Why does calling setCount(count + 1) twice not add two?
Because state updates are batched and count is the same stale value for both calls. Use setCount(prev => prev + 1) so each update receives the correct previous value, even when batched together.
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More FAQs in Common useState Mistakes That Break Your React App
Usually because you mutated the state variable directly or mutated an object in place instead of creating a new one. React compares references, so always use the setter and replace objects and arrays with new copies.
Because React tracks hooks by the order they are called. If a hook is inside a condition, the order can change between renders, which breaks React's internal tracking and causes bugs. Always call hooks at the top level.
Pass a value for simple initial state. For expensive initial computations, pass a function that returns the value so it only runs once on the first render instead of on every render.
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