OnPush change detection strategy in Angular - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the OnPush change detection strategy in Angular?

OnPush is a change detection strategy that tells Angular to check a component only when its input properties change or when an event happens inside it. This helps improve performance by reducing unnecessary checks.

intermediate

How does Angular detect changes with OnPush strategy?

Angular runs change detection for an OnPush component only when:
1. An input property changes by reference.
2. An event originated from the component or its children.
3. An observable or async pipe inside the component emits a new value.

intermediate

What happens if you mutate an object input without changing its reference in an OnPush component?

Angular will NOT detect the change because OnPush checks only for new object references. Mutating the object without changing its reference means Angular thinks nothing changed.

advanced

How can you trigger change detection manually in an OnPush component?

You can inject ChangeDetectorRef and call its markForCheck() or detectChanges() methods to tell Angular to check the component for updates.

beginner

Why is OnPush strategy good for performance?

OnPush reduces how often Angular checks components by only running change detection when needed. This lowers CPU work and speeds up apps, especially with many components.

When does Angular run change detection for an OnPush component?

A Never, it disables change detection

B Every time any data changes anywhere in the app

C Only when the user clicks a button

D Only when input references change or events happen inside the component

OnPush runs change detection only when input references change or events inside the component occur.

What happens if you mutate an input object without changing its reference in an OnPush component?

A Angular ignores the change and does not update the view

B Angular throws an error

C Angular detects the change and updates the view

D Angular reloads the entire app

OnPush checks input references, so mutating without changing reference is ignored.

Which Angular service can you use to manually trigger change detection in an OnPush component?

A HttpClient

B Router

C ChangeDetectorRef

D Renderer2

ChangeDetectorRef lets you manually trigger change detection.

What is a main benefit of using OnPush strategy?

A Improves app performance by reducing checks

B Simplifies template syntax

C Automatically fixes bugs

D Disables event handling

OnPush improves performance by reducing unnecessary change detection.

Which of these will NOT trigger change detection in an OnPush component?

A Input property reference changes

B Mutating an input object without changing its reference

C An observable emits a new value used in the template

D User clicks a button inside the component

Mutating an input object without changing its reference does not trigger change detection with OnPush.

Explain how the OnPush change detection strategy works in Angular and why it improves performance.

Think about when Angular decides to update the component view.

Describe how you can handle a situation where an OnPush component does not update after mutating an input object.

Consider how Angular detects changes with OnPush.

Practice

1. What does the OnPush change detection strategy do in Angular?

easy

A. It disables change detection completely.

B. It checks for changes only when input properties change or events occur.

C. It forces Angular to check for changes on every event regardless of inputs.

D. It updates the component only when manually triggered by the developer.

Solution

1. Step 1: Understand OnPush behavior

   OnPush strategy tells Angular to run change detection only when input properties change or events happen inside the component.

2. Step 2: Compare with other options

   Options A, C, and D describe behaviors that are not how OnPush works. It does not disable detection, nor does it check on every event or require manual triggers.

3. Final Answer:

   It checks for changes only when input properties change or events occur. -> Option B

4. Quick Check:

   OnPush triggers on input or event changes = B [OK]

Hint: OnPush triggers only on input or event changes [OK]

Common Mistakes:

• Thinking OnPush disables change detection
• Believing OnPush checks on every event
• Assuming manual triggers are required

2. Which is the correct way to set the OnPush change detection strategy in an Angular component?

easy

A. @Component({ selector: 'app', changeDetection: ChangeDetectionStrategy.OnPush })

B. @Component({ selector: 'app', changeDetection: ChangeDetectionStrategy.Default })

C. @Component({ selector: 'app', changeDetection: 'OnPush' })

D. @Component({ selector: 'app', changeDetectionStrategy: OnPush })

Solution

1. Step 1: Recall Angular syntax for change detection

   The correct property name is changeDetection and the value is ChangeDetectionStrategy.OnPush.

2. Step 2: Analyze options

   @Component({ selector: 'app', changeDetection: ChangeDetectionStrategy.OnPush }) uses the correct property and enum value. @Component({ selector: 'app', changeDetection: ChangeDetectionStrategy.Default }) uses Default strategy, C uses a string instead of enum, and D uses a wrong property name.

3. Final Answer:

   @Component({ selector: 'app', changeDetection: ChangeDetectionStrategy.OnPush }) -> Option A

4. Quick Check:

   Use changeDetection with enum OnPush = A [OK]

Hint: Use changeDetection: ChangeDetectionStrategy.OnPush exactly [OK]

Common Mistakes:

• Using string 'OnPush' instead of enum
• Wrong property name like changeDetectionStrategy
• Confusing Default with OnPush

3. Given this Angular component with OnPush strategy:

@Component({
  selector: 'counter',
  template: `{{count}}`,
  changeDetection: ChangeDetectionStrategy.OnPush
})
export class CounterComponent {
  @Input() count = 0;
}

If the parent updates count from 0 to 1, what will happen?

medium

A. The component updates only if an event inside it triggers change detection.

B. The component view does not update until manually triggered.

C. The component view updates to show 1 immediately.

D. The component throws an error because OnPush disallows input changes.

Solution

1. Step 1: Understand OnPush input update behavior

   OnPush triggers change detection when input properties change, so updating count from parent triggers view update.

2. Step 2: Evaluate other options

   The component view does not update until manually triggered. is wrong because manual trigger is not needed for input changes. The component throws an error because OnPush disallows input changes. is false; OnPush allows input changes. The component updates only if an event inside it triggers change detection. is incomplete because input changes alone trigger detection.

3. Final Answer:

   The component view updates to show 1 immediately. -> Option C

4. Quick Check:

   Input change triggers OnPush update = D [OK]

Hint: Input changes trigger OnPush updates automatically [OK]

Common Mistakes:

• Thinking manual detection is needed for input changes
• Believing OnPush blocks input updates
• Confusing event triggers with input triggers

4. You have an Angular component using OnPush strategy. Inside it, you update a local variable without changing any input. The view does not update. What is the likely cause?

medium

A. The component must use Default strategy to update local variables.

B. OnPush disables all change detection, so no updates happen.

C. You must call detectChanges() manually for every change.

D. OnPush only checks for input changes or events, so local changes don't trigger detection.

Solution

1. Step 1: Recall OnPush detection triggers

   OnPush triggers change detection only on input changes or events inside the component.

2. Step 2: Understand local variable update effect

   Updating a local variable without input change or event does not trigger detection, so view stays the same.

3. Step 3: Analyze other options

   OnPush disables all change detection, so no updates happen. is false; OnPush does not disable detection completely. You must call detectChanges() manually for every change. is sometimes needed but not always. The component must use Default strategy to update local variables. is incorrect; Default strategy is not required for local updates.

4. Final Answer:

   OnPush only checks for input changes or events, so local changes don't trigger detection. -> Option D

5. Quick Check:

   Local changes need input/event to trigger OnPush = A [OK]

Hint: OnPush ignores local changes without input or event triggers [OK]

Common Mistakes:

• Assuming OnPush disables all detection
• Forgetting to trigger detection manually when needed
• Thinking Default strategy is required for local updates

5. You have a parent component passing an object as input to a child component using OnPush strategy. The parent updates a property inside the object without changing the object reference. Will the child detect the change and update its view?

hard

A. No, because OnPush only detects changes when the input reference changes.

B. Yes, but only if you call markForCheck() manually.

C. Yes, because OnPush detects deep changes inside objects automatically.

D. No, because OnPush disables all change detection for objects.

Solution

1. Step 1: Understand OnPush input change detection

   OnPush detects changes by checking if the input reference changes, not deep object mutations.

2. Step 2: Analyze object mutation effect

   Changing a property inside the object without changing the reference does not trigger OnPush detection.

3. Step 3: Evaluate other options

   Yes, because OnPush detects deep changes inside objects automatically. is false because OnPush does not do deep checks. Yes, but only if you call markForCheck() manually. is partially true but not automatic. No, because OnPush disables all change detection for objects. is incorrect; OnPush does not disable detection for objects.

4. Final Answer:

   No, because OnPush only detects changes when the input reference changes. -> Option A

5. Quick Check:

   OnPush detects input reference changes only = C [OK]

Hint: OnPush tracks input references, not deep object changes [OK]

Common Mistakes:

• Assuming deep object changes trigger OnPush
• Thinking OnPush disables detection for objects
• Forgetting to update object reference to trigger detection