WordPress at a glance
function is not described

WP_Nav_Menu_Widget::update() public WP 3.0.0

Handles updating settings for the current Navigation Menu widget instance.

{} It's a method of the class: WP_Nav_Menu_Widget{}

No Hooks.

Return

Array. Updated settings to save.

Usage

$WP_Nav_Menu_Widget = new WP_Nav_Menu_Widget();
$WP_Nav_Menu_Widget->update( $new_instance, $old_instance );
$new_instance(array) (required)
New settings for this instance as input by the user via WP_Widget::form().
$old_instance(array) (required)
Old settings for this instance.

Changelog

Since 3.0.0 Introduced.

Code of WP_Nav_Menu_Widget::update() WP 5.7.2

<?php
public function update( $new_instance, $old_instance ) {
	$instance = array();
	if ( ! empty( $new_instance['title'] ) ) {
		$instance['title'] = sanitize_text_field( $new_instance['title'] );
	}
	if ( ! empty( $new_instance['nav_menu'] ) ) {
		$instance['nav_menu'] = (int) $new_instance['nav_menu'];
	}
	return $instance;
}