Check and verify a nonce in WordPress plugin from JavaScript code

secure-plugin.php

<?php

add_action('wp_ajax_get_custom_data', 'get_custom_data');

function get_custom_data() {
	// Ready for the magic to protect your code?
	check_ajax_referer('secure-plugin-nonce');

	/* That's it - the check_ajax_referer function verifies the nonce is correct or it dies and stops code execution if it fails.

	If you want to customize the error handling and perhaps return an error to your JS code, you could change the code to something like:

	if ( ! check_ajax_referer( 'secure-plugin-nonce', false, false ) ) {	
		wp_send_json_error( 'Invalid nonce' );
	}

	*/
	
	$sanitized_user_data = sanitize_text_field( $_POST['user_data'] );
	// ... continue with the rest of your plugin code ... 
}