setState hook function taking prevSate code example

Example 1: when to use previous state in useState

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);

Example 2: simple usestate example

// First: import useState. It's a named export from 'react'
// Or we could skip this step, and write React.useState
import React, { useState } from 'react';
import ReactDOM from 'react-dom';

// This component expects 2 props:
//   text - the text to display
//   maxLength - how many characters to show before "read more"
function LessText({ text, maxLength }) {
  // Create a piece of state, and initialize it to `true`
  // `hidden` will hold the current value of the state,
  // and `setHidden` will let us change it
  const [hidden, setHidden] = useState(true);

  // If the text is short enough, just render it
  if (text.length <= maxLength) {
    return <span>{text}</span>;
  }

  // Render the text (shortened or full-length) followed by
  // a link to expand/collapse it.
  // When a link is clicked, update the value of `hidden`,
  // which will trigger a re-render
  return (
    <span>
      {hidden ? `${text.substr(0, maxLength).trim()} ...` : text}
      {hidden ? (
        <a onClick={() => setHidden(false)}> read more</a>
      ) : (
        <a onClick={() => setHidden(true)}> read less</a>
      )}
    </span>
  );
}

ReactDOM.render(
  <LessText
    text={`Focused, hard work is the real key
      to success. Keep your eyes on the goal, 
      and just keep taking the next step 
      towards completing it.`}
    maxLength={35}
  />,
  document.querySelector('#root')
);