

Fill both blanks to get the FormArray 'skills' and add a new FormControl with empty string.

Angular

const skillsArray = this.form.get('[1]') as [2];
skillsArray.push(new FormControl(''));

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

Askills

BFormGroup

CFormArray

Dcontrols

Attempts:

3 left

💡 Hint

Common Mistakes

Casting to FormGroup instead of FormArray

Using wrong control name

Not casting at all causing errors

Use get('skills') to access the FormArray and cast it as FormArray to use push.

5fill in blank

hard

Fill all three blanks to create a FormArray with two FormControls initialized with 'Angular' and 'TypeScript'.

Angular

this.form = new FormGroup({ skills: new [1]([new FormControl('[2]'), new FormControl('[3]')]) });

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

AFormArray

BReact

CTypeScript

DAngular

Attempts:

3 left

💡 Hint

Common Mistakes

Using FormGroup instead of FormArray

Swapping the strings 'Angular' and 'TypeScript'

Using unrelated strings like 'React'

Create a FormArray with two FormControl instances initialized with 'Angular' and 'TypeScript'.

Practice

1. What is the main purpose of FormArray in Angular forms?

easy

A. To manage a dynamic list of form controls that can grow or shrink

B. To create a single form control with a fixed value

C. To style form inputs dynamically

D. To handle form submission events

Solution

1. Step 1: Understand what FormArray does

   FormArray is designed to hold multiple form controls dynamically, allowing adding or removing controls as needed.

2. Step 2: Compare options

   Only To manage a dynamic list of form controls that can grow or shrink correctly describes this dynamic list management. Others describe unrelated features.

3. Final Answer:

   To manage a dynamic list of form controls that can grow or shrink -> Option A

4. Quick Check:

   FormArray = dynamic list of controls [OK]

Hint: FormArray is for lists of controls, not single controls [OK]

Common Mistakes:

• Confusing FormArray with FormControl
• Thinking FormArray handles styling
• Assuming FormArray manages form submission

2. Which of the following is the correct way to add a new control to a FormArray named emails?

easy

A. emails.insertControl(new FormControl(''))

B. emails.addControl(new FormControl(''))

C. emails.push(new FormControl(''))

D. emails.appendControl(new FormControl(''))

Solution

1. Step 1: Recall FormArray methods

   The method to add a control to FormArray is push().

2. Step 2: Check each option

   Only emails.push(new FormControl('')) uses push() correctly. Others are invalid methods for FormArray.

3. Final Answer:

   emails.push(new FormControl('')) -> Option C

4. Quick Check:

   Use push() to add control [OK]

Hint: Use push() to add controls to FormArray [OK]

Common Mistakes:

• Using addControl() which is for FormGroup
• Trying appendControl() or insertControl() which don't exist
• Confusing FormArray with FormGroup methods

3. Given this code snippet, what will be the length of the FormArray after execution?

const tags = new FormArray([]);
tags.push(new FormControl('angular'));
tags.push(new FormControl('typescript'));
tags.removeAt(0);

medium

A. 1

B. 2

C. 0

D. 3

Solution

1. Step 1: Track push operations

   Two controls are added, so length becomes 2.

2. Step 2: Apply removeAt(0)

   The first control is removed, reducing length by 1.

3. Final Answer:

   1 -> Option A

4. Quick Check:

   2 added - 1 removed = 1 [OK]

Hint: Count pushes minus removes for length [OK]

Common Mistakes:

• Forgetting removeAt reduces length
• Assuming removeAt(0) removes last item
• Counting pushes only without removals

4. Identify the error in this code snippet that tries to remove a control from a FormArray named phones:

phones.remove(1);

medium

A. remove() requires a FormControl argument, not an index

B. No error; remove() works fine

C. remove() should be called with a string index

D. remove() is not a method of FormArray; should use removeAt()

Solution

1. Step 1: Check FormArray removal methods

   FormArray uses removeAt(index) to remove controls by position.

2. Step 2: Analyze the code

   Calling remove(1) is invalid because remove is not a FormArray method.

3. Final Answer:

   remove() is not a method of FormArray; should use removeAt() -> Option D

4. Quick Check:

   Use removeAt() to remove by index [OK]

Hint: Use removeAt() to remove by index, not remove() [OK]

Common Mistakes:

• Using remove() instead of removeAt()
• Passing wrong argument types to removal methods
• Assuming remove() exists on FormArray

5. You want to create a form where users can add multiple email addresses dynamically. Which approach correctly initializes the FormArray and adds a new empty email control when a button is clicked?

hard

A. Initialize with new FormGroup([]) and add with emails.push('')

B. Initialize with new FormArray([]) and add with emails.push(new FormControl(''))

C. Initialize with new FormControl([]) and add with emails.addControl(new FormControl(''))

D. Initialize with new FormArray(['']) and add with emails.insertControl(new FormControl(''))

Solution

1. Step 1: Correct initialization of FormArray

   FormArray should be initialized with an empty array: new FormArray([]).

2. Step 2: Adding new controls properly

   Use push(new FormControl('')) to add a new empty control dynamically.

3. Final Answer:

   Initialize with new FormArray([]) and add with emails.push(new FormControl('')) -> Option B

4. Quick Check:

   FormArray + push(new FormControl('')) = dynamic emails [OK]

Hint: Use FormArray([]) and push(new FormControl('')) to add fields [OK]

Common Mistakes:

• Using FormControl instead of FormArray for multiple inputs
• Trying to add controls with addControl() on FormArray
• Initializing FormArray with values instead of controls