A React class component receives values from its parent as props and reads them with this.props. For example, a parent can render <Greeting name="Taylor" />, and the class can display that value with this.props.name. Class components remain supported, though React recommends function components for new code.
What is a React class component?
A class component is a JavaScript class that extends React’s Component base class. It must define a render() method that returns the UI. Other methods, such as lifecycle methods, are optional.
In render(), a class can read its props, state, and context. The example below imports Component from React and displays the name prop:
import { Component } from 'react';
class Greeting extends Component {
render() {
return <h1>Hello, {this.props.name}!</h1>;
}
}The parent supplies the value in JSX:
<Greeting name="Taylor" />React passes that attribute to the component as a prop. Inside the class, this.props.name is how the component reads the passed value, so this example renders Hello, Taylor!. See React’s guide to passing props and the Component API reference.
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How to pass props to a component
Write the prop as an attribute on the component in the parent’s JSX. A quoted value such as name="Taylor" passes a string. Props can also carry other JavaScript values, including objects, arrays, and functions.
<Greeting
name="Taylor"
details={{ city: 'Toronto' }}
tags={['welcome', 'new']}
onSelect={() => console.log('Selected')}
/>For values other than strings, use braces to put a JavaScript expression in JSX. The receiving class reads them as properties of this.props, such as this.props.details or this.props.onSelect.
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How to read props from a component
Read a prop by its name from this.props, commonly inside render(). For example, this.props.name accesses the value provided by name in the parent’s JSX. Props are inputs from outside the component; they are not the component’s own state.
React treats props as immutable inputs. Do not assign to this.props.name or mutate an object or array received through a prop. If the component needs a transformed value, calculate it without changing the original, or create a copy. React describes props and state as immutable snapshots for a render in its Rules of React and guide to keeping components pure.
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How to specify default values for props
A class can declare static defaultProps for props that are missing or explicitly set to undefined:
class Greeting extends Component {
static defaultProps = {
name: 'friend',
};
render() {
return <h1>Hello, {this.props.name}!</h1>;
}
}With this default, <Greeting /> renders “Hello, friend!” and an undefined value also falls back to the default. An explicit null is not replaced, so <Greeting name={null} /> leaves this.props.name as null. This behavior is documented in the Component reference.
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How props change over time—and how they differ from state
When a parent renders the component again with different props, React supplies the new inputs and the class renders from them. Avoid copying props into state just to use them: derived state can make components harder to reason about. Prefer calculating a value from current props during rendering. If a value is meant to be controlled by the parent, keep it as a prop; if the component must manage its own changing data, use state for that distinct purpose.
Props come from the parent; state belongs to the component. In a class, state is held in this.state and updated with this.setState(), not by directly mutating the state object. Both props and state should be treated as immutable snapshots for a render.
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Keep render pure
The render() method should calculate and return UI from the component’s inputs rather than perform side effects. Do not put subscriptions, browser API work, or other side-effecting operations in it. React may call render at different times or more than once in development Strict Mode, so one call does not necessarily correspond to one committed display. Put class-based effects in the appropriate lifecycle methods instead; React’s Component reference explains the class lifecycle APIs.
Class components and function components today
Class components still work, but React recommends function components for new code. The key differences relevant to props and component behavior are:
| Concern | Class component | Function component |
|---|---|---|
| Reading inputs | Read values through this.props. |
Receive props as a function parameter. |
| Local state | Use this.state and update it with this.setState(). |
Use state Hooks such as useState. |
| Hooks | Hooks cannot be called inside a class component. | Hooks are available in function components. |
| Lifecycle behavior | Use class lifecycle methods. | Use Hooks such as useEffect for many common effect patterns. |
This is current guidance, not a removal notice: classes remain supported. Some class APIs do not have a direct function-component equivalent. For a new component that needs Hooks, use a function component rather than trying to call Hooks from a class.
React 19 changes that do not remove props
React 19 removed class static propTypes, legacy context APIs, and this.refs, according to React’s legacy API reference. That version-specific change does not remove ordinary props or access through this.props. React recommends a type system such as TypeScript for prop typing.
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Common mistakes to avoid
- Changing a prop: Treat it as an input. Compute a new value or copy a collection instead of modifying the value received from the parent.
- Doing work in render: Keep
render()focused on returning UI; put side effects in lifecycle methods. - Copying props into state automatically: Derive display values from props unless the component genuinely needs a separate state value.
- Using outdated lifecycle names:
componentWillMount,componentWillReceiveProps, andcomponentWillUpdateare deprecated. TheirUNSAFE_-prefixed forms exist for historical compatibility and should not be used in new code. - Calling Hooks in a class: Hooks are not supported in class components; migrate the component to a function if it needs them.
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