Controlled components in React - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a controlled component in React?

A controlled component is a form element (like input, textarea, select) whose value is controlled by React state. The component's displayed value is set by React, and user input updates the state.

beginner

How do you update the value of a controlled input in React?

You update the value by changing the state that controls the input's value, usually inside an event handler like onChange.

intermediate

Why are controlled components preferred over uncontrolled components in React?

Controlled components keep React as the single source of truth for form data, making it easier to validate, modify, or submit form data and keep UI consistent.

intermediate

What happens if you do not provide an onChange handler to a controlled input?

The input becomes read-only because React controls its value but has no way to update it from user input.

beginner

Show a simple React controlled input example.

Example: <br>const [name, setName] = React.useState('');<br><input value={name} onChange={e => setName(e.target.value)} />

In React, what controls the value of a controlled input?

A React state

B The DOM directly

C Browser default behavior

D CSS styles

Controlled inputs get their value from React state, not directly from the DOM.

What must you provide to a controlled input to allow user typing?

A A defaultValue attribute

B A ref to the input element

C An onChange handler that updates state

D A CSS class

Without an onChange handler updating state, the input cannot change its value.

Which is a benefit of controlled components?

A React manages form data centrally

B They require less code than uncontrolled components

C They automatically validate input

D They do not need event handlers

Controlled components keep form data in React state, making management easier.

What happens if you set a controlled input's value but omit onChange?

A React throws an error

B Input updates normally

C Input clears automatically

D Input becomes read-only

Without onChange, React cannot update state, so input cannot change.

Which React hook is commonly used to create controlled components?

A useEffect

B useState

C useRef

D useContext

useState holds the current value and updates it on user input.

Explain what a controlled component is and why it is useful in React forms.

Think about who controls the input's value and how React uses state.

Describe how you would implement a controlled text input in React.

Remember the flow: user types → onChange → update state → input value 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