wp-includes/class-wp-query.php:4558Determines whether the query is for an existing single page.
$pageint|string|int[]|string[]optional''bool public function is_page( $page = '' ) { if ( ! $this->is_page ) { return false; } if ( empty( $page ) ) { return true; } $page_obj = $this->get_queried_object(); if ( ! $page_obj ) { return false; } $page = array_map( 'strval', (array) $page ); if ( in_array( (string) $page_obj->ID, $page, true ) ) { return true; } elseif ( in_array( $page_obj->post_title, $page, true ) ) { return true; } elseif ( in_array( $page_obj->post_name, $page, true ) ) { return true; } else { foreach ( $page as $pagepath ) { if ( ! strpos( $pagepath, '/' ) ) { continue; } $pagepath_obj = get_page_by_path( $pagepath ); if ( $pagepath_obj && ( $pagepath_obj->ID === $page_obj->ID ) ) { return true; } } } return false; }Introduced in 3.1.0. Unchanged from 6.7.7 through 7.1.0.
Signature, return type and hooks compared across 5 parsed releases.
src/wp-includes/class-wp-query.php, and regenerated for each WordPress release so it tracks the code rather than a snapshot of it.