Reusable UI components in React - Performance & Optimization

Performance: Reusable UI components

MEDIUM IMPACT

Reusable UI components affect rendering speed and bundle size by controlling how often components re-render and how much code is reused.

Creating a button component used multiple times with different labels

React

const handleSave = () => save();
const handleCancel = () => cancel();

function Button({ label, onClick }) {
  return <button onClick={onClick}>{label}</button>;
}

function App() {
  return (
    <>
      <Button label="Save" onClick={handleSave} />
      <Button label="Cancel" onClick={handleCancel} />
    </>
  );
}

Passing stable function references avoids unnecessary re-renders and reduces layout recalculations.

📈 Performance GainReduces re-renders to only when props actually change

Creating a button component used multiple times with different labels

React

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

// Used inline with anonymous functions causing re-renders
function App() {
  return (
    <>
      <Button label="Save" onClick={() => save()} />
      <Button label="Cancel" onClick={() => cancel()} />
    </>
  );
}

Inline anonymous functions cause new function instances on every render, triggering unnecessary re-renders of Button components.

📉 Performance CostTriggers multiple re-renders and layout recalculations on each parent render

Performance Comparison

Pattern	DOM Operations	Reflows	Paint Cost	Verdict
Inline anonymous functions in props	High (new functions cause re-renders)	Multiple per parent render	High due to repeated layout	[X] Bad
Stable function references with memoized components	Low (re-renders only on prop change)	Single or minimal	Low due to fewer layout recalculations	[OK] Good

Rendering Pipeline

Reusable components affect the rendering pipeline by controlling how often React reconciles and updates the DOM nodes, impacting style calculation, layout, and paint.

→Reconciliation

→Layout

→Paint

⚠️ BottleneckReconciliation stage is most expensive when components re-render unnecessarily.

Core Web Vital Affected

INP

Reusable UI components affect rendering speed and bundle size by controlling how often components re-render and how much code is reused.

Optimization Tips

1Avoid inline functions in props to prevent unnecessary re-renders.

2Use React.memo to memoize components that receive stable props.

3Keep component props stable and simple to maximize memoization benefits.

Performance Quiz - 3 Questions

What is a common cause of unnecessary re-renders in reusable React components?

A Using React.memo to wrap components

B Using keys on list items

C Passing inline anonymous functions as props

D Splitting components into smaller pieces

DevTools: React DevTools Profiler

How to check: Record a profiling session while interacting with the UI, then inspect component render counts and durations.

What to look for: Look for components that re-render frequently without prop changes indicating unnecessary work.

Practice

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

easy

A. They automatically fix bugs

B. They make the app run faster

C. They save time and keep the app consistent

D. They replace the need for CSS styling

Solution

1. Step 1: Understand the purpose of reusable components

   Reusable components are designed to be used multiple times, reducing repeated code.

2. Step 2: Identify the main benefit

   Using reusable components saves development time and keeps the app's look and behavior consistent.

3. Final Answer:

   They save time and keep the app consistent -> Option C

4. Quick Check:

   Reusable components = save time + consistency [OK]

Hint: Reusable means use again to save time and stay consistent [OK]

Common Mistakes:

• Thinking reusable components speed up app runtime
• Believing they fix bugs automatically
• Confusing styling with component reuse

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

easy

A.

B.

C.

D.

Solution

1. Step 1: Recall JSX prop syntax

   Props are passed as attributes with name=value pairs, using quotes for strings.

2. Step 2: Check each option

   uses correct JSX syntax: title="Hello". Others have syntax errors.

3. Final Answer:

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

4. Quick Check:

   Props in JSX use name="value" [OK]

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

Common Mistakes:

• Using curly braces without variable (Option B)
• Adding extra quotes around prop name (Option C)
• Using colon instead of equals (Option A)

3. Given this React component:

function Button({ label }) {
  return <button>{label}</button>;
}

// Usage:
<Button label="Click me" />

What will be rendered on the page?

medium

A. label

B. Nothing will render

C. {label}

D. Click me

Solution

1. Step 1: Understand prop usage in the component

   The component receives label as a prop and inserts it inside the button element.

2. Step 2: Check the usage

   The component is used with label="Click me", so the button shows "Click me" text.

3. Final Answer:

   <button>Click me</button> -> Option D

4. Quick Check:

   Prop label = "Click me" shown inside button [OK]

Hint: Props passed show as content inside component tags [OK]

Common Mistakes:

• Thinking the literal word 'label' appears
• Confusing JSX expression with string
• Assuming component renders nothing without explicit return

4. What is wrong with this reusable React component code?

function Alert(props) {
  return <div>{message}</div>;
}

// Usage:
<Alert message="Warning!" />

medium

A. The variable 'message' is not defined inside the component

B. The component does not receive props correctly

C. The usage of the component is incorrect

D. The component should return a button instead of a div

Solution

1. Step 1: Check how props are used inside the component

   The component uses {message} but does not extract it from props.

2. Step 2: Identify the error

   Since message is not defined or extracted, this causes a reference error.

3. Final Answer:

   The variable 'message' is not defined inside the component -> Option A

4. Quick Check:

   Props must be accessed via props or destructuring [OK]

Hint: Always get props from function parameter before use [OK]

Common Mistakes:

• Using prop names directly without props or destructuring
• Assuming usage syntax is wrong
• Thinking component must return button

5. You want to create a reusable Input component that can show a label and accept user input. Which approach correctly allows customizing the label text and input placeholder using props?

hard

A. function Input() {
return (
<label>
label
<input placeholder="placeholder" />
</label>
);
}

B. function Input({ label, placeholder }) {
return (
<label>
{label}
<input placeholder={placeholder} />
</label>
);
}

C. function Input(props) {
return (
<label>
props.label
<input placeholder="props.placeholder" />
</label>
);
}

D. function Input({ label, placeholder }) {
return (
<label>
<input placeholder={label} />
{placeholder}
</label>
);
}

Solution

1. Step 1: Check prop usage for label and placeholder

   Props should be destructured and used inside JSX expressions with curly braces.

2. Step 2: Verify correct placement of label and placeholder

   function Input({ label, placeholder }) { return ( <label> {label} <input placeholder={placeholder} /> </label> ); } correctly places label text and uses placeholder prop inside input element.

3. Final Answer:

   function Input({ label, placeholder }) { return ( <label> {label} <input placeholder={placeholder} /> </label> ); } -> Option B

4. Quick Check:

   Destructure props and use inside JSX with curly braces [OK]

Hint: Destructure props and use inside JSX with curly braces [OK]

Common Mistakes:

• Using prop names as plain text instead of expressions
• Putting placeholder text in wrong prop
• Mixing label and placeholder positions