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

function App() {
  console.log("render");
  let [val, setVal] = useState(0);
  return <button onClick={() => setVal(1)}><Child/></button>;
}

function Child() {
  console.log('render the child')
  return <span>go</span>
}

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

render
render the child
render
render the child
render