ngDoCheck for custom change detection in Angular - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to implement the ngDoCheck lifecycle hook in an Angular component.

Angular

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

@Component({
  selector: 'app-check',
  template: `<p>Check example</p>`
})
export class CheckComponent implements [1] {
  ngDoCheck() {
    console.log('Custom change detection running');
  }
}

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

ADoCheck

BOnChanges

CAfterViewInit

DOnInit

Attempts:

3 left

💡 Hint

Common Mistakes

Using OnInit instead of DoCheck

Forgetting to implement the DoCheck interface

The DoCheck interface allows you to implement the ngDoCheck method for custom change detection.

2fill in blank

medium

Complete the code to detect changes manually inside ngDoCheck.

Angular

import { Component, DoCheck, ChangeDetectorRef } from '@angular/core';

@Component({
  selector: 'app-manual-check',
  template: `<p>{{message}}</p>`
})
export class ManualCheckComponent implements DoCheck {
  message = 'Initial';

  constructor(private cdr: [1]) {}

  ngDoCheck() {
    this.message = 'Checked';
    this.cdr.detectChanges();
  }
}

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

AChangeDetectorRef

BNgZone

CRenderer2

DElementRef

Attempts:

3 left

💡 Hint

Common Mistakes

Injecting NgZone instead of ChangeDetectorRef

Calling detectChanges on the wrong service

The ChangeDetectorRef service allows manual triggering of change detection.

3fill in blank

hard

Fix the error in the ngDoCheck method to avoid infinite loops.

Angular

ngDoCheck() {
  if (this.value !== this.previousValue) {
    this.previousValue = this.value;
    this.message = 'Value changed';
    [1];
  }
}

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

Athis.cdr.reattach()

Bthis.cdr.markForCheck()

Cthis.cdr.detach()

Dthis.cdr.detectChanges()

Attempts:

3 left

💡 Hint

Common Mistakes

Calling detectChanges inside ngDoCheck causing infinite loops

Not marking the component for check

Using markForCheck() schedules change detection without causing an immediate cycle, preventing infinite loops.

4fill in blank

hard

Fill both blanks to create a custom change detection that compares old and new values.

Angular

export class CustomCheckComponent implements DoCheck {
  oldValue = '';
  newValue = 'Hello';

  ngDoCheck() {
    if (this.newValue [1] this.oldValue) {
      this.oldValue = this.newValue;
      console.log('Value changed:', this.newValue);
    }
  }
}

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

A===

B!=

C==

D!==

Attempts:

3 left

💡 Hint

Common Mistakes

Using == instead of !== causing false positives

Using === which checks for equality, not difference

The strict inequality operator !== checks if values are different, triggering the change logic.

5fill in blank

hard

Fill all three blanks to implement ngDoCheck with manual change detection and previous value tracking.

Angular

export class TrackValueComponent implements DoCheck {
  previousValue = '';
  currentValue = 'Angular';

  constructor(private [1]: ChangeDetectorRef) {}

  ngDoCheck() {
    if (this.currentValue [2] this.previousValue) {
      this.previousValue = this.currentValue;
      this.[3].markForCheck();
    }
  }
}

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

Acdr

B!==

D===

EchangeDetector

FdetectChanges

GmarkForCheck

HcdRef

Attempts:

3 left

💡 Hint

Common Mistakes

Using === instead of !== for comparison

Calling detectChanges inside ngDoCheck causing loops

Using wrong variable names for ChangeDetectorRef

Inject ChangeDetectorRef as cdr, compare values with !==, and call cdr.markForCheck() to schedule change detection.

Practice

1. What is the main purpose of the ngDoCheck lifecycle hook in Angular?

easy

A. To run custom change detection logic when Angular's default detection misses changes.

B. To initialize component data before rendering.

C. To handle user input events like clicks and keypresses.

D. To destroy the component and clean up resources.

Solution

1. Step 1: Understand Angular's default change detection

   Angular automatically detects changes for simple data types but can miss changes in complex objects or arrays.

2. Step 2: Role of ngDoCheck

   ngDoCheck allows developers to add custom checks to detect changes Angular might miss.

3. Final Answer:

   To run custom change detection logic when Angular's default detection misses changes. -> Option A

4. Quick Check:

   ngDoCheck = custom change detection [OK]

Hint: Remember: ngDoCheck is for custom checks Angular misses [OK]

Common Mistakes:

• Confusing ngDoCheck with ngOnInit or ngOnDestroy
• Thinking ngDoCheck handles user events
• Assuming ngDoCheck replaces Angular's default detection

2. Which of the following is the correct syntax to implement ngDoCheck in an Angular component?

easy

A. ngDoCheck() { console.log('Checking changes'); }

B. ngDoCheck: function() { console.log('Checking changes'); }

C. function ngDoCheck() { console.log('Checking changes'); }

D. ngDoCheck => { console.log('Checking changes'); }

Solution

1. Step 1: Recognize Angular lifecycle hook syntax

   Angular lifecycle hooks are methods inside the component class with no special syntax like arrow functions or function keywords.

2. Step 2: Identify correct method declaration

   ngDoCheck() { console.log('Checking changes'); } shows the correct method syntax inside a class: ngDoCheck() { ... }.

3. Final Answer:

   ngDoCheck() { console.log('Checking changes'); } -> Option A

4. Quick Check:

   ngDoCheck is a class method, not a function expression [OK]

Hint: Use method syntax inside class for lifecycle hooks [OK]

Common Mistakes:

• Using arrow functions for lifecycle hooks
• Adding function keyword inside class
• Using object literal syntax for hooks

3. Consider this Angular component snippet:

items = [1, 2, 3];

ngDoCheck() {
  if (this.items.length !== this.oldLength) {
    console.log('Items changed');
    this.oldLength = this.items.length;
  }
}

What will be logged if items.push(4) is called and Angular runs change detection?

medium

A. Error: oldLength is undefined

B. Nothing is logged

C. Items changed twice

D. Items changed

Solution

1. Step 1: Check initialization of oldLength

   The code uses this.oldLength but it is never initialized, so it's undefined initially.

2. Step 2: Runtime behavior after items.push(4)

   this.items.length becomes 4, and 4 !== undefined is true, so 'Items changed' is logged. No error occurs.

3. Final Answer:

   Items changed -> Option D

4. Quick Check:

   4 !== undefined -> true, logs [OK]

Hint: Always initialize variables used in ngDoCheck before use [OK]

Common Mistakes:

• Assuming oldLength is automatically defined
• Expecting ngDoCheck to detect changes without setup
• Thinking accessing undefined properties causes runtime errors

4. Given this ngDoCheck implementation:

ngDoCheck() {
  if (this.data !== this.oldData) {
    this.processData();
    this.oldData = this.data;
  }
}

Why might processData() never be called even if data changes internally (like modifying a property inside an object)?

medium

A. Because ngDoCheck only runs once per component.

B. Because !== compares object references, not internal changes.

C. Because processData() is not a valid method.

D. Because oldData is not declared.

Solution

1. Step 1: Understand object reference comparison

   The !== operator checks if two variables point to different objects, not if the content inside objects changed.

2. Step 2: Effect on ngDoCheck logic

   If data is the same object but its properties change, this.data !== this.oldData is false, so processData() is not called.

3. Final Answer:

   Because !== compares object references, not internal changes. -> Option B

4. Quick Check:

   Object reference check misses internal mutations [OK]

Hint: Use deep checks or immutable objects for internal changes [OK]

Common Mistakes:

• Thinking !== detects internal object changes
• Assuming ngDoCheck runs only once
• Ignoring method existence or variable declaration

5. You want to detect changes inside a nested object profile in ngDoCheck. Which approach correctly detects changes to any property inside profile without replacing the whole object?

hard

A. Call detectChanges() manually inside ngDoCheck.

B. Only check if profile !== oldProfile using reference equality.

C. Use a deep comparison function inside ngDoCheck to compare profile with a saved copy.

D. Replace profile with a new object every time a property changes.

Solution

1. Step 1: Understand limitations of reference checks

   Reference checks only detect if the object itself changed, not internal property changes.

2. Step 2: Use deep comparison for nested changes

   A deep comparison function compares all nested properties to detect any internal changes.

3. Step 3: Avoid unnecessary object replacement

   Replacing the whole object every time is inefficient; deep comparison allows detecting changes without replacement.

4. Final Answer:

   Use a deep comparison function inside ngDoCheck to compare profile with a saved copy. -> Option C

5. Quick Check:

   Deep compare nested objects for internal changes [OK]

Hint: Deep compare objects to detect internal changes [OK]

Common Mistakes:

• Relying on reference equality for nested objects
• Manually calling detectChanges inside ngDoCheck
• Replacing whole object unnecessarily