Source

File: wp-includes/customize/class-wp-customize-nav-menu-setting.php

public function value() {
		if ( $this->is_previewed && get_current_blog_id() === $this->_previewed_blog_id ) {
			$undefined  = new stdClass(); // Symbol.
			$post_value = $this->post_value( $undefined );

			if ( $undefined === $post_value ) {
				$value = $this->_original_value;
			} else {
				$value = $post_value;
			}
		} else {
			$value = false;

			// Note that a term_id of less than one indicates a nav_menu not yet inserted.
			if ( $this->term_id > 0 ) {
				$term = wp_get_nav_menu_object( $this->term_id );

				if ( $term ) {
					$value = wp_array_slice_assoc( (array) $term, array_keys( $this->default ) );

					$nav_menu_options  = (array) get_option( 'nav_menu_options', array() );
					$value['auto_add'] = false;

					if ( isset( $nav_menu_options['auto_add'] ) && is_array( $nav_menu_options['auto_add'] ) ) {
						$value['auto_add'] = in_array( $term->term_id, $nav_menu_options['auto_add'], true );
					}
				}
			}

			if ( ! is_array( $value ) ) {
				$value = $this->default;
			}
		}

		return $value;
	}