Should keys be globally unique?
Interviewer Expectations for this Question
01
Core Competency
Assesses fundamental understanding of React conventions, runtime behavior, and memory/performance considerations.
02
Evaluation Criteria
Hiring managers look for precision, avoidance of ambiguous jargon, and ability to explain trade-offs under real production conditions.
Comprehensive Model Answer
Verified Solution
The keys used within arrays should be unique among their siblings but they don’t need to be globally unique. i.e, You can use the same keys with two different arrays.
For example, the below Book component uses two arrays with different arrays,
function Book(props) {
const index = (
<ul>
{props.pages.map((page) => (
<li key={page.id}>{page.title}</li>
))}
</ul>
);
const content = props.pages.map((page) => (
<div key={page.id}>
<h3>{page.title}</h3>
<p>{page.content}</p>
<p>{page.pageNumber}</p>
</div>
));
return (
<div>
{index}
<hr />
{content}
</div>
);
}
Candidate Response Strategy & Interview Tips
- Start with a concise one-sentence summary: Deliver a direct, confident answer first before expanding into nuances.
- Demonstrate real-world trade-offs: Discuss where this approach excels and when you would avoid it in production systems.
- Discuss complexity & edge cases: Proactively explain time/space complexity or boundary conditions (null values, scale limits).
- Prepare for interviewer follow-ups: Technical hiring panels frequently probe deeper into concurrency, backward compatibility, or alternative libraries.