Source

File: wp-includes/widgets/class-wp-widget-recent-posts.php

public function update( $new_instance, $old_instance ) {
		$instance              = $old_instance;
		$instance['title']     = sanitize_text_field( $new_instance['title'] );
		$instance['number']    = (int) $new_instance['number'];
		$instance['show_date'] = isset( $new_instance['show_date'] ) ? (bool) $new_instance['show_date'] : false;
		return $instance;
	}