Is this.setState synchronous?
No. this.setState is asynchronous and batched. Reading this.state right after calling it shows the old value. If the new state depends on the previous, pass a function to this.setState to get the correct previous value.
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More FAQs in Common Class Component Mistakes Developers Still Make
Because in JavaScript, this is determined by how a function is called, not where it is defined. When you pass a class method as a callback, it loses its binding and this becomes undefined. You must bind in the constructor or use arrow function class fields.
Because React tracks state changes through this.setState. Mutating this.state directly does not trigger a re-render, so the UI does not update. Always use this.setState to update state in class components.
To clean up anything you set up in componentDidMount, like subscriptions, timers, and listeners. Forgetting cleanup causes memory leaks and warnings about updating state on an unmounted component.
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