@ViewChild decorator usage in Angular - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What will be the output of this Angular component using @ViewChild?

Consider this Angular component code snippet:

import { Component, ViewChild, ElementRef, AfterViewInit } from '@angular/core';

@Component({
  selector: 'app-sample',
  template: `<input #inputRef type='text' value='Hello' />` 
})
export class SampleComponent implements AfterViewInit {
  @ViewChild('inputRef') inputElement!: ElementRef;

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

What will be logged to the console when this component renders?

Angular

import { Component, ViewChild, ElementRef, AfterViewInit } from '@angular/core';

@Component({
  selector: 'app-sample',
  template: `<input #inputRef type='text' value='Hello' />` 
})
export class SampleComponent implements AfterViewInit {
  @ViewChild('inputRef') inputElement!: ElementRef<HTMLInputElement>;

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

A undefined

B null

C "Hello"

D Error: inputElement is undefined

💡 Hint

Remember when @ViewChild references are available in the component lifecycle.

The @ViewChild decorator gets a reference to the input element after the view initializes. The value property of the native input element is 'Hello', so it logs "Hello".

📝 Syntax

intermediate

Which @ViewChild syntax correctly queries a component by its class type?

Given a child component named ChildComponent, which @ViewChild declaration correctly gets its instance?

Angular

import { Component, ViewChild } from '@angular/core';
import { ChildComponent } from './child.component';

@Component({
  selector: 'app-parent',
  template: `<child-component></child-component>`
})
export class ParentComponent {
  // Which @ViewChild is correct?
}

A @ViewChild('ChildComponent') child!: ChildComponent;

B @ViewChild(ChildComponent) child!: ChildComponent;

C @ViewChild() child!: ChildComponent;

D @ViewChild('child') child!: ChildComponent;

💡 Hint

When querying by component class, you pass the class itself, not a string.

To get a child component instance by its class, you pass the class type to @ViewChild, like @ViewChild(ChildComponent). Using a string looks for a template reference variable, which is not present here.

❓ lifecycle

advanced

When is the @ViewChild reference available in the component lifecycle?

In which lifecycle hook is the @ViewChild property guaranteed to be initialized and safe to use?

A ngAfterViewInit()

B ngOnInit()

C constructor()

D ngAfterContentInit()

💡 Hint

Think about when the component's view is fully rendered.

@ViewChild references are set after Angular initializes the component's view. This happens in the ngAfterViewInit() lifecycle hook. Before this, the reference may be undefined.

🔧 Debug

advanced

Why does this @ViewChild usage cause an error?

Examine this Angular component code:

import { Component, ViewChild, ElementRef } from '@angular/core';

@Component({
  selector: 'app-debug',
  template: `
Content
`
})
export class DebugComponent {
  @ViewChild('myDiv') myDiv!: ElementRef;

  ngOnInit() {
    console.log(this.myDiv.nativeElement.textContent);
  }
}

What error will occur and why?

Angular

import { Component, ViewChild, ElementRef } from '@angular/core';

@Component({
  selector: 'app-debug',
  template: `<div #myDiv>Content</div>`
})
export class DebugComponent {
  @ViewChild('myDiv') myDiv!: ElementRef;

  ngOnInit() {
    console.log(this.myDiv.nativeElement.textContent);
  }
}

A No output, silent failure

B Logs 'Content' correctly

C SyntaxError due to missing semicolon

D Error: Cannot read property 'nativeElement' of undefined

💡 Hint

Consider when @ViewChild properties are initialized relative to lifecycle hooks.

The @ViewChild property is not yet initialized in ngOnInit, so accessing this.myDiv.nativeElement causes a runtime error because myDiv is undefined at that point.

❓ state_output

expert

What is the final value of the input after this interaction with @ViewChild?

Given this Angular component:

import { Component, ViewChild, ElementRef, AfterViewInit } from '@angular/core';

@Component({
  selector: 'app-expert',
  template: `` 
})
export class ExpertComponent implements AfterViewInit {
  @ViewChild('inputRef') inputElement!: ElementRef;

  ngAfterViewInit() {
    this.inputElement.nativeElement.value = 'Changed';
  }

  changeValue() {
    this.inputElement.nativeElement.value = 'Clicked';
  }
}

If the user clicks a button that calls changeValue() after the view loads, what is the input's value?

Angular

import { Component, ViewChild, ElementRef, AfterViewInit } from '@angular/core';

@Component({
  selector: 'app-expert',
  template: `<input #inputRef type='text' value='Start' />` 
})
export class ExpertComponent implements AfterViewInit {
  @ViewChild('inputRef') inputElement!: ElementRef<HTMLInputElement>;

  ngAfterViewInit() {
    this.inputElement.nativeElement.value = 'Changed';
  }

  changeValue() {
    this.inputElement.nativeElement.value = 'Clicked';
  }
}

A "Clicked"

B "Changed"

C "Start"

D undefined

💡 Hint

Think about the order of operations: initial value, lifecycle hook, and user interaction.

The input starts with value 'Start'. After the view initializes, ngAfterViewInit sets it to 'Changed'. When the user clicks the button, changeValue() sets it to 'Clicked'. So the final value is 'Clicked'.

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