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What are low and high-priority updates? | Reconciliation | react Interview Questions | NextRound.in

Questions

10 of 13

1What is reconciliation in React?

2What is react fibre?

3What is fibre in react fiber?

4Discuss fibre lifecycle.

5Discuss stack reconciler.

6What is concurrent rendering in react?

7How does React Fiber improve the rendering performance?

8What is the purpose of the `dangerouslySetInnerHTML` attribute in React?

9When rendering a list what is a key and what is its purpose?

10What are low and high-priority updates?

11What is work in progress tree in react fibre?

12Is React Fiber backward compatible with previous versions of React?

13How does React Fiber handle errors?

react

State and Props

Class Components

Function Components

Lifecycle functions

Performance Optimisation

10 / 13

What are low and high-priority updates?

Difficulty: 5/10

scheduler, concurrent mode, state updates

In Fiber, there are two trees: the current tree and the work-in-progress tree.

Low Priority:

1
Background Tasks: Updates related to non-critical components (e.g., off-screen elements, data fetching) fall into this category.
2
Idle Time Tasks: Work that can be deferred during idle periods, such as preloading data or optimising performance, is considered low priority.

High Priority:

1
User Interactions: Updates triggered by user actions like clicks, and keyboard input receive high priority to maintain responsiveness.
2
Animations: Smooth animations like scrolling, and transitions require timely updates for a pleasant user experience.

Scenario Questions

0-2 years experience

1You need a button click to update a counter instantly, without any delay. How would you make sure that update is treated as high priority in a functional component?
2If you wrap a state update inside startTransition, what change would you notice in the UI compared to a normal setState call?

2-5 years experience

1While typing in a search box, the UI feels sluggish because a large list is being filtered on each keystroke. How would you diagnose whether the filtering is running as a low‑priority update and fix the lag?

Follow-up Questions

What happens if you accidentally treat a user‑initiated update as low priority?
Can you combine high‑ and low‑priority updates in the same render cycle? How?
How does this priority model interact with React.memo or useCallback?

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2Your team added a background data sync that uses useDeferredValue, but now a modal that depends on that data sometimes shows stale information. What could be causing this and how would you address it?

5-8 years experience

1Design a component that fetches data in the background (low priority) but also handles user‑initiated refreshes (high priority). Explain the APIs you would use and the trade‑offs involved.
2You have a dashboard with many widgets that update every second. Some widgets are critical for the user, others are decorative. How would you structure the updates to keep the critical ones high priority while deferring the rest?

8+ years experience

1Your organization is migrating a large legacy codebase to React 18 with concurrent features. How would you plan the rollout of low‑ vs high‑priority updates across multiple teams to minimize regressions and maintain performance?
2When introducing startTransition across a suite of micro‑frontends, what architectural considerations do you need to address to ensure consistent scheduling behavior and avoid priority inversion?