Common list rendering mistakes in React - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What is the output of this React component?

Consider this React component that renders a list of items:

function ItemList() {
  const items = ['apple', 'banana', 'cherry'];
  return (
    <ul>
      {items.map((item) => <li>{item}</li>)}
    </ul>
  );
}

What problem will this cause in the browser console?

React

function ItemList() {
  const items = ['apple', 'banana', 'cherry'];
  return (
    <ul>
      {items.map((item) => <li key={item}>{item}</li>)}
    </ul>
  );
}

A No problem; the list renders fine without warnings.

B Warning: Each child in a list should have a unique "key" prop.

C Error: JSX elements cannot be returned without a wrapping element.

D Warning: The map function must return a string, not JSX elements.

💡 Hint

React needs a way to identify each list item uniquely for efficient updates.

React requires a unique key prop on each list item to track changes efficiently. Without keys, React shows a warning in the console.

📝 Syntax

intermediate

Which option correctly fixes the syntax error in this list rendering code?

This React code tries to render a list but has a syntax error:

const numbers = [1, 2, 3];
return (
  <ul>
    {numbers.map(number => <li key={number}>{number}<li>)}
  </ul>
);

Which option fixes the syntax error?

React

const numbers = [1, 2, 3];
return (
  <ul>
    {numbers.map(number => <li key={number}>{number}</li>)}
  </ul>
);

A {numbers.map(number => <li key={number}>{number}</li>)}

B {numbers.map(number => <li key={number}>{number}</li></li>)}

C {numbers.map(number => <li key={number}>{number}</li>);}

D {numbers.map(number => <li key={number}>{number})}</li>}

💡 Hint

Check that all JSX tags are properly closed.

The original code misses the closing </li> tag. Option A correctly closes the <li> tag.

🔧 Debug

advanced

Why does this React list rendering cause unexpected behavior?

Look at this React component:

function TodoList() {
  const todos = [
    { id: 1, task: 'Wash dishes' },
    { id: 2, task: 'Do laundry' },
    { id: 3, task: 'Cook dinner' }
  ];

  return (
    <ul>
      {todos.map((todo, index) => (
        <li key={index}>{todo.task}</li>
      ))}
    </ul>
  );
}

What problem can arise from using index as the key?

React

function TodoList() {
  const todos = [
    { id: 1, task: 'Wash dishes' },
    { id: 2, task: 'Do laundry' },
    { id: 3, task: 'Cook dinner' }
  ];

  return (
    <ul>
      {todos.map((todo, index) => (
        <li key={index}>{todo.task}</li>
      ))}
    </ul>
  );
}

A Using index as key causes React to ignore the list and render nothing.

B Using index as key causes a syntax error in React.

C Using index as key will prevent the list from rendering at all.

D Using index as key can cause incorrect item updates when the list changes order or items are added/removed.

💡 Hint

Think about what happens if the list items reorder or change.

Keys should be stable and unique. Using index as key can cause React to confuse items when the list changes, leading to bugs in UI updates.

❓ state_output

advanced

What will be the output after clicking the button in this React component?

Examine this React component:

import { useState } from 'react';

function CounterList() {
  const [counts, setCounts] = useState([0, 0, 0]);

  function incrementFirst() {
    counts[0] += 1;
    setCounts(counts);
  }

  return (
    <div>
      <ul>
        {counts.map((count, i) => <li key={i}>Count {i + 1}: {count}</li>)}
      </ul>
      <button onClick={incrementFirst}>Increment First</button>
    </div>
  );
}

What happens when the button is clicked?

React

import { useState } from 'react';

function CounterList() {
  const [counts, setCounts] = useState([0, 0, 0]);

  function incrementFirst() {
    counts[0] += 1;
    setCounts(counts);
  }

  return (
    <div>
      <ul>
        {counts.map((count, i) => <li key={i}>Count {i + 1}: {count}</li>)}
      </ul>
      <button onClick={incrementFirst}>Increment First</button>
    </div>
  );
}

A The first count number does not update in the UI because React state was mutated directly.

B The component crashes with a runtime error when clicking the button.

C The first count number updates and the UI re-renders showing the incremented value.

D All count numbers reset to zero after clicking the button.

💡 Hint

React state should not be mutated directly; think about how React detects changes.

Mutating the state array directly and passing the same reference to setState does not trigger React to re-render. The UI stays the same.

🧠 Conceptual

expert

Which option best explains why keys are critical in React list rendering?

Why does React require a unique key for each element in a list?

A Keys prevent React from rendering duplicate list items by filtering them out automatically.

B Keys are used to style list items differently based on their position.

C Keys help React identify which items have changed, been added, or removed to optimize rendering performance.

D Keys allow React to convert lists into arrays internally for faster processing.

💡 Hint

Think about how React updates the UI efficiently when data changes.

React uses keys to track elements between renders. This helps React update only the changed elements instead of re-rendering the entire list.