ngDoCheck for custom change detection in Angular - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the purpose of ngDoCheck in Angular?

ngDoCheck is a lifecycle hook that lets you run custom change detection logic in a component. It runs during every change detection cycle, allowing you to check for changes Angular might miss.

beginner

When is ngDoCheck called in the Angular component lifecycle?

ngDoCheck is called after ngOnChanges and before ngAfterContentChecked during every change detection run.

intermediate

Why would you use ngDoCheck instead of relying on Angular's default change detection?

You use ngDoCheck when Angular's default change detection does not detect changes, such as when mutating objects or arrays directly instead of replacing them.

intermediate

What is a common pattern to detect changes inside ngDoCheck?

A common pattern is to store a previous value and compare it with the current value inside ngDoCheck. If they differ, you know a change happened.

beginner

What should you be careful about when using ngDoCheck?

Because ngDoCheck runs very often, avoid heavy or slow operations inside it to keep your app fast and responsive.

What does ngDoCheck allow you to do in Angular?

A Initialize component inputs

B Destroy component resources

C Run custom change detection logic

D Handle user events

ngDoCheck is specifically for custom change detection logic during Angular's change detection cycle.

When is ngDoCheck called?

A After every change detection cycle

B Only when inputs change

C Before <code>ngOnInit</code>

D Only once when the component is created

ngDoCheck runs after every change detection cycle, regardless of input changes.

Which scenario is a good reason to use ngDoCheck?

A When you want to detect changes in a mutated array

B When you want to listen to click events

C When you want to fetch data from a server

D When you want to style a component

Angular's default change detection may miss changes if you mutate arrays or objects directly. ngDoCheck helps detect those changes.

What is a recommended practice inside ngDoCheck?

A Run heavy calculations

B Make HTTP requests

C Modify DOM elements directly

D Compare previous and current values to detect changes

Comparing previous and current values is a lightweight way to detect changes inside ngDoCheck.

What should you avoid inside ngDoCheck to keep your app fast?

A Simple value comparisons

B Heavy or slow operations

C Logging to console

D Updating component inputs

Heavy or slow operations inside ngDoCheck can slow down your app because it runs very often.

Explain how ngDoCheck helps with custom change detection in Angular.

Think about when Angular's default detection might not notice changes.

Describe best practices to follow when implementing ngDoCheck.

Consider performance impact since it runs often.

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