useState hook, setState function. Accessing previous state value

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If the next state value depends on the previous value, as in this example of an increment button, it’s better to use the functional version of setState (setCount(prevCount => prevCount + 1)).

You can run into errors if you’re passing the setter function into a callback function, like an onChange or HTTP Request response handler.

As an explicit example, in the below page, if you click “Delayed Counter (basic)” and then click “Immediate Counter”, the count will only increment by 1. However, if you click “Delayed Counter (functional)”, followed by “Immediate Counter”, the count will eventually increment by 2.

import React, { useState } from "react";
import ReactDOM from "react-dom";

function Counter() {
  const [count, setCount] = useState(0);

  return (
    <div>
      <h1>{count}</h1>
      <button onClick={() => setTimeout(() => setCount(count + 1), 2000)}>
        Delayed Counter (basic)
      </button>
      <button onClick={() => setTimeout(() => setCount(x => x + 1), 2000)}>
        Delayed Counter (functional)
      </button>
      <button onClick={() => setCount(count + 1)}>Immediate Counter</button>
    </div>
  );
}

const rootElement = document.getElementById("root");
ReactDOM.render(<Counter />, rootElement);

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