How does React.js useState hook work under the hood?

Under the hood, the useState hook leverages closures and a data structure managed by React to provide stateful behavior in functional components. Here's a simplified explanation of how useState works internally:

1. Initialization: When you call useState(initialState) in a functional component, React creates a "state variable" associated with that component. It allocates memory for storing the current state value and a reference to a function that can update that state.

2. Component Instance: Each time the component is rendered, React creates a new instance of the component. This instance includes the state variables and their associated updater functions.

3. Persistent Identity: React uses the component's identity (i.e., the reference to the function component itself) to maintain the association between the component instance and its state variables. This ensures that the state remains consistent across re-renders of the same component.

4. State Updates: When you call the state updater function (e.g., setCount), React schedules a re-render of the component with the new state value. React internally updates the state variable associated with the component instance.

5. Batching Updates: React batches multiple state updates that occur within the same event loop iteration to improve performance. This means that if you call setCount multiple times within the same function execution, React will only perform one re-render with the final state value.

6. Lazy Initialization: React initializes the state lazily. This means that the initial state value provided to useState is only evaluated during the initial render of the component, not on every render. Subsequent renders reuse the previously initialized state value.

7. Preservation of State: React ensures that state updates are preserved between re-renders of the component. Even if the component function is called multiple times (e.g., due to re-renders triggered by parent component updates), the state remains consistent because it's associated with the component instance.

Overall, the useState hook provides a simple and efficient way to introduce stateful behavior in functional components by leveraging closures and React's internal mechanisms for managing component instances and state updates. This abstraction allows developers to write declarative code without worrying about the underlying implementation details.