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import logo from './logo.svg';
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import './App.css';
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function App() {
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  return (
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    <div className="App">
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      <header className="App-header">
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        <img src={logo} className="App-logo" alt="logo" />
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        <p>
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          Edit <code>src/App.js</code> and save to reload.
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        </p>
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        <a
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          className="App-link"
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          href="https://reactjs.org"
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          target="_blank"
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          rel="noopener noreferrer"
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        >
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          Learn React
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        </a>
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      </header>
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    </div>
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  );
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}
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export default App;
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⬇
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function App() {
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  return (
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    <div className="App">
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      Hello, world!
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    </div>
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  );
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}
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export default App;

1
import React from 'react';
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import ReactDOM from 'react-dom';
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import './index.css';
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import App from './App';
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import reportWebVitals from './reportWebVitals';
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ReactDOM.render(
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  <React.StrictMode>
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    <App />
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  </React.StrictMode>,
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  document.getElementById('root')
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);
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// If you want to start measuring performance in your app, pass a function
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// to log results (for example: reportWebVitals(console.log))
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// or send to an analytics endpoint. Learn more: https://bit.ly/CRA-vitals
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reportWebVitals();
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⬇
20

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import React from 'react';
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import ReactDOM from 'react-dom';
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import App from './App';
24

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