Do these 3 things before closing this tab:
1Fix the driver behind crashes, sound loss and screen glitches2Repair Windows errors before they cause bigger problems3Scan for outdated or missing drivers - takes under a minuteIn a React class component, initialize local state as an object, read it with this.state, and request changes with this.setState(). Don’t assign to state directly: React queues updates, so code immediately after setState may still see the old value. Classes remain supported, although React recommends function components for new code.
How to initialize and read class state
A class component extends Component and implements render(). With modern class-field syntax, declare state on the class:
import { Component } from 'react';
class Counter extends Component {
state = { age: 42 };
render() {
return <p>>You are {this.state.age}.</p>;
}
}Use this.state.age in the component to read the current value. State is component-specific data that React uses when rendering.
Using a constructor instead
You can initialize state in a constructor when needed:
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class Counter extends Component {
constructor(props) {
super(props);
this.state = { age: 42 };
}
render() {
return <p>>You are {this.state.age}.</p>;
}
}Call super(props) before using the instance. The constructor is the place to assign this.state directly; do not call setState there. In modern code, a class field is often simpler. See React’s Component reference.
How to update state without mutating it
Never change a state property by assigning to it, such as this.state.age = 43. State should be treated as immutable; use this.setState() to request an update.
Set a value that does not depend on current state
this.setState({ name: newName });For class components, an object passed to setState is shallowly merged into the current state object. Unmentioned top-level properties are retained.
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Calculate from pending state
If a new value depends on the previous one, pass an updater function rather than calculating from a possibly stale read:
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age: prevState.age + 1
}));React applies updater functions to pending state when processing the update. Keep an updater pure: calculate and return the next state rather than performing side effects inside it. The updater form is especially useful when updates may be queued close together.
Update a nested object by copying changed levels
The merge is shallow, not recursive. For nested state, copy the object levels you change:
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this.setState(prevState => ({
profile: {
...prevState.profile,
name: newName
}
}));Here the new top-level state is merged, while profile is replaced with a copied object containing the updated name and the other existing profile fields. Mutating prevState.profile in place would violate the immutability rule.
Why state may look unchanged right after setState
setState queues a change; it is not a promise that this.state changes synchronously on the next line. The handler that called it continues with the state snapshot it started with, and React makes the new state available starting with a subsequent render. React may batch updates and render them together.
this.setState({ name: 'Ada' });
console.log(this.state.name); // May still be the previous valueIf work must happen after the update has been committed, use componentDidUpdate or the optional callback argument to setState. Don’t force synchronous rendering as a routine fix; React notes that synchronous flushing is an unusual option that can hurt performance. The details are in the React Component reference.
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How state persists or resets when props change
React associates state with a component’s identity and position in the rendered UI tree, not merely with a line of source code. When the same component remains at the same position, its state generally persists. Changing the component type, its position, or its key can change that identity and reset or move the state associated with the UI.
Reset a subtree by changing its key
If a changed prop means the whole child subtree should start fresh, give that child a key based on the identity that should define its state. When the key changes, React treats it as a different component and creates fresh state. Use this deliberately: it resets all state below that keyed component, not just one field.
React explains this identity model in Preserving and Resetting State and State: A Component’s Memory.
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Avoid copying props into state by default
Copying a prop into state can leave two values that must be kept in sync. If a value can be calculated from current props or state, calculate it during rendering instead. If you need to perform a side effect after a prop changes, compare the current and previous props in componentDidUpdate before acting; triggering another update without a guard can create an update loop. For a value that should be controlled by a parent, consider making the child controlled rather than maintaining a duplicate.
What belongs in render and lifecycle methods
render() should be pure: derive the UI from props, state, and context, without changing state, accessing browser APIs, or performing other side effects. React can render more than once or discard a render attempt, so do not make application behavior depend on a particular render call being committed. Handle user-triggered effects in event handlers and lifecycle-related work in the appropriate lifecycle methods. See React’s rules for pure components and Hooks.
componentDidMountis commonly used for setup after mounting.componentDidUpdateis commonly used to respond to committed updates; compare previous props or state before triggering another update.componentWillUnmountis commonly used to clean up work started by the component.
These lifecycle methods overlap with many effect-and-cleanup use cases in function components, but a migration needs to preserve behavior rather than mechanically rename methods. React’s Component reference describes the class lifecycle APIs.
Should you use a class or Hooks?
React continues to support class components, but recommends functions instead of classes for new components. Hooks such as useState cannot be called inside a class component; they are used in function components.
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Scan for outdated or missing drivers - takes under a minuteDriver Scan →Repair Windows errors before they cause bigger problemsFix Now →| Concern | Class component | Function component |
|---|---|---|
| State API | this.state and this.setState |
useState and its setter |
| Object update behavior | An object passed to setState shallowly merges into existing state |
A Hook setter replaces that state value; explicitly merge objects when that is the intended behavior |
| Work around updates and cleanup | Lifecycle methods such as componentDidMount, componentDidUpdate, and componentWillUnmount |
Hooks such as useEffect for effects and cleanup, organized around the behavior being managed |
| React’s guidance | Supported; useful in existing code | Recommended for new components |
For a new component, follow the current React guidance and use a function component. When maintaining a class, its state and update semantics remain supported. A migration should account for the difference between class merging and Hook replacement, as well as lifecycle behavior. The official references are Component and useState.
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