Question 1

What is the difference between the Real DOM and the Virtual DOM in React?

Accepted Answer

The Real DOM is compared to a clunky, slow-moving city where changing a small detail requires rebuilding large sections of the environment. In contrast, the Virtual DOM acts as a lightweight "magical blueprint" or stunt double. When data changes, React uses a process called "diffing" to spot the specific differences between the old and new versions, updating only the necessary parts of the real website rather than rebuilding the entire page.

Question 2

How do "Props" and "State" differ in managing data?

Accepted Answer

Props and State are the two primary ways React components handle information. Props (short for properties) are read-only instructions passed down from a parent component to a child, creating a predictable "unidirectional data flow." State, however, is a component's internal memory that handles data that changes over time, such as user input or a toggle switch. When state is updated using a specific setter function, React automatically re-renders the component to reflect the new "source of truth."

Question 3

Why is it considered a mistake to use an array index as a "key" in a list?

Accepted Answer

Keys serve as unique identity cards that help React track individual elements as they move or change. If you use an array index as a key, the identity of the items becomes tied to their position rather than their data. If the list is reordered or an item is added to the top, every index changes, which confuses React. This can lead to significant performance drops and "nightmare bugs," such as user text disappearing or jumping between input fields.

Question 4

What is "Fiber" and how does it improve the user experience?

Accepted Answer

Fiber is React's internal architecture designed to make the rendering process "interruptible." Before Fiber, React had to finish a rendering task once it started, which could cause the UI to feel "janky" if a user tried to interact with the page during a large update. Fiber breaks rendering work into tiny units and uses "time slicing" to pause background work for high-priority events, like clicks or keystrokes, ensuring the application remains fluid and responsive.

Question 5

What are React Hooks and which ones are most commonly used?

Accepted Answer

Hooks are functions introduced to allow developers to use state and other React features inside simple functional components instead of complex classes. The "Big Three" hooks are useState for managing internal data, useEffect for handling side effects like API data fetching, and useContext for sharing data across many components without "prop drilling." Modern updates have also introduced specialized hooks like useOptimistic, which allows the UI to show a successful state before a server even confirms the action.

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