React Hooks useRef and useMemo

Question 1

What does useRef return?

Accepted Answer

A mutable ref object with a .current property

Answer

useRef returns a plain JavaScript object { current: initialValue } that persists for the full lifetime of the component.

Question 2

Does mutating a ref's .current property cause a re-render?

Accepted Answer

No, mutations to ref.current do not trigger re-renders

Answer

Refs are outside React's state system, so changing .current does not notify React or trigger a re-render.

Question 3

How do you attach a ref to a DOM element?

Accepted Answer

Pass it as the ref prop on the JSX element

Answer

React automatically sets ref.current to the DOM node when you pass the ref object to the ref prop.

Question 4

What is useMemo used for?

Accepted Answer

Memoizing expensive computed values between renders

Answer

useMemo caches the result of a computation and only re-runs it when specified dependencies change.

Question 5

What are the two arguments to useMemo?

Accepted Answer

A factory function and a dependency array

Answer

useMemo takes a factory function (returning the value to memoize) and a dependency array.

React Practice Test