React: useState

To use useState, first import it with import React, { useState } from 'react';

Basic Syntax

const [varName, setVar] = useState(initial value);

Why do we write like this? Because useState as a function returns two variables. the first one is the var_name, and the second one is the function to change the variable.

"Advanced" Usage

we can get the initial value from a function:

const [varName, setVar] = useState(() => {
  return 1+2-3+4;
});

we can set the indirectly customize the setVar function. For this example, we made the setVar function can only accept the values which are smaller than 10.

const [var, dummy] = useState(0);
const setVar = (value) => {
  if (value > 10) {
    dummy(10);
    return;
  }
  dummy(value);
}

An Example

import React, { useState } from 'react';
import './App.css';


function MyButton({ updateFunc }) {
    return (
    <div>
        <button onClick={updateFunc} className='button'> Click Me</button>
    </div>
    );
}

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

    return (
    <div className="App">
        <p>This is a counter</p>
        <p>Currently the count is {count}</p>
        <MyButton updateFunc={() => { setCount(count + 1); }}/>
        <MyButton updateFunc={() => { setCount(count - 1); }}/>
    </div>
    );
}

export default App;