Question 1

Why does my React component render twice in development?

Accepted Answer

In development, Strict Mode deliberately renders components twice and runs effects twice, mounting, unmounting and remounting them. This does not happen in production builds. It is not a bug and not a performance problem in your code; it is a deliberate check.

The purpose is to surface impurity. A component is expected to be a pure function of its props and state, and an effect is expected to clean up after itself well enough that setting it up again produces the same result. Running things twice makes violations obvious immediately rather than as a subtle bug later, when concurrent rendering may legitimately do something similar.

The productive response is therefore to ask what the double invocation revealed rather than to suppress it. Anything that breaks under it was already fragile.

- An effect that fetches data and appends to a list will duplicate entries. That means it lacks a cleanup discarding the stale result, which is also what protects it from out-of-order responses in real use.
- A subscription created without a matching teardown leaks one on every remount, and Strict Mode exposes it on the very first mount.
- A counter incremented during render, or a value mutated outside state, jumps by two, which correctly identifies a side effect happening during rendering.

The wrong fixes are the tempting ones: a flag making the effect run only once, or disabling Strict Mode. Both hide the signal without addressing what it found, and the underlying fragility remains for production concurrency to expose later.

If you have verified an effect is genuinely idempotent and correctly cleaned up, the double run is harmless and can be ignored. The behaviour disappears in a production build regardless.

Question 2

What is a race condition in a React useEffect and how do I avoid it?

Accepted Answer

The problem appears whenever an effect starts an asynchronous request that depends on a changing value. If the value changes again before the first request finishes, two requests are in flight, and

Question 3

What is the useEffect dependency array for?

Accepted Answer

The dependency array tells React which values the effect uses, so it can decide whether to run it again after a render. Omitting the array runs the effect after every render. Passing an empty array

Question 4

Why is my React state not updating immediately?

Accepted Answer

State updates are not applied immediately. Calling the setter schedules a re-render; it does not change the variable you are currently holding. Reading that variable on the next line gives you the

Question 5

What are some effective React performance optimization techniques?

Accepted Answer

Optimizing React performance involves techniques like code splitting, memoization, and lazy loading to enhance speed and responsiveness. Implement these methods based on your app's needs.

Question 6

How to optimize React app performance?

Accepted Answer

Optimizing React app performance involves strategies like code splitting, memoization, lazy loading, and optimizing state management. These techniques enhance rendering speed and reduce resource consumption.

Question 7

What is the React component lifecycle explained?

Accepted Answer

The React component lifecycle consists of Mounting, Updating, and Unmounting phases, each with specific methods that manage component behavior and performance.

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