What is a component in React - Performance Impact

Performance: What is a component

MEDIUM IMPACT

This concept affects how quickly the page can render and update UI elements by managing reusable pieces of the interface.

Building UI with reusable pieces

React

function Title() { return <h1>Title</h1>; }
function Description() { return <p>Description</p>; }
function ClickButton() { return <button>Click me</button>; }
function App() {
  return (
    <div>
      <Title />
      <Description />
      <ClickButton />
    </div>
  );
}

Splitting UI into small components limits re-render scope and improves interaction speed.

📈 Performance GainOnly affected components re-render, reducing unnecessary work and improving INP.

Building UI with reusable pieces

React

function App() {
  return (
    <div>
      <h1>Title</h1>
      <p>Description</p>
      <button>Click me</button>
    </div>
  );
}

All UI elements are in one big component causing large re-renders and slower updates.

📉 Performance CostTriggers full component re-render on any state change, increasing INP.

Performance Comparison

Pattern	DOM Operations	Reflows	Paint Cost	Verdict
Single large component	Many DOM nodes updated at once	Multiple reflows per update	High paint cost due to full re-render	[X] Bad
Multiple small components	Only changed components update DOM nodes	Minimal reflows limited to changed parts	Lower paint cost due to partial updates	[OK] Good

Rendering Pipeline

React components are parsed, their JSX converted to virtual DOM, then React compares changes and updates the real DOM efficiently.

→Virtual DOM Diffing

→DOM Updates

→Paint

→Composite

⚠️ BottleneckDOM Updates stage is most expensive when many components re-render unnecessarily.

Core Web Vital Affected

INP

This concept affects how quickly the page can render and update UI elements by managing reusable pieces of the interface.

Optimization Tips

1Split UI into small, reusable components to limit re-render scope.

2Avoid putting too much UI in one component to prevent large re-renders.

3Use React memoization techniques to prevent unnecessary updates.

Performance Quiz - 3 Questions

Why is splitting UI into multiple React components better for performance?

A It bundles all code into one file, reducing network requests.

B It limits re-rendering to only changed components, improving responsiveness.

C It increases the number of DOM nodes, making rendering slower.

D It disables React's virtual DOM, speeding up updates.

DevTools: React DevTools and Performance panel

How to check: Use React DevTools to inspect component tree and see which components re-render on interaction. Use Performance panel to record and analyze rendering times.

What to look for: Look for unnecessary re-renders of large components and long scripting times that delay input responsiveness.

Practice

1. What is a React component?

easy

A. A reusable piece of UI built with a function or class

B. A special type of variable that stores data

C. A tool to style HTML elements

D. A command to run React apps

Solution

1. Step 1: Understand React components

   A React component is a building block of UI that can be reused multiple times.

2. Step 2: Identify the correct definition

   Only A reusable piece of UI built with a function or class describes a reusable UI piece built with a function or class.

3. Final Answer:

   A reusable piece of UI built with a function or class -> Option A

4. Quick Check:

   Component = reusable UI block [OK]

Hint: Components are reusable UI parts made with functions [OK]

Common Mistakes:

• Confusing components with variables
• Thinking components are styling tools
• Believing components are commands

2. Which of the following is the correct way to define a React functional component?

easy

A. const MyComponent = 'Hello';

B. function MyComponent() { return Hello; }

C. var MyComponent = Hello;

D. class MyComponent extends React.Component { }

Solution

1. Step 1: Review functional component syntax

   A functional component is a function returning JSX, like function MyComponent() { return
   Hello; }.

2. Step 2: Check each option

   function MyComponent() { return
   Hello
   ; } matches the correct syntax. var MyComponent =
   Hello
   ; assigns JSX directly to a variable, which is not a component. const MyComponent = 'Hello'; assigns a string, not JSX. class MyComponent extends React.Component { } is a class component, not functional.

3. Final Answer:

   function MyComponent() { return <div>Hello</div>; } -> Option B

4. Quick Check:

   Functional component = function returning JSX [OK]

Hint: Functional components are functions returning JSX [OK]

Common Mistakes:

• Assigning JSX directly to variables
• Using strings instead of JSX
• Confusing functional and class components

3. What will this React component render?

function Greeting() {
  return <h1>Hello, world!</h1>;
}

medium

A. Nothing, because the function is not called

B. An error because of missing parentheses

C. <h1>Hello, world!</h1> displayed on the page

D. A blank page with no content

Solution

1. Step 1: Understand component return value

   The component returns an <h1> element with text "Hello, world!" which React renders as HTML.

2. Step 2: Confirm rendering behavior

   When used in JSX, this component will display the heading on the page. No syntax errors exist.

3. Final Answer:

   <h1>Hello, world!</h1> displayed on the page -> Option C

4. Quick Check:

   Return JSX renders UI [OK]

Hint: Returned JSX shows on page when component is used [OK]

Common Mistakes:

• Thinking function must be called manually
• Assuming missing parentheses cause error here
• Believing component renders nothing by default

4. Identify the error in this React component:

function Welcome() {
  return <div>Welcome to React</div>
}

medium

A. Missing semicolon after return statement

B. JSX must be wrapped in parentheses if on multiple lines

C. Function name should start with lowercase letter

D. No error, component is correct

Solution

1. Step 1: Check JSX syntax

   The JSX is a single line and correctly returned without parentheses, which is valid.

2. Step 2: Verify naming and semicolon rules

   Function names for components should start uppercase, which is true. Semicolons are optional in JavaScript here.

3. Final Answer:

   No error, component is correct -> Option D

4. Quick Check:

   Valid JSX and naming means no error [OK]

Hint: JSX on one line doesn't need parentheses [OK]

Common Mistakes:

• Thinking semicolons are mandatory after return
• Believing lowercase function names are required
• Assuming JSX always needs parentheses

5. You want to create a reusable button component that shows different text based on a property. Which is the best way to do this?

hard

A. function Button(props) { return {props.text}; }

B. function Button() { return Click me; }

C. const Button = Click me;

D. function Button(text) { return {text}; }

Solution

1. Step 1: Understand passing data to components

   Props are used to pass data to components. Using props.text allows dynamic button text.

2. Step 2: Evaluate each option

   function Button(props) { return {props.text}; } correctly uses props to show different text. function Button() { return Click me; } always shows fixed text. const Button = Click me; is not a function component. function Button(text) { return {text}; } incorrectly uses a parameter instead of props object.

3. Final Answer:

   function Button(props) { return <button>{props.text}</button>; } -> Option A

4. Quick Check:

   Use props for dynamic content [OK]

Hint: Use props to pass dynamic text to components [OK]

Common Mistakes:

• Hardcoding text inside component
• Not using props object for parameters
• Assigning JSX directly to variables