Q1. What is setState in React?
setState is a method used in class components to update the component's state. It schedules an update and tells React that this component and its children need to re-render with the new state. setState can accept an object or a function.
Q2. Is setState synchronous or asynchronous?
setState is asynchronous. React may batch multiple setState calls for performance reasons. If you need to execute code after the state has been updated, you can pass a callback as the second argument to setState. In functional components, useEffect can be used to react to state changes.
Q3. How do you use the functional form of setState?
When the new state depends on the previous state, you should use the functional form: this.setState((prevState, props) => ({ count: prevState.count + 1 })). This ensures you get the most up-to-date state, especially when multiple updates are batched.
Q4. What happens when you call setState with the same value?
If the new value is the same as the current state (by shallow comparison), React may skip re-rendering the component and its children. This optimization improves performance. However, in some cases, React might still re-render, but it's generally safe to rely on this behavior.
Q5. Can you call setState in the render method?
No, calling setState in render creates an infinite loop because it triggers a re-render, which calls render again. setState should only be called in event handlers, lifecycle methods, or hooks like useEffect.