TextInput with controlled state in React Native - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a controlled TextInput in React Native?

A controlled TextInput is one where the input value is managed by React state. The displayed text is set by a state variable, and changes update that state.

beginner

Why use controlled TextInput instead of uncontrolled?

Controlled TextInput lets you validate, format, or respond to user input immediately by keeping the input value in React state.

beginner

Which React hook is commonly used to manage TextInput state?

The useState hook is used to create a state variable and update function to control the TextInput value.

beginner

How do you update the state when the user types in a controlled TextInput?

You pass a function to the TextInput's onChangeText prop that receives the new text and calls the state update function.

intermediate

What happens if you forget to update the state in a controlled TextInput?

The TextInput will not update its displayed text because the value prop stays the same, making the input appear frozen.

In React Native, which prop sets the displayed text in a controlled TextInput?

A text

B defaultValue

C value

D input

The value prop controls the displayed text in a controlled TextInput.

Which hook is used to create a state variable for TextInput value?

A useState

B useEffect

C useContext

D useRef

useState creates state variables and update functions.

What prop do you use to detect text changes in a TextInput?

A onTextChange

B onChangeText

C onInput

D onChange

onChangeText is the correct prop for text change events in React Native TextInput.

What happens if you set a fixed value prop but do not update state on text change?

A TextInput becomes read-only

B TextInput clears text

C TextInput crashes

D TextInput updates normally

Without updating state, the TextInput appears read-only because value never changes.

Why might you prefer controlled TextInput over uncontrolled?

A To avoid using state

B To improve performance

C To disable keyboard

D To handle input validation easily

Controlled inputs let you validate or format input as the user types.

Explain how to create a controlled TextInput in React Native.

Think about how React state controls the input text.

Describe what happens if the onChangeText handler does not update the state in a controlled TextInput.

Consider the connection between state and displayed text.

Practice

1. What does it mean when a TextInput in React Native is "controlled"?

easy

A. The input value is managed by React state using value and onChangeText.

B. The input automatically saves data to a database.

C. The input cannot be edited by the user.

D. The input uses default native keyboard settings only.

Solution

1. Step 1: Understand controlled input concept

   A controlled TextInput means React state holds the current text value.

2. Step 2: Identify how React manages input

   React uses value prop to set input text and onChangeText to update state on typing.

3. Final Answer:

   The input value is managed by React state using value and onChangeText. -> Option A

4. Quick Check:

   Controlled input = React state control [OK]

Hint: Controlled means React state controls input text [OK]

Common Mistakes:

• Thinking controlled means input is read-only
• Confusing controlled with auto-saving
• Assuming native keyboard controls the value

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

easy

A. <TextInput value={text} onChange={setText} />

B. <TextInput value={text} onChangeText={setText} />

C. <TextInput defaultValue={text} onChange={setText} />

D. <TextInput defaultValue={text} onChangeText={text} />

Solution

1. Step 1: Identify correct props for controlled input

   The value prop sets the input text, and onChangeText updates state on typing.

2. Step 2: Check each option's syntax

   <TextInput value={text} onChangeText={setText} /> uses value and onChangeText correctly. Others misuse onChange or defaultValue.

3. Final Answer:

   <TextInput value={text} onChangeText={setText} /> -> Option B

4. Quick Check:

   Use value + onChangeText for controlled input [OK]

Hint: Use value and onChangeText props for controlled TextInput [OK]

Common Mistakes:

• Using onChange instead of onChangeText
• Using defaultValue for controlled input
• Passing wrong handler to onChangeText

3. What will be displayed inside the TextInput after typing "Hi" in this code?

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

<TextInput value={text} onChangeText={setText} />

medium

A. The input will show "undefined".

B. The input will remain empty.

C. The input will show "Hi" as typed.

D. The input will show "" (empty string) always.

Solution

1. Step 1: Understand state updates on typing

   Typing "Hi" triggers onChangeText which calls setText to update state to "Hi".

2. Step 2: Value prop reflects state

   The value prop uses text state, so input shows "Hi" as typed.

3. Final Answer:

   The input will show "Hi" as typed. -> Option C

4. Quick Check:

   State updates input value = typed text [OK]

Hint: Value prop shows current state text instantly [OK]

Common Mistakes:

• Thinking input won't update without defaultValue
• Confusing onChangeText with onChange
• Assuming input clears after typing

4. Identify the error in this controlled TextInput code:

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

<TextInput value={text} onChange={setText} />

medium

A. Using onChange instead of onChangeText causes no updates.

B. State should be initialized with null, not empty string.

C. Missing defaultValue prop causes error.

D. The value prop should be omitted for controlled input.

Solution

1. Step 1: Check event handler prop

   TextInput uses onChangeText for text changes, not onChange.

2. Step 2: Effect of wrong handler

   Using onChange means setText never runs, so state stays empty and input doesn't update.

3. Final Answer:

   Using onChange instead of onChangeText causes no updates. -> Option A

4. Quick Check:

   Use onChangeText, not onChange, for TextInput [OK]

Hint: Use onChangeText, not onChange, for TextInput updates [OK]

Common Mistakes:

• Confusing onChange with onChangeText
• Thinking defaultValue is required for controlled input
• Removing value prop breaks control

5. You want to create a controlled TextInput that only accepts uppercase letters. Which code snippet correctly implements this behavior?

hard

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

<TextInput
value={text}
onChangeText={t => setText(t.toLowerCase())}
/>

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

<TextInput
value={text.toUpperCase()}
onChangeText={setText}
/>

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

<TextInput
defaultValue={text.toUpperCase()}
onChangeText={setText}
/>

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

<TextInput
value={text}
onChangeText={t => setText(t.toUpperCase())}
/>

Solution

1. Step 1: Understand uppercase enforcement

   To force uppercase, transform input text to uppercase before saving state.

2. Step 2: Check each option's logic

   const [text, setText] = React.useState('');
   
   <TextInput
     value={text}
     onChangeText={t => setText(t.toUpperCase())}
   />

   converts input to uppercase in onChangeText before calling setText.

   const [text, setText] = React.useState('');
   
   <TextInput
     value={text.toUpperCase()}
     onChangeText={setText}
   />

   only uppercases value but stores raw input, so user can type lowercase.

   const [text, setText] = React.useState('');
   
   <TextInput
     defaultValue={text.toUpperCase()}
     onChangeText={setText}
   />

   uses defaultValue which is uncontrolled.

   const [text, setText] = React.useState('');
   
   <TextInput
     value={text}
     onChangeText={t => setText(t.toLowerCase())}
   />

   converts to lowercase, opposite of requirement.

3. Final Answer:

   <TextInput value={text} onChangeText={t => setText(t.toUpperCase())} /> -> Option D

4. Quick Check:

   Transform input in onChangeText for controlled uppercase [OK]

Hint: Transform text in onChangeText before setting state [OK]

Common Mistakes:

• Trying to uppercase value prop only
• Using defaultValue instead of value
• Transforming text to lowercase by mistake