Reusable components in React - Step-by-Step Execution

Concept Flow - Reusable components

Create Component Function

↓

Use Props to Customize

↓

Render JSX Output

↓

Use Component Multiple Times

↓

Each Instance Shows Customized Output

This flow shows how a reusable React component is created, customized with props, rendered, and used multiple times with different data.

Execution Sample

React

function Greeting(props) {
  return <h1>Hello, {props.name}!</h1>;
}

ReactDOM.render(
  <>
    <Greeting name="Alice" />
    <Greeting name="Bob" />
  </>,
  document.getElementById('root')
);

This code defines a reusable Greeting component that shows a hello message with a name, then uses it twice with different names.

Execution Table

Step	Action	Props Received	Rendered Output	Notes
1	Call Greeting with name='Alice'	{ name: 'Alice' }	<h1>Hello, Alice!</h1>	First instance renders with Alice
2	Call Greeting with name='Bob'	{ name: 'Bob' }	<h1>Hello, Bob!</h1>	Second instance renders with Bob
3	No more calls	N/A	N/A	Rendering stops after all instances

💡 All component instances rendered with their respective props, no more calls to render.

Variable Tracker

Variable	Start	After 1	After 2	Final
props.name	undefined	Alice	Bob	Bob

Key Moments - 2 Insights

Why does each Greeting show a different name even though the component code is the same?

Because each call to Greeting passes different props.name values, as shown in execution_table steps 1 and 2, customizing the output.

What happens if we call Greeting without a name prop?

The props.name would be undefined, so the output would show 'Hello, !' or could be handled with default props, but this example does not show that.

Visual Quiz - 3 Questions

Look at the execution table, what is the rendered output at step 2?

A <h1>Hello, Alice!</h1>

B <h1>Hello, Bob!</h1>

C <h1>Hello, !</h1>

D <h1>Hello, React!</h1>

💡 Hint

Check the 'Rendered Output' column at step 2 in the execution_table.

At which step does the component receive props.name equal to 'Alice'?

A Step 3

B Step 2

C Step 1

D Never

💡 Hint

Look at the 'Props Received' column in the execution_table.

If we add a third call <Greeting name="Charlie" />, what will happen in the variable_tracker after that?

A props.name will have values: undefined, Alice, Bob, Charlie

B props.name will stay the same as before

C props.name will only show Charlie

D props.name will be undefined

💡 Hint

Variable tracker shows props.name values after each call, so adding a call adds a new value.

Concept Snapshot

Reusable components in React:
- Define a function that returns JSX
- Use props to customize output
- Call component multiple times with different props
- Each instance renders independently
- Helps avoid repeating code

Full Transcript

This visual execution shows how reusable components work in React. We start by defining a component function called Greeting that takes props and returns a heading with a hello message including props.name. Then we use this component twice with different names: Alice and Bob. Each time React calls the Greeting function with the given props and renders the customized output. The execution table tracks each call, the props received, and the rendered output. The variable tracker shows how props.name changes with each call. Key moments clarify why the same component code shows different outputs and what happens if props are missing. The quiz tests understanding of props and rendering steps. This helps beginners see how reusable components save effort and keep code clean.

Practice

1. What is the main benefit of using reusable components in React?

easy

A. They make your app run faster by default

B. They let you write less code and keep your app organized

C. They automatically fix bugs in your code

D. They prevent users from clicking buttons

Solution

1. Step 1: Understand the purpose of reusable components

   Reusable components are designed to be used multiple times to avoid repeating code.

2. Step 2: Identify the main benefit

   Using reusable components helps keep the app organized and reduces the amount of code you write.

3. Final Answer:

   They let you write less code and keep your app organized -> Option B

4. Quick Check:

   Reusable components = less code + organized app [OK]

Hint: Reusable means use again to save code and stay organized [OK]

Common Mistakes:

• Thinking reusable components speed up the app automatically
• Believing they fix bugs without coding
• Confusing reusable components with user interaction controls

2. Which of the following is the correct way to pass a prop named title with value "Hello" to a reusable component Card?

easy

A. <Card title=Hello />

B. <Card title={Hello} />

C. <Card "Hello"=title />

D. <Card title="Hello" />

Solution

1. Step 1: Recall JSX prop syntax

   Props are passed as attributes with name and value inside quotes or curly braces for expressions.

2. Step 2: Check each option

   <Card title="Hello" /> uses correct syntax: title="Hello". <Card title={Hello} /> misses quotes around string. <Card title=Hello /> lacks quotes or braces. <Card "Hello"=title /> has incorrect attribute format.

3. Final Answer:

   <Card title="Hello" /> -> Option D

4. Quick Check:

   Props need name="value" syntax [OK]

Hint: Props look like HTML attributes with quotes [OK]

Common Mistakes:

• Forgetting quotes around string prop values
• Using curly braces without variables
• Swapping prop name and value order

3. What will be the output of this React component usage?

function Greeting(props) {
  return <h1>Hello, {props.name}!</h1>;
}

<Greeting name="Alice" />
<Greeting name="Bob" />

medium

A. Hello, Alice!Hello, Bob!

B. Hello, Alice!

C. Hello, Bob!

D. Error: Cannot render two components like this

Solution

1. Step 1: Understand component usage

   Each <Greeting /> renders an <h1> with the passed name prop.

2. Step 2: Check the two usages

   First renders Hello, Alice!, second renders Hello, Bob!. Both outputs appear in order.

3. Final Answer:

   <h1>Hello, Alice!</h1><h1>Hello, Bob!</h1> -> Option A

4. Quick Check:

   Two components with different props render two greetings [OK]

Hint: Each component instance renders separately with its props [OK]

Common Mistakes:

• Thinking only one component renders
• Believing React errors on multiple components
• Confusing props with state causing wrong output

4. Identify the error in this reusable component code:

function Button(props) {
  return <button>Click {props.label}</button>
}

// Usage
<Button label=Submit />

medium

A. Button component must return a div, not button

B. Props cannot be used inside JSX

C. Missing quotes around the prop value in usage

D. Component name must start with lowercase

Solution

1. Step 1: Check prop usage syntax

   In JSX, string prop values must be in quotes or curly braces.

2. Step 2: Identify the error

   The usage <Button label=Submit /> lacks quotes around "Submit" causing syntax error.

3. Final Answer:

   Missing quotes around the prop value in usage -> Option C

4. Quick Check:

   String props need quotes [OK]

Hint: String props always need quotes in JSX [OK]

Common Mistakes:

• Thinking button must be wrapped in div
• Believing props can't be used inside JSX
• Using lowercase component names

5. You want to create a reusable List component that shows items with a custom renderItem prop function. Which approach correctly uses renderItem inside the component?

hard

A. function List(props) {
return <ul>{
props.items.map(item => <li>{props.renderItem(item)}</li>)
}</ul>;
}

B. function List(props) {
return <ul>{
props.items.map(props.renderItem(item))
}</ul>;
}

C. function List(props) {
return <ul>{
props.renderItem(props.items)
}</ul>;
}

D. function List(props) {
return <ul>{
props.items.forEach(item => props.renderItem(item))
}</ul>;
}

Solution

1. Step 1: Understand how to use renderItem with map

   To render a list, use map to transform each item into JSX, calling renderItem for each.

2. Step 2: Check each option

   function List(props) {
     return <ul>{
       props.items.map(item => <li>{props.renderItem(item)}</li>)
     }</ul>;
   }

   correctly maps items and calls renderItem inside <li>.

   function List(props) {
     return <ul>{
       props.items.map(props.renderItem(item))
     }</ul>;
   }

   calls renderItem incorrectly inside map.

   function List(props) {
     return <ul>{
       props.renderItem(props.items)
     }</ul>;
   }

   calls renderItem once with whole array.

   function List(props) {
     return <ul>{
       props.items.forEach(item => props.renderItem(item))
     }</ul>;
   }

   uses forEach which returns undefined, so no output.

3. Final Answer:

   Option A uses map and renderItem correctly inside list -> Option A

4. Quick Check:

   Use map with renderItem for lists [OK]

Hint: Use map with renderItem inside JSX to render lists [OK]

Common Mistakes:

• Using forEach instead of map for rendering
• Calling renderItem outside map or with wrong argument
• Passing whole array to renderItem instead of single item