React useState from scratch

useState.js

import React from 'react';
import ReactDOM from 'react-dom';

let callCount = -1
let states = []

function useState(initValue) {
  const id = ++callCount
  
  if (states[id]) return states[id]
  
  const setValue = (newValue) => {
    states[id][0] = newValue
    reRender()
  }
  
  let tuple = [initValue, setValue]
  states.push(tuple)
  return tuple
}

const App = () => {
  // creating variable
  const [counter, setCounter] = useState(0);
  
  return (
    <section>
      <h2>{counter}</h2>
      <button onClick={()=> setCounter(counter + 1)}>+</button>
      <button onClick={()=> setCounter(counter - 1)}>-</button>
    </section>
  )
}

function reRender() {
  // reset callCounter
  callCount = -1
  ReactDOM.render(<App />, document.getElementById('root'));
}

// initial render
reRender()

The setState should be something like: states[id][0] = newValue instanceof Function ? newValue(states[id][0]) : newValue;