What are props in React - Performance Impact

Performance: What are props

MEDIUM IMPACT

Props affect how components receive data and can impact rendering performance when changed frequently or deeply nested.

Passing data to child components in React

React

function Parent() {
  const data = { name: 'Alice', age: 30 };
  return <Child info={data} />;
}

function Child({ info }) {
  return <div>{info.name}</div>;
}

Passing objects directly avoids unnecessary serialization and parsing.

📈 Performance GainReduces render blocking and improves interaction responsiveness.

Passing data to child components in React

React

function Parent() {
  const data = { name: 'Alice', age: 30 };
  return <Child info={JSON.stringify(data)} />;
}

function Child({ info }) {
  const parsed = JSON.parse(info);
  return <div>{parsed.name}</div>;
}

Serializing and parsing props causes extra work and blocks rendering.

📉 Performance CostBlocks rendering for extra milliseconds due to JSON operations on each render.

Performance Comparison

Pattern	DOM Operations	Reflows	Paint Cost	Verdict
Passing serialized props	Extra JS parsing	Triggers re-render	Higher paint cost	[X] Bad
Passing direct props	Minimal JS work	Only necessary re-render	Lower paint cost	[OK] Good

Rendering Pipeline

Props flow from parent to child components triggering React's render process. Changes in props cause React to re-render affected components and update the DOM.

→JavaScript Execution

→Virtual DOM Diffing

→DOM Update

→Paint

⚠️ BottleneckVirtual DOM Diffing and DOM Update when props change frequently or deeply.

Core Web Vital Affected

INP

Props affect how components receive data and can impact rendering performance when changed frequently or deeply nested.

Optimization Tips

1Avoid creating new object or array props on every render.

2Pass only the data child components need to minimize re-renders.

3Use React.memo or useMemo to prevent unnecessary updates.

Performance Quiz - 3 Questions

What happens if you pass new object literals as props on every render?

A It improves performance by caching props automatically.

B It causes React to re-render child components even if data is unchanged.

C It prevents any re-rendering of child components.

D It reduces JavaScript execution time.

DevTools: React DevTools and Performance panel

How to check: Use React DevTools to inspect component props and check if components re-render unnecessarily. Use Performance panel to record and analyze render times.

What to look for: Look for frequent re-renders caused by prop changes and long scripting times blocking interaction.

Practice

1. What are props in React?

easy

A. Data passed from a parent component to a child component

B. A way to store component state internally

C. Functions that update the component's state

D. A method to style components

Solution

1. Step 1: Understand the role of props

   Props are used to send data from a parent component down to a child component in React.

2. Step 2: Differentiate props from state and methods

   Props are not for storing state or styling; they are inputs like function arguments.

3. Final Answer:

   Data passed from a parent component to a child component -> Option A

4. Quick Check:

   Props = Parent to child data [OK]

Hint: Props are like function arguments passed down [OK]

Common Mistakes:

• Confusing props with state
• Thinking props change inside the child
• Believing props are used for styling

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

easy

A.

B.

C.

D.

Solution

1. Step 1: Recognize JSX prop syntax

   Props are passed as attributes with key-value pairs inside JSX tags, using quotes for strings.

2. Step 2: Check each option's syntax

   correctly uses title="Hello". misses the key, C uses unquoted value, D misses quotes.

3. Final Answer:

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

4. Quick Check:

   Props syntax = key="value" [OK]

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

Common Mistakes:

• Omitting the prop name
• Not quoting string values
• Using curly braces without variables

3. Given this component:

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

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

What will be rendered on the screen?

medium

A. Hello, props.name!

B. Hello, !

C. Hello, Alice!

D. Error: props is undefined

Solution

1. Step 1: Identify how props are used in the component

   The component uses props.name inside JSX to display the passed name.

2. Step 2: Check the usage of the component

   The component is called with name="Alice", so props.name equals "Alice".

3. Final Answer:

   Hello, Alice! -> Option C

4. Quick Check:

   props.name = Alice [OK]

Hint: Props values appear where you use {props.key} [OK]

Common Mistakes:

• Printing the literal string 'props.name'
• Forgetting to pass the prop
• Assuming props is undefined

4. What is wrong with this React component usage?

function Message(props) {
  return <p>{props.text}</p>;
}

// Usage:
<Message text=Hello />

medium

A. The function should return a string, not JSX

B. The prop value Hello should be in quotes

C. Props cannot be used inside <p> tags

D. The component name should be lowercase

Solution

1. Step 1: Check the prop syntax in usage

   The prop text=Hello is missing quotes around the string value.

2. Step 2: Recall JSX rules for string props

   String props must be quoted like text="Hello" or wrapped in curly braces if a variable.

3. Final Answer:

   The prop value Hello should be in quotes -> Option B

4. Quick Check:

   String props need quotes [OK]

Hint: Always quote string props in JSX [OK]

Common Mistakes:

• Leaving string props unquoted
• Using lowercase component names
• Misunderstanding JSX return types

5. You want to create a reusable Button component that shows different labels based on props. Which approach correctly uses props to achieve this?

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

// Usage:
<Button label="Save" />
<Button label="Cancel" />

hard

A. This approach works because props.label changes the button text dynamically

B. This approach is wrong because props cannot be used inside button tags

C. You must use state instead of props to change button labels

D. Button labels must be hardcoded inside the component

Solution

1. Step 1: Understand props usage for dynamic content

   Props allow passing different values to the same component to show varied content.

2. Step 2: Verify the example usage

   The Button component uses props.label inside the button, and different labels are passed in usage.

3. Final Answer:

   This approach works because props.label changes the button text dynamically -> Option A

4. Quick Check:

   Props enable reusable components [OK]

Hint: Use props to customize component content easily [OK]

Common Mistakes:

• Thinking props can't change displayed text
• Confusing props with state for static labels
• Hardcoding values inside components