ngDoCheck for custom change detection in Angular

Introduction

Sometimes Angular's automatic change detection does not catch all changes. ngDoCheck lets you check for changes yourself and react to them.

You want to detect changes in objects or arrays that Angular does not track by default.

You need to run custom logic when some data changes, but Angular's default checks miss it.

You want to optimize performance by controlling when and how change detection runs.

You have complex data structures and want to detect deep changes manually.

Syntax

Angular

ngDoCheck(): void {
  // Your custom change detection logic here
}

This method is part of Angular's DoCheck lifecycle interface.

Angular calls ngDoCheck frequently, so keep logic efficient.

Examples

Check if a simple value changed and react to it.

Angular

ngDoCheck(): void {
  if (this.oldValue !== this.newValue) {
    console.log('Value changed!');
    this.oldValue = this.newValue;
  }
}

Detect changes in an array's length manually.

Angular

ngDoCheck(): void {
  const currentLength = this.items.length;
  if (this.oldLength !== currentLength) {
    console.log('Array length changed!');
    this.oldLength = currentLength;
  }
}

Sample Program

This component uses ngDoCheck to detect when the items array changes length. When you click the button, a new item is added, and the console logs the change.

Angular

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

@Component({
  selector: 'app-custom-check',
  template: `
    <h2>Custom Change Detection Example</h2>
    <button (click)="addItem()">Add Item</button>
    <ul>
      <li *ngFor="let item of items">{{ item }}</li>
    </ul>
  `
})
export class CustomCheckComponent implements DoCheck {
  items: string[] = ['Apple', 'Banana'];
  oldLength = this.items.length;

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

  addItem() {
    this.items.push('Orange');
  }
}

Important Notes

Do not put heavy or slow code inside ngDoCheck because it runs often.

Use ngDoCheck only when Angular's default change detection is not enough.

Summary

ngDoCheck lets you run your own checks for changes.

It is useful for detecting changes Angular misses, like in arrays or objects.

Keep the code inside ngDoCheck fast and simple.

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