PrevState

PrevState is a conventional name for a parameter that represents the previous value of a component's state in React programming. It appears in updater functions passed to setState in class components and to the setter function returned by useState in function components. The concept helps ensure that state updates are computed based on the most recent state, which is important when updates are batched or triggered asynchronously.

In class components, setState can receive a function instead of an object: the function is called with (prevState, props) and should return an object representing the state changes. prevState is a snapshot of the state before the update. The returned object is then merged into the current state. This pattern prevents errors when multiple updates occur in quick succession.

In function components using useState, the setter can also accept a functional updater: setValue(prevValue => newValue). Here, prevValue serves the same purpose as prevState, providing the latest state value needed to compute the next one. This approach similarly guards against stale closures and batched updates.

Best practices emphasize treating prevState as read-only. Do not mutate it; instead, produce and return a new state object or value. When updating nested state, shallow-copy the relevant portions (for example, spreading prevState and then overwriting the needed fields) to maintain immutability.

PrevState, as a pattern, is a fundamental aspect of writing predictable state updates in React. It is not a distinct feature or API but a naming convention for the updater function’s parameter that carries the prior state.