Controlled components in React

Introduction

Controlled components let you keep form data inside React state. This helps you manage and use the data easily.

When you want to update the UI based on user input immediately.

When you need to validate or format input as the user types.

When you want to enable or disable buttons based on form data.

When you want to submit form data from React state.

When you want to reset or clear form fields programmatically.

Syntax

React

function MyComponent() {
  const [value, setValue] = React.useState('');

  function handleChange(event) {
    setValue(event.target.value);
  }

  return (
    <input type="text" value={value} onChange={handleChange} />
  );
}

The value attribute sets the input's current value from React state.

The onChange event updates the state when the user types.

Examples

Simple controlled input with inline event handler.

React

const [name, setName] = React.useState('');

<input value={name} onChange={e => setName(e.target.value)} />

Controlled checkbox input using checked attribute.

React

const [checked, setChecked] = React.useState(false);

<input type="checkbox" checked={checked} onChange={e => setChecked(e.target.checked)} />

Controlled textarea element with state.

React

const [text, setText] = React.useState('Hello');

<textarea value={text} onChange={e => setText(e.target.value)} />

Sample Program

This component shows a text input controlled by React state. As you type, the text below updates instantly.

React

import React from 'react';

export default function ControlledInput() {
  const [inputValue, setInputValue] = React.useState('');

  function handleChange(event) {
    setInputValue(event.target.value);
  }

  return (
    <div>
      <label htmlFor="nameInput">Name: </label>
      <input
        id="nameInput"
        type="text"
        value={inputValue}
        onChange={handleChange}
        aria-label="Name input"
      />
      <p>You typed: {inputValue}</p>
    </div>
  );
}

Important Notes

Always set the value or checked attribute to keep input controlled.

Without onChange, the input will be read-only.

Use aria-label or label for accessibility.

Summary

Controlled components keep form data in React state.

Use value and onChange to sync input and state.

This helps with validation, formatting, and dynamic UI updates.

Practice

1. What is a controlled component in React?

easy

A. A component that does not use state at all

B. A component that controls other components' rendering

C. A form input element whose value is controlled by React state

D. A component that uses refs to access DOM elements

Solution

1. Step 1: Understand controlled components

   Controlled components are form inputs whose values are managed by React state, not by the DOM alone.

2. Step 2: Identify the correct definition

   A form input element whose value is controlled by React state correctly describes this concept as React controlling the input value via state.

3. Final Answer:

   A form input element whose value is controlled by React state -> Option C

4. Quick Check:

   Controlled component = React state controls input value [OK]

Hint: Controlled means React state holds the input value [OK]

Common Mistakes:

• Confusing controlled with uncontrolled components
• Thinking refs control input value
• Assuming controlled means no state usage

2. Which of the following is the correct way to create a controlled input in React?

easy

A.

B.

C.

D.

Solution

1. Step 1: Check controlled input syntax

   A controlled input must have its value set from state and update state on change.

2. Step 2: Analyze options

   <input value={this.state.text} onChange={this.handleChange} /> uses value from state and onChange handler to update it, which is correct.

3. Final Answer:

   <input value={this.state.text} onChange={this.handleChange} /> -> Option A

4. Quick Check:

   Value + onChange = controlled input [OK]

Hint: Controlled input needs value and onChange props [OK]

Common Mistakes:

• Using defaultValue instead of value for controlled inputs
• Omitting onChange handler
• Not linking value to state

3. What will be the output when typing in this controlled input?

function MyInput() {
  const [text, setText] = React.useState('');
  return (
    <input value={text} onChange={e => setText(e.target.value)} />
  );
}

medium

A. The input updates and shows what the user types

B. The input stays empty and does not update

C. The input throws a runtime error

D. The input shows the initial empty string and ignores typing

Solution

1. Step 1: Understand the controlled input behavior

   The input's value is linked to state variable 'text'. The onChange updates 'text' with user input.

2. Step 2: Predict the input behavior

   As user types, onChange updates state, which updates value, so input shows typed characters.

3. Final Answer:

   The input updates and shows what the user types -> Option A

4. Quick Check:

   State updates input value = visible typing [OK]

Hint: State + onChange updates input value dynamically [OK]

Common Mistakes:

• Thinking input won't update without defaultValue
• Assuming input is uncontrolled without onChange
• Confusing initial state with input behavior

4. Identify the error in this controlled component code:

function Form() {
  const [name, setName] = React.useState('');
  return (
    <input value={name} onChange={e => setName} />
  );
}

medium

A. The useState hook is missing an initial value

B. The value prop should be defaultValue

C. The input element is missing a closing tag

D. The onChange handler does not call setName with the new value

Solution

1. Step 1: Check onChange handler usage

   The onChange handler is set to 'e => setName' which is a function reference, not a call with the new value.

2. Step 2: Correct onChange handler

   It should be 'e => setName(e.target.value)' to update state with input value.

3. Final Answer:

   The onChange handler does not call setName with the new value -> Option D

4. Quick Check:

   onChange must call setState with event value [OK]

Hint: onChange must call setState with event target value [OK]

Common Mistakes:

• Passing setName without calling it
• Using defaultValue instead of value in controlled input
• Forgetting to pass initial state to useState

5. You want to create a controlled textarea that converts all input to uppercase as the user types. Which code snippet correctly implements this?

hard

A. const [text, setText] = React.useState('');
return

B. const [text, setText] = React.useState('');
return

C. const [text, setText] = React.useState('');
return

D. const [text, setText] = React.useState('');
return

Solution

1. Step 1: Understand controlled textarea with uppercase transformation

   The textarea's value must come from state, and onChange must update state with uppercase input.

2. Step 2: Analyze each option

   const [text, setText] = React.useState(''); return <textarea value={text} onChange={e => setText(e.target.value.toUpperCase())} />; updates state with uppercase input on change and sets value from state, so textarea shows uppercase as typed.

3. Final Answer:

   const [text, setText] = React.useState(''); return <textarea value={text} onChange={e => setText(e.target.value.toUpperCase())} />; -> Option B

4. Quick Check:

   Update state with uppercase input onChange = uppercase textarea [OK]

Hint: Transform input in onChange before setting state [OK]

Common Mistakes:

• Transforming value prop instead of input value
• Using defaultValue in controlled component
• Not updating state with transformed input