When to use OnPush in Angular - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the OnPush change detection strategy do in Angular?

OnPush tells Angular to check a component only when its input properties change or when an event happens inside it, improving performance by skipping unnecessary checks.

beginner

When should you consider using OnPush in your Angular components?

Use OnPush when your component's inputs are immutable or rarely change, and you want to optimize performance by reducing how often Angular checks for changes.

intermediate

How does OnPush affect Angular's change detection compared to the default strategy?

With OnPush, Angular skips checking the component unless input references change or an event occurs inside it, unlike the default strategy which checks every component every time.

intermediate

What kind of data changes are detected by OnPush strategy?

OnPush detects changes when input properties receive new object references or primitive values change, but not when internal mutable data changes without new references.

intermediate

Why is immutability important when using OnPush?

Because OnPush relies on detecting new references to inputs, using immutable data ensures Angular notices changes and updates the component correctly.

What triggers Angular to check a component with OnPush strategy?

A When input properties change by reference or an event occurs inside the component

B Every time any component changes in the app

C Only when the user clicks a button

D Never, OnPush disables change detection

OnPush triggers change detection only when input references change or an event happens inside the component.

Which scenario is best suited for OnPush strategy?

A Component with immutable inputs that rarely change

B Component with many internal mutable state changes

C Component that updates every millisecond

D Component with no inputs

OnPush works best when inputs are immutable and changes are infrequent, improving performance.

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

A Angular will throw an error

B Angular will detect the change and update the view

C Angular will NOT detect the change and will NOT update the view

D Angular will reload the entire app

OnPush detects changes by reference, so mutating without changing reference is not detected.

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

A By adding more inputs

B By refreshing the browser

C By changing the component's selector

D By injecting ChangeDetectorRef and calling markForCheck() or detectChanges()

You can manually trigger detection using ChangeDetectorRef methods in OnPush components.

What is a main benefit of using OnPush strategy?

A Disables Angular's change detection

B Improved performance by reducing unnecessary checks

C Automatic detection of all changes

D Simpler code with no inputs

OnPush improves performance by limiting when Angular checks the component.

Explain when and why you would use the OnPush change detection strategy in Angular.

Think about how Angular decides to update components and how OnPush changes that.

Describe how OnPush affects Angular's change detection compared to the default strategy.

Compare the frequency and triggers of change detection in both strategies.

Practice

1. What is the main reason to use ChangeDetectionStrategy.OnPush in an Angular component?

easy

A. To improve performance by checking for changes only when inputs change or events occur

B. To disable change detection completely

C. To force Angular to check for changes on every event

D. To automatically update the component without any input changes

Solution

1. Step 1: Understand OnPush behavior

   OnPush tells Angular to check for changes only when input properties change or events inside the component happen.

2. Step 2: Compare with other options

   It does not disable change detection or force checks on every event, but optimizes performance by limiting checks.

3. Final Answer:

   To improve performance by checking for changes only when inputs change or events occur -> Option A

4. Quick Check:

   OnPush improves performance = A [OK]

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

Common Mistakes:

• Thinking OnPush disables change detection
• Assuming OnPush checks on every event
• Believing OnPush updates without input changes

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

easy

A. @Component({ selector: 'app-example', changeDetection: ChangeDetectionStrategy.Default })

B. @Component({ selector: 'app-example', changeDetection: ChangeDetectionStrategy.OnPush })

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

D. @Component({ selector: 'app-example', changeDetection: true })

Solution

1. Step 1: Recall Angular syntax for change detection

   The correct syntax uses the enum ChangeDetectionStrategy.OnPush inside the component decorator.

2. Step 2: Eliminate incorrect options

   A uses Default strategy, B uses a boolean which is invalid, C uses a string instead of enum.

3. Final Answer:

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

4. Quick Check:

   Use enum OnPush in decorator = D [OK]

Hint: Use ChangeDetectionStrategy.OnPush enum in @Component [OK]

Common Mistakes:

• Using string 'OnPush' instead of enum
• Setting changeDetection to true or false
• Confusing Default with OnPush

3. Given this component using OnPush:

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

What happens if the parent updates count by mutating an object property instead of replacing the input?

medium

A. The component updates only after a manual change detection call

B. The component updates and shows the new count

C. Angular throws a runtime error

D. The component does not update because OnPush requires immutable input changes

Solution

1. Step 1: Understand OnPush input change detection

   OnPush detects changes only when input references change (immutable updates), not when internal properties mutate.

2. Step 2: Analyze mutation effect

   Mutating an object property keeps the same reference, so Angular does not detect the change and does not update the view.

3. Final Answer:

   The component does not update because OnPush requires immutable input changes -> Option D

4. Quick Check:

   OnPush needs new input references = A [OK]

Hint: OnPush needs new input objects, not mutated ones [OK]

Common Mistakes:

• Assuming mutation triggers update
• Expecting runtime errors on mutation
• Thinking manual detection is always needed

4. You have an Angular component with OnPush strategy but the view does not update after input changes. Which fix will solve this problem?

medium

A. Manually call ChangeDetectorRef.detectChanges() after input mutation

B. Switch to Default change detection strategy

C. Replace the input object with a new reference instead of mutating it

D. Remove the @Input() decorator

Solution

1. Step 1: Identify why view doesn't update

   OnPush requires new input references to detect changes; mutating inputs keeps the same reference, so no update.

2. Step 2: Choose the best fix

   Replacing the input with a new object triggers change detection properly without needing manual calls or strategy changes.

3. Final Answer:

   Replace the input object with a new reference instead of mutating it -> Option C

4. Quick Check:

   New input references trigger OnPush updates = C [OK]

Hint: Use new input objects, not mutations, to update OnPush views [OK]

Common Mistakes:

• Relying on manual detectChanges() calls unnecessarily
• Switching to Default without reason
• Removing @Input() breaks data flow

5. You have a parent component passing a list of items to a child component with OnPush strategy. The parent updates the list by adding a new item using items.push(newItem). The child does not update. How should you fix this to ensure the child updates efficiently?

hard

A. Use items = [...items, newItem] to create a new array reference

B. Call markForCheck() on the child's ChangeDetectorRef after push

C. Change child's strategy to Default

D. Manually trigger detectChanges() in the child after push

Solution

1. Step 1: Understand OnPush and input references

   OnPush detects changes only when input references change, so mutating the array with push keeps the same reference.

2. Step 2: Apply immutable update

   Using items = [...items, newItem] creates a new array reference, triggering OnPush change detection in the child.

3. Final Answer:

   Use items = [...items, newItem] to create a new array reference -> Option A

4. Quick Check:

   Immutable input updates trigger OnPush = B [OK]

Hint: Create new input arrays to trigger OnPush updates [OK]

Common Mistakes:

• Mutating arrays with push expecting updates
• Relying on manual detectChanges() calls
• Changing strategy unnecessarily