@ViewChild decorator usage in Angular - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to import the @ViewChild decorator from Angular core.

Angular

import { [1] } from '@angular/core';

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

AViewChild

BComponent

CNgModule

DInjectable

Attempts:

3 left

💡 Hint

Common Mistakes

Importing Component instead of @ViewChild

Forgetting to import @ViewChild

Importing from wrong Angular package

The @ViewChild decorator is imported from '@angular/core' to access child components or elements.

2fill in blank

medium

Complete the code to declare a @ViewChild property named 'myInput' that references a template variable 'inputRef'.

Angular

@ViewChild('[1]') myInput: ElementRef;

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

ArefInput

BmyInput

CinputRef

DinputElement

Attempts:

3 left

💡 Hint

Common Mistakes

Using the property name instead of the template variable

Missing quotes around the variable name

Using a wrong variable name

The string passed to @ViewChild must match the template variable name to get the reference.

3fill in blank

hard

Fix the error in the @ViewChild declaration to correctly get a reference to a child component named ChildComponent.

Angular

@ViewChild([1]) childComp!: ChildComponent;

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

AChildComponent

B'ChildComponent'

CchildComp

D'childComp'

Attempts:

3 left

💡 Hint

Common Mistakes

Passing component name as a string instead of class

Using the property name instead of the class

Adding quotes around the class name

When referencing a component class, pass the class name without quotes to @ViewChild.

4fill in blank

hard

Fill both blanks to declare a @ViewChild property that reads the native element of a template variable 'myDiv'.

Angular

@ViewChild('[1]', { [2]: true }) myDivRef!: ElementRef;

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

AmyDiv

Bstatic

Cread

DelementRef

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'read' instead of 'static' in the options

Using wrong template variable name

Omitting the options object

The first blank is the template variable name. The second blank is the 'static' option to control when the query resolves.

5fill in blank

hard

Fill all three blanks to declare a @ViewChild property that reads the native element of a template variable 'header' with static query enabled.

Angular

@ViewChild('[1]', { [2]: true, [3]: ElementRef }) headerRef!: ElementRef;

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

Aheader

Bstatic

Cread

DmyHeader

Attempts:

3 left

💡 Hint

Common Mistakes

Mixing up 'static' and 'read' options

Using wrong template variable name

Omitting the 'read' option when needed

The first blank is the template variable name 'header'. The second blank is the 'static' option set to true. The third blank is the 'read' option specifying ElementRef.

Practice

1. What is the primary purpose of the @ViewChild decorator in Angular?

easy

A. To get a reference to a template element or child component

B. To create a new Angular module

C. To define routes in the application

D. To style components dynamically

Solution

1. Step 1: Understand the role of @ViewChild

   @ViewChild is used to access a child element or component from the template directly in the class.

2. Step 2: Compare with other options

   Creating modules, defining routes, or styling are unrelated to @ViewChild's purpose.

3. Final Answer:

   To get a reference to a template element or child component -> Option A

4. Quick Check:

   @ViewChild = Reference to template element [OK]

Hint: Remember: @ViewChild grabs template elements or child components [OK]

Common Mistakes:

• Confusing @ViewChild with routing or module decorators
• Thinking @ViewChild creates components
• Assuming @ViewChild styles elements

2. Which of the following is the correct syntax to use @ViewChild to get a reference to a template element with template variable #myInput?

easy

A. @ViewChild(myInput) inputElement: ElementRef;

B. @ViewChild('myInput') inputElement: ElementRef;

C. @ViewChild(#myInput) inputElement: ElementRef;

D. @ViewChild('inputElement') myInput: ElementRef;

Solution

1. Step 1: Check correct string usage in @ViewChild

   The template variable name must be passed as a string, so it should be '@ViewChild('myInput')'.

2. Step 2: Validate variable names and types

   The variable name in the class can be anything, here 'inputElement', and type should be ElementRef for native elements.

3. Final Answer:

   @ViewChild('myInput') inputElement: ElementRef; -> Option B

4. Quick Check:

   Template variable name as string in @ViewChild [OK]

Hint: Always pass template variable name as a string in @ViewChild [OK]

Common Mistakes:

• Omitting quotes around template variable name
• Using # inside @ViewChild argument
• Swapping variable names and template references

3. Given the following Angular component code, what will be logged to the console after the view initializes?

  <input #myInput type="text" value="Hello">

  export class MyComponent implements AfterViewInit {
    @ViewChild('myInput') inputRef!: ElementRef<HTMLInputElement>;

    ngAfterViewInit() {
      console.log(this.inputRef.nativeElement.value);
    }
  }

medium

A. undefined

B. Error: inputRef is undefined

C. null

D. Hello

Solution

1. Step 1: Understand @ViewChild and ngAfterViewInit timing

   @ViewChild gets the input element reference, and ngAfterViewInit runs after the view is ready, so inputRef is defined.

2. Step 2: Access nativeElement value

   inputRef.nativeElement.value accesses the input's value attribute, which is 'Hello'.

3. Final Answer:

   Hello -> Option D

4. Quick Check:

   Access input value via @ViewChild after view init [OK]

Hint: Access @ViewChild only after ngAfterViewInit to get values [OK]

Common Mistakes:

• Trying to access @ViewChild in constructor or ngOnInit
• Assuming inputRef is null or undefined after view init
• Confusing nativeElement with the ElementRef itself

4. Identify the error in the following Angular component code using @ViewChild:

  export class MyComponent {
    @ViewChild('myButton') buttonRef: ElementRef;

    ngOnInit() {
      this.buttonRef.nativeElement.click();
    }
  }

medium

A. Accessing @ViewChild reference before view initialization

B. Missing quotes around 'myButton' in @ViewChild

C. Using ElementRef instead of TemplateRef

D. No error, code is correct

Solution

1. Step 1: Check lifecycle hook used

   ngOnInit runs before the view is fully initialized, so @ViewChild references are not ready yet.

2. Step 2: Correct lifecycle hook for @ViewChild access

   Accessing @ViewChild should be done in ngAfterViewInit to ensure the element exists.

3. Final Answer:

   Accessing @ViewChild reference before view initialization -> Option A

4. Quick Check:

   @ViewChild ready only after ngAfterViewInit [OK]

Hint: Use ngAfterViewInit, not ngOnInit, to access @ViewChild [OK]

Common Mistakes:

• Accessing @ViewChild in ngOnInit causing undefined errors
• Confusing lifecycle hooks for view queries
• Ignoring the timing of template element availability

5. You want to call a method focusInput() on a child component ChildComponent using @ViewChild. Which of the following is the correct way to do this safely after the view initializes?

hard

A. Use @ViewChild(ChildComponent) child: ElementRef; and call this.child.nativeElement.focusInput() inside ngAfterViewInit()

B. Use @ViewChild('child') child: ChildComponent; and call this.child.focusInput() inside constructor()

C. Use @ViewChild(ChildComponent) child!: ChildComponent; and call this.child.focusInput() inside ngAfterViewInit()

D. Use @ViewChild('child') child: ElementRef; and call this.child.focusInput() inside ngOnInit()

Solution

1. Step 1: Correct @ViewChild usage for child component

   Use the component class type in @ViewChild to get the child component instance, e.g. @ViewChild(ChildComponent).

2. Step 2: Call child method after view initialization

   Call the method inside ngAfterViewInit to ensure the child component is ready.

3. Final Answer:

   Use @ViewChild(ChildComponent) child!: ChildComponent; and call this.child.focusInput() inside ngAfterViewInit() -> Option C

4. Quick Check:

   Child component method call via @ViewChild after view init [OK]

Hint: Use component class in @ViewChild and call methods after ngAfterViewInit [OK]

Common Mistakes:

• Calling child methods in constructor or ngOnInit
• Using ElementRef instead of component class for child components
• Trying to access nativeElement for component methods