Can you sit the React Certification today?

What happens to `props.children` if a component is used with no content between its tags, e.g. `<Card />`?

• AReact inserts a placeholder div
• BIt throws an error immediately
• CIt defaults to an empty string automatically in every case
• DIt's undefined

What is an AbortController used for inside a data-fetching effect's cleanup?

• ACancelling an in-flight request if the component unmounts or dependencies change before it finishes, preventing a stale response from overwriting fresher state
• BMaking the fetch execute faster
• CAutomatically retrying a failed request
• DIt has no relevant use inside useEffect

Why is 'this component re-renders a lot' not automatically a performance problem worth fixing?

• ABecause a component function re-running is ordinary JavaScript execution, often cheap; real cost — DOM mutation — only happens if the output actually differs
• BBecause re-renders are always entirely free, with no cost at all
• CBecause React allows only a fixed number of re-renders per second
• DBecause unnecessary re-renders always crash the app

Is prop drilling through one or two layers always something that needs fixing?

• AYes, any prop drilling at all is a code smell that must be eliminated
• BYes, but only inside class components
• COnly if the prop is a function, not a plain value
• DNo — a prop threaded through one or two layers is often easy to trace by just reading the code; the pain scales with depth and prop count, not mere existence

After lifting state up, how does a child that no longer owns the state actually change it?

• AThe parent passes down a callback prop (often wrapping its own setter), and the child calls that callback to request the change
• BIt can't — only the parent that owns the state may ever change it
• CThe child directly imports the parent's setState function from another module
• DThe child dispatches a raw browser event that the parent happens to listen for