Controlled components in React - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What is the output of this controlled input component?

Consider this React component that uses a controlled input. What will be displayed when the user types 'hello'?

React

import React, { useState } from 'react';

function ControlledInput() {
  const [text, setText] = useState('');

  return (
    <div>
      <input
        type="text"
        value={text}
        onChange={e => setText(e.target.value)}
        aria-label="input-text"
      />
      <p>{text}</p>
    </div>
  );
}

export default ControlledInput;

A The input will not accept any typing.

B The paragraph will display 'hello' as the user types.

C The paragraph will remain empty regardless of typing.

D The paragraph will display the reversed string of the input.

💡 Hint

Think about how the state updates on input change and how it affects the paragraph.

The component uses a controlled input where the input's value is tied to the state variable text. When the user types, onChange updates text, which then updates the paragraph content. So the paragraph shows exactly what the user types.

❓ state_output

intermediate

What is the value of state after this input change?

Given this controlled component, what will be the value of name state after the user types 'React'?

React

import React, { useState } from 'react';

function NameInput() {
  const [name, setName] = useState('');

  function handleChange(e) {
    setName(e.target.value.toUpperCase());
  }

  return <input type="text" value={name} onChange={handleChange} aria-label="name-input" />;
}

export default NameInput;

A "React"

B "react"

C "REACT"

D "" (empty string)

💡 Hint

Look at how the input value is transformed before setting state.

The handleChange function converts the input value to uppercase before setting the state. So typing 'React' results in state being 'REACT'.

📝 Syntax

advanced

Which option causes a syntax error in this controlled component?

Identify which code snippet will cause a syntax error when used as a controlled input in React.

A const [value, setValue] = useState(''); return <input value={value} onChange={e => setValue(e.target.value) />;

B const [value, setValue] = useState(''); return <input value={value} onChange={e => setValue(e.target.value)} />;

C const [value, setValue] = useState(''); return <input value={value} onChange={e => setValue(e.target.value) } />;

D ;>/ })eulav.tegrat.e(eulaVtes >= e{=egnahCno }eulav{=eulav tupni< nruter ;)''(etatSesu = ]eulaVtes ,eulav[ tsnoc

💡 Hint

Check the JSX syntax carefully, especially the closing of the input tag.

Option A has an incorrect closing of the input tag: / > instead of />. This causes a syntax error in JSX.

🔧 Debug

advanced

Why does this controlled input not update on typing?

Look at this React component. Why does the input not show any typed characters?

React

import React, { useState } from 'react';

function BrokenInput() {
  const [text, setText] = useState('');

  return <input type="text" value={text} onChange={() => setText('')} aria-label="broken-input" />;
}

export default BrokenInput;

A The value prop is missing, so input is uncontrolled and does not update.

B The input type is incorrect and does not accept text.

C The useState hook is not initialized properly.

D The onChange handler resets the state to empty string on every change, so input never updates.

💡 Hint

Check what the onChange function does to the state.

The onChange handler sets the state to an empty string every time the input changes. This means the input value is always reset to empty, so the user cannot type anything visible.

🧠 Conceptual

expert

What is the main advantage of controlled components in React?

Why do React developers prefer controlled components for form inputs?

A They allow React state to fully control the form data, enabling validation and conditional rendering easily.

B They automatically optimize performance by avoiding re-renders.

C They require less code than uncontrolled components.

D They allow direct DOM manipulation for faster updates.

💡 Hint

Think about how controlled components relate to React state and UI updates.

Controlled components keep the form data in React state, making it easy to validate input, conditionally render UI, and keep data consistent. They do not inherently optimize performance or reduce code length.