Why state and props drive component behavior in React Native

Introduction

State and props control what a component shows and how it acts. They help the app update and respond to user actions.

When you want a button to change its label after being pressed.

When you need to show different text based on user input.

When passing information from a parent screen to a child screen.

When you want to update a list after adding or removing items.

When you want to keep track of user choices inside a screen.

Syntax

React Native

import React, { useState } from 'react';
import { View, Text, Button } from 'react-native';

function MyComponent(props) {
  const [stateValue, setStateValue] = useState(initialValue);
  return (
    <View>
      <Text>{props.someProp}</Text>
      <Button onPress={() => setStateValue(newValue)} title="Press me" />
    </View>
  );
}

props are values passed from a parent component to a child component.

state is data managed inside the component that can change over time.

Examples

This component uses props to show a name passed from its parent.

React Native

import React from 'react';
import { Text } from 'react-native';

function Greeting(props) {
  return <Text>Hello, {props.name}!</Text>;
}

This component uses state to keep track of a number that changes when the button is pressed.

React Native

import React, { useState } from 'react';
import { View, Text, Button } from 'react-native';

function Counter() {
  const [count, setCount] = useState(0);
  return (
    <View>
      <Text>Count: {count}</Text>
      <Button onPress={() => setCount(count + 1)} title="Add" />
    </View>
  );
}

Sample App

This app shows text that changes color when you press the button. The color is stored in state. It also shows a message passed from the parent using props.

React Native

import React, { useState } from 'react';
import { View, Text, Button } from 'react-native';

export default function ColorChanger(props) {
  const [color, setColor] = useState('blue');

  return (
    <View style={{ padding: 20, alignItems: 'center' }}>
      <Text style={{ color: color, fontSize: 24 }}>This text changes color!</Text>
      <Button
        title="Change to red"
        onPress={() => setColor('red')}
        accessibilityLabel="Change text color to red"
      />
      <Text style={{ marginTop: 10 }}>Parent says: {props.message}</Text>
    </View>
  );
}

Important Notes

Changing state causes the component to update and show new data.

props are read-only inside the component; you cannot change them there.

Use state for data that changes inside the component, and props for data passed from outside.

Summary

State holds data that can change inside a component.

Props pass data from parent to child components.

Together, they control what the component shows and how it behaves.

Practice

1. What is the main role of state in a React Native component?

easy

A. To define the component's layout structure

B. To pass data from a parent component to a child component

C. To style the component with CSS-like rules

D. To hold data that can change over time inside the component

Solution

1. Step 1: Understand what state represents

   State is used to store data that can change during the life of a component, like user input or toggles.

2. Step 2: Differentiate state from props

   Props are for passing data from parent to child, not for internal changes.

3. Final Answer:

   To hold data that can change over time inside the component -> Option D

4. Quick Check:

   State = internal changing data [OK]

Hint: State holds changing data inside component [OK]

Common Mistakes:

• Confusing state with props
• Thinking state is for styling
• Believing state is static

2. Which of the following is the correct way to define a state variable using React hooks in a functional component?

easy

A. var count = useState(0);

B. let count = useState(0);

C. const [count, setCount] = useState(0);

D. state = { count: 0 };

Solution

1. Step 1: Recall React hook syntax for state

   useState returns an array with the current value and a function to update it, usually destructured.

2. Step 2: Check each option

   const [count, setCount] = useState(0); correctly uses array destructuring with useState. Others misuse syntax or do not destructure.

3. Final Answer:

   const [count, setCount] = useState(0); -> Option C

4. Quick Check:

   useState with destructuring = correct [OK]

Hint: useState returns [value, setter] array [OK]

Common Mistakes:

• Not destructuring useState result
• Using var or let instead of const
• Assigning useState directly to a variable

3. Consider this React Native component code:

function Greeting(props) {
  return Hello, {props.name}!;
}

function App() {
  return ;
}

What will be displayed on the screen?

medium

A. Hello, Alice!

B. Hello, name!

C. Hello, !

D. Error: props not defined

Solution

1. Step 1: Understand how props work in this example

   The Greeting component receives name as a prop from App with value "Alice".

2. Step 2: Check the rendered output

   The component returns Hello, {props.name}! which becomes "Hello, Alice!" on screen.

3. Final Answer:

   Hello, Alice! -> Option A

4. Quick Check:

   Props pass data = Hello, Alice! [OK]

Hint: Props pass data from parent to child [OK]

Common Mistakes:

• Assuming props is undefined
• Ignoring the passed name value
• Confusing props with state

4. What is wrong with this React Native code snippet?

function Counter() {
  const [count, setCount] = useState(0);
  return (
    
      {count}
       count = count + 1} />
    
  );
}

medium

A. You cannot assign directly to the state variable; must use setCount

B. Button component does not have a title prop

C. useState is not imported

D. Text component cannot display numbers

Solution

1. Step 1: Identify how state updates should happen

   State variables are read-only; to change state, use the setter function like setCount.

2. Step 2: Check the onPress handler

   The code tries to assign directly to count, which is invalid and won't update the UI.

3. Final Answer:

   You cannot assign directly to the state variable; must use setCount -> Option A

4. Quick Check:

   State updates require setter function [OK]

Hint: Always use setter to update state, never assign directly [OK]

Common Mistakes:

• Trying to change state variable directly
• Forgetting to import useState
• Misusing Button props

5. You want a component to show a message that changes when a button is pressed. Which approach correctly uses state and props to achieve this?

function Message({ initial }) {
  const [text, setText] = useState(initial);
  return (
    
      {text}
       setText('Updated!')} />
    
  );
}

function App() {
  return ;
}

hard

A. This fails because Button cannot change state

B. This works because state holds the message and props provide the initial value

C. This fails because props cannot be used inside useState

D. This fails because Text cannot display state variables

Solution

1. Step 1: Analyze how props and state are used

   The component receives an initial message via props and stores it in state for changes.

2. Step 2: Check the button's onPress behavior

   Pressing the button calls setText to update the state, which updates the displayed message.

3. Final Answer:

   This works because state holds the message and props provide the initial value -> Option B

4. Quick Check:

   Props initialize state; state changes update UI [OK]

Hint: Use props for initial data, state for changes [OK]

Common Mistakes:

• Thinking props can change after initial render
• Trying to update props directly
• Assuming Button can't trigger state changes