WP - create custom json feed

register_feed.php

// register json feed
class custom_feed {
  
	public $feed = 'json';
	public function __construct() {
	
		add_action( 'init', array( $this, 'init' ) );
		
	}
  
	public function init() {
	
		// feed name to access in URL eg. /feed/custom-xml/
		add_feed( $this->feed, array( $this, 'json' ) );
	
	}
 
	public function json() {
		global $wp_query;
		$n = (!empty($wp_query->query_vars['number'])) ? $wp_query->query_vars['_post'] : 1;
		$range = (!empty($wp_query->query_vars['range'])) ? $wp_query->query_vars['range'] : 'all';
		$data = get_data($n, $range, 'json');
		header('Content-Type: application/json');
		echo $data;		
	}
  
}
$custom_feed = new custom_feed();