Custom components in React Native - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a custom component in React Native?

A custom component is a reusable piece of UI built by combining React Native elements or other components. It helps organize code and reuse UI parts easily.

beginner

How do you create a simple functional custom component in React Native?

You create a function that returns JSX. For example: <br><pre>function MyButton() {<br>  return (<br>    <Button title="Click me" /> <br>  );<br>}</pre>

beginner

Why use props in custom components?

Props let you pass data or settings into a component, so it can show different content or behave differently each time you use it.

beginner

What is the benefit of breaking UI into custom components?

It makes your app easier to read, maintain, and test. You can reuse components in many places without rewriting code.

beginner

How do you use a custom component inside another component?

You import or define the custom component, then use it like a tag in JSX. For example: <br><pre>&lt;MyButton /&gt;</pre>

What does a React Native custom component return?

A Database queries

B Plain text only

C CSS styles

D JSX elements

Custom components return JSX elements that describe the UI to render.

How do you pass data to a custom component?

A Using CSS classes

B Using global variables

C Using props

D Using HTML attributes

Props are the way to pass data into custom components in React Native.

Which of these is a correct way to define a custom component?

A var MyComp = 5;

B function MyComp() { return <View></View>; }

C const MyComp = 'Hello';

D let MyComp = true;

A custom component is a function returning JSX, like a View element.

Why should you create custom components?

A To reuse UI parts and keep code clean

B To make the app slower

C To avoid using props

D To write more code

Custom components help reuse UI and organize code better.

How do you include a custom component named MyButton in JSX?

A <MyButton />

B <mybutton />

C <button />

D <Mybutton />

Component names must start with uppercase and used as .

Explain how to create and use a custom component in React Native.

Think about making a small reusable UI piece.

Why is breaking your app UI into custom components helpful?

Imagine building with Lego blocks.

Practice

1. What is a custom component in React Native?

easy

A. A CSS style sheet

B. A built-in React Native library

C. A special type of variable

D. A reusable function that returns UI elements

Solution

1. Step 1: Understand what a component is

   A component is a piece of UI that can be reused in different parts of the app.

2. Step 2: Identify custom components

   Custom components are user-defined functions that return UI elements, unlike built-in libraries or styles.

3. Final Answer:

   A reusable function that returns UI elements -> Option D

4. Quick Check:

   Custom component = reusable function [OK]

Hint: Custom components are functions returning UI [OK]

Common Mistakes:

• Confusing components with styles
• Thinking components are variables
• Mixing built-in libraries with custom components

2. Which of the following is the correct syntax to define a simple custom component named Greeting in React Native?

easy

A. function Greeting() { return Hello; }

B. var Greeting = Hello;

C. const Greeting = () => { Hello }

D. class Greeting extends Component { render() { return 'Hello'; } }

Solution

1. Step 1: Check function component syntax

   Function components return JSX inside the function body with a return statement.

2. Step 2: Evaluate each option

   function Greeting() { return Hello; } correctly defines a function returning JSX. var Greeting = Hello; assigns JSX directly to a variable, which is invalid. const Greeting = () => { Hello } misses the return statement. class Greeting extends Component { render() { return 'Hello'; } } uses class syntax but returns a string, not JSX.

3. Final Answer:

   function Greeting() { return Hello; } -> Option A

4. Quick Check:

   Function component must return JSX with return [OK]

Hint: Function components need a return statement [OK]

Common Mistakes:

• Omitting return in arrow functions
• Assigning JSX directly to variables
• Returning strings instead of JSX

3. What will be displayed on the screen when this component is rendered?

function Welcome(props) {
  return Hi, {props.name}!;
}

// Usage:
<Welcome name="Anna" />

medium

A. Hi, {props.name}!

B. Hi, Anna!

C. Hi, name!

D. Error: props not defined

Solution

1. Step 1: Understand props usage

   The component receives a prop called 'name' and inserts it inside the Text element using curly braces.

2. Step 2: Substitute the prop value

   Since the component is used with name="Anna", the output will be 'Hi, Anna!'.

3. Final Answer:

   Hi, Anna! -> Option B

4. Quick Check:

   Props insert values inside JSX [OK]

Hint: Props values show inside curly braces in JSX [OK]

Common Mistakes:

• Treating props as plain text
• Forgetting curly braces for props
• Assuming props are undefined

4. Identify the error in this custom component code:

function Button() {
  return (
    <TouchableOpacity>
      <Text>Click me</Text>
    </TouchableOpacity>
  )
}

export default Button;

medium

A. Text component cannot be inside TouchableOpacity

B. Function must be named with uppercase first letter

C. Missing import of TouchableOpacity

D. Missing return statement

Solution

1. Step 1: Check component usage

   The component uses TouchableOpacity and Text, which must be imported from 'react-native'.

2. Step 2: Identify missing import

   The code snippet does not show imports, so the likely error is missing import of TouchableOpacity.

3. Final Answer:

   Missing import of TouchableOpacity -> Option C

4. Quick Check:

   All used components must be imported [OK]

Hint: Always import components you use [OK]

Common Mistakes:

• Forgetting to import React Native components
• Thinking Text can't be inside TouchableOpacity
• Confusing function naming rules

5. You want to create a reusable Card component that accepts a title and content as props and displays them inside styled View and Text components. Which of the following is the best way to define this component?

hard

A. function Card({ title, content }) {
return (

{title}
{content}

);
}

B. function Card(props) {
return (

{props.title}
{props.content}

);
}

C. function Card() {
return (

title
content

);
}

D. function Card({ title, content }) {
return (

{title}

);
}

Solution

1. Step 1: Check props usage and destructuring

   function Card({ title, content }) { return ( {title} {content} ); } uses destructuring to get title and content props, which is clear and concise.

2. Step 2: Verify UI structure and styling

   function Card({ title, content }) { return ( {title} {content} ); } wraps texts in a styled View with padding, border, and rounded corners, making it reusable and visually distinct.

3. Step 3: Compare with other options

   function Card(props) { return ( {props.title} {props.content} ); } lacks styling, while the option without props uses literal strings instead of props, and the option missing content display omits the content Text.

4. Final Answer:

   function Card({ title, content }) { return ( {title} {content} ); } -> Option A

5. Quick Check:

   Destructure props and add styles for reusable UI [OK]

Hint: Destructure props and add styles for reusable components [OK]

Common Mistakes:

• Not using props to customize content
• Forgetting to style reusable components
• Using literal strings instead of props