What is the Role of Keys in a List Rendering Scenario?
💡 Concept Name
Keys in React Lists – Unique identifiers that help React track elements in a list during rendering and updating processes.
📘 Quick Intro
Keys help React efficiently update the UI by identifying which items have changed, been added, or removed from a list. Without keys, React may unnecessarily re-render elements or cause unexpected behavior.
🧠 Analogy / Short Story
Imagine a classroom seating chart where each student has a unique roll number. When students move around, the teacher can still identify them correctly by their number rather than their seat.
Keys in React serve the same purpose—they let React keep track of each component even when their positions or contents change.
🔧 Technical Explanation
- 🔑 Keys must be unique among siblings to ensure accurate identification.
- ⚡ They allow React's diffing algorithm to minimize DOM manipulation and improve performance.
- 🚫 Using array indexes as keys is discouraged if the list changes order or has dynamic content.
- 🧩 Without keys, React may destroy and recreate elements, causing loss of state or bugs.
💻 Real Code Example
const fruits = ['Apple', 'Banana', 'Orange'];
function FruitList() {
return (
<ul>
{fruits.map((fruit, index) => (
<li key={fruit}>{fruit}</li>
))}
</ul>
);
}
❓ Interview Q&A
Q1: Why are keys important in React list rendering?
A: Keys help React identify which list items have changed, added, or removed, improving performance.
Q2: What is the best practice for assigning keys?
A: Use a unique and stable identifier like a database ID or unique string.
Q3: Can we use index as a key?
A: Only if the list is static and will not change order or length.
Q4: What happens if keys are not unique?
A: React may re-render components incorrectly or lose internal state.
Q5: Do keys get passed as props to the component?
A: No, keys are only used internally by React and are not accessible in the child component.
Q6: Are keys required in React lists?
A: Yes, they are required when using lists with dynamic rendering (e.g., map function).
Q7: Can you change keys during re-rendering?
A: Changing keys unnecessarily can hurt performance by forcing re-mounting.
Q8: How does key help in reconciliation?
A: Keys let React compare previous and current elements efficiently to update only what's needed.
Q9: What happens if you omit the key prop entirely?
A: React logs a warning and may not render the list correctly.
Q10: Do keys affect component state?
A: Indirectly, yes—changing keys can force components to reset state if re-mounted.
📝 MCQs
Q1. What is the main purpose of keys in React lists?
- Style list items
- Help React identify items for efficient updates
- Attach click events
- Enable animations
Q2. Which of these is a good key choice?
- Array index
- Math.random()
- Timestamp
- Unique database ID
Q3. What can happen if you use array index as a key in a dynamic list?
- Performance boost
- CSS issues
- Incorrect rendering or loss of state
- Nothing happens
Q4. Are keys accessible as props in child components?
- Yes
- Only in React 18
- No
- Only for class components
Q5. Where should the key prop be placed?
- Inside component
- In parent div
- On the outermost element in the list
- On any HTML tag
Q6. What does React do when keys change?
- Ignores it
- Saves state
- Re-mounts the component
- Logs an error
Q7. Are keys optional when using map to render lists?
- Yes
- No
- Only in class components
- Only in strict mode
Q8. Which one causes inefficient re-rendering?
- Memoization
- Static keys
- Non-unique keys
- List of numbers
Q9. How does React use keys during diffing?
- To reorder DOM
- To attach styles
- To compare previous and next items
- To apply keys to CSS
Q10. Why shouldn't you use random values as keys?
- They cause syntax errors
- They aren't allowed in JSX
- They change on every render and defeat diffing
- They’re too slow
💡 Bonus Insight
When rendering child components in a loop, using meaningful keys helps preserve internal component state and prevents bugs like input value reset, incorrect animations, or UI flickers.
📄 PDF Download
Need a handy summary for your notes? Download this topic as a PDF!