This is example of two ways how you can include and run React app in WordPress.

If you don't want to use WordPress REST API and then read JSON in React, there are still some simple options how to get data from WordPress to React.

Copy theme file to folder

{root_folder}/wp-content/themes/

Edit your app in {root_folder}/wp-content/themes/twentynineteen/js/my-react-app.js

Include js file and pass data to react js in theme file {root_folder}/wp-content/themes/twentynineteen/template-parts/content/content-page.php like this:

// Register the script
wp_register_script( 'my-react-app', get_stylesheet_directory_uri() . '/js/my-react-app.js' );

// Localize the script with new data
$data_array = array(
	'yourName' => 'Radek',
	'yourWebsite' => 'http://www.mezulanik.cz',
);
//Pass data to Javascript
wp_localize_script( 'my-react-app', 'personData', json_encode($data_array) );

// Enqueued script with localized data.
wp_enqueue_script( 'my-react-app' );

Then in content-page.php create app root container <div id="myApp"></div> so the app will run inside.

In React app you will be able to access variable personData easily

constructor(props) {
    super(props);
    this.state = personData;
}

LAST STEP: We're not building this app and JS file will stay uncompiled. To be able to run JSX code in WP, you have to change include type to babel in end of the functions.php file:

add_filter('style_loader_tag', 'javascript_to_babel', 10, 2);
add_filter('script_loader_tag', 'javascript_to_babel', 10, 2);
function javascript_to_babel($tag, $handle) {
	if ( 'my-react-app' !== $handle) : return $tag; endif;
	return str_replace( "<script type='text/javascript'", "<script type='text/babel'", $tag );
}

Edit your app in {root_folder}/wp-content/themes/twentynineteen/js-src/my-react-app.js and build it using

npm run build

you can also use

npm run watch

to run build automatically after every change.

Then you don't need to do last step from 1a because babel will be compiled already. It's better to move your js-src folder outside from theme folder so uncompiled files wouldn't sit on the server after deploy. I recommend this way as it's safe with compiled output and lightweight to build one page apps.

Edit your classic React app in {root_folder}/wp-content/themes/twentynineteen/myReactApp/ and build it using

yarn build

Go to end of the functions.php and include all JS and CSS files to WordPress:

add_action( 'wp_enqueue_scripts', 'enqueue_my_react_app' );
function enqueue_my_react_app(){
	foreach( glob( get_template_directory(). '/myReactApp/build/static/js/*.js' ) as $file ) {
		// $file contains the name and extension of the file
		$filename = substr($file, strrpos($file, '/') + 1);
		wp_enqueue_script( $filename, get_template_directory_uri().'/myReactApp/build/static/js/'.$filename);
	}
	foreach( glob( get_template_directory(). '/myReactApp/build/static/css/*.css' ) as $file ) {
		// $file contains the name and extension of the file
		$filename = substr($file, strrpos($file, '/') + 1);
		wp_enqueue_style( $filename, get_template_directory_uri().'/myReactApp/build/static/css/'.$filename);
	}
}

Then in content-page.php create app root container <div id="root"></div> so the app will run inside.

You can pass data from PHP to React by inserting script into this theme file:

<script>
    window.reactInit = {
			scriptData: "<?php echo get_template_directory_uri().'/myReactApp/'; ?>"
		};
</script>

In React app you will be able to access variable scriptData like this

<img src={window.reactInit.scriptData+'public/media/logo.svg'} className="App-logo" alt="logo" />

I'm using a public folder for logo here, because relative path to the logo is different in WP than in React, so you can pass custom path as parameter.

If you know about some better way how to do this, feel free to let me know on www.mezulanik.cz.