App.js

import React, { useState } from "react";
import logo from "./logo.svg";
import "./App.css";

function App() {
  const [counter, setCounter] = useState(0);

  const increment = () => setCounter(counter + 1);
  const decrement = () => setCounter(counter - 1);

  return (
    <div className="App">
      <header className="App-header">
        <img src={logo} className="App-logo" alt="logo" />
        <p>
          Edit <code>src/App.js</code> and save to reload.
        </p>

        <p>Counter: {counter}</p>

        <button onClick={increment} title="Increment">
          Increment
        </button>
        <button onClick={decrement} title="Decrement">
          Decrement
        </button>

        <p>{new Date(Date.now()).toLocaleDateString()}</p>

        <a
          className="App-link"
          href="https://reactjs.org"
          target="_blank"
          rel="noopener noreferrer"
        >
          Learn React
        </a>
      </header>
    </div>
  );
}

export default App;

App.test.js

import React from "react";
import ReactDOM from "react-dom";
import App from "./App";
import { render, cleanup, fireEvent } from "@testing-library/react";
// import 'jest-dom/extend-expect'

afterEach(cleanup);

it("renders without crashing", () => {
  const div = document.createElement("div");
  ReactDOM.render(<App />, div);
  ReactDOM.unmountComponentAtNode(div);
});

it("start to count at zero", async () => {
  const { getByText } = render(<App />);
  return expect(getByText(/Counter: \d/).innerHTML).toBe("Counter: 0");
});

it("should increment counter when clicking on +", async () => {
  const { getByText, getByTitle } = render(<App />);

  const button = getByTitle("Increment", { selector: "button" });
  fireEvent.click(button);
  fireEvent.click(button);

  return expect(getByText(/Counter: \d/).innerHTML).toBe("Counter: 2");
});

it("should decrement counter when clicking on -", async () => {
  const { getByText, getByTitle } = render(<App />);

  const button = getByTitle("Decrement", { selector: "button" });
  fireEvent.click(button);
  fireEvent.click(button);

  return expect(getByText(/Counter: -?\d/).innerHTML).toBe("Counter: -2");
});