Question 1

What is Angular change detection?

Accepted Answer

The process that syncs component state to the DOM by checking bindings when the app may have changed. Traditionally Zone.js patches async APIs and triggers CD. Default strategy checks the whole tree; OnPush checks when inputs/events/signals mark the view dirty.

Question 2

How does ChangeDetectionStrategy.OnPush improve performance?

Accepted Answer

OnPush skips checking a subtree unless inputs change by reference, events fire, or the view is marked dirty. Combine with immutable inputs and async pipe/signals. Mutating objects in place without marking dirty won't update the view.

Question 3

What does markForCheck vs detectChanges do?

Accepted Answer

markForCheck marks the path to root dirty for the next CD; detectChanges runs CD immediately on that view. Use markForCheck inside OnPush when updating state outside Angular's knowledge. detectChanges is sharper and can be used carefully in tests/directives.

Question 4

Why can mutating an @Input object fail to update an OnPush child?

Accepted Answer

OnPush compares input references; in-place mutation keeps the same reference. Clone/update immutably or call markForCheck. Prefer signals which notify dependents on set.

Question 5

How do signals interact with change detection?

Accepted Answer

Updating a signal marks dependent views dirty so Angular can refresh only what needs updating. Signal-based CD reduces reliance on Zone.js. computed/effect form a reactive graph. Templates reading signals establish dependencies.

Question 6

When would you detach a change detector?

Accepted Answer

Rare cases like high-frequency updates where you manually control when to reattach/detect. Detaching skips automatic checks. Easy to forget reattach—prefer OnPush/signals first. Useful for virtual scroll internals sometimes.

Change Detection Angular Interview Questions

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