WordPress at a glance
function is not described

WP_Query::is_feed() WP 3.1.0

Is the query for a feed?

This is a method of the class: WP_Query

No Hooks.

Return

true/false.

Usage

$WP_Query = new WP_Query();
$var = $WP_Query->is_feed( $feeds );
$feeds(string/array)
Optional feed types to check.
Default: ''

Code of WP Query::is feed: wp-includes/class-wp-query.php WP 5.2

<?php
public function is_feed( $feeds = '' ) {
	if ( empty( $feeds ) || ! $this->is_feed ) {
		return (bool) $this->is_feed;
	}
	$qv = $this->get( 'feed' );
	if ( 'feed' == $qv ) {
		$qv = get_default_feed();
	}
	return in_array( $qv, (array) $feeds );
}