Pass parameters from PHP to React component - global variables

blablabla.js

/// user.php ///

// initialize global variable in a script added to your php file
<script>
    window.reactData = {
        postId: <?php echo get_the_id(); ?>
    };
</script>


/// userContainer.jsx ///

class User extends React.Component {
    constructor(props) {
        super(props);
        this.postId = reactData.postId; // here it is!
    }
}