Functional component renders four times - why not twice?

Question

I came across this simple React functional component, which renders four times, while I would expect it to render once initially, execute the useEffect which updates the state and therefore render again. Instead the console sends 4 log outputs, indicating that it renders four times. Any idea why and any resources on the basic lifecycle of react functional components?

const { useState, useEffect } = React;

function App() {
  const [jobs, setJobs] = useState([]);

  useEffect(() => setJobs(["test"]), []);

  console.log("APP", jobs);

  return (
    <div className="App">
      <h1>Hello CodeSandbox</h1>
      <h2>Start editing to see some magic happen!</h2>
      <div>{jobs}</div>
    </div>
  );
}

ReactDOM.render(
    <React.StrictMode>
        <App/>
    </React.StrictMode>,
    document.getElementById("root")
);

<div id="root"></div>
<script src="https://cdnjs.cloudflare.com/ajax/libs/react/16.12.0/umd/react.development.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/react-dom/16.12.0/umd/react-dom.development.js"></script>

https://codesandbox.io/s/solitary-tree-t120d?file=/src/App.js:149-191

Answer 1

This happens because in your codesandbox you use <React.StrictMode> to wrap your <App />.

The docs specifically state (https://reactjs.org/docs/strict-mode.html):

Strict mode can’t automatically detect side effects for you, but it can help you spot them by making them a little more deterministic. This is done by intentionally double-invoking the following functions:

Class component constructor, render, and shouldComponentUpdate methods
....