Source

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

protected function populate_value() {
		if ( ! is_array( $this->value ) ) {
			return;
		}

		if ( isset( $this->value['menu_order'] ) ) {
			$this->value['position'] = $this->value['menu_order'];
			unset( $this->value['menu_order'] );
		}
		if ( isset( $this->value['post_status'] ) ) {
			$this->value['status'] = $this->value['post_status'];
			unset( $this->value['post_status'] );
		}

		if ( ! isset( $this->value['original_title'] ) ) {
			$this->value['original_title'] = $this->get_original_title( (object) $this->value );
		}

		if ( ! isset( $this->value['nav_menu_term_id'] ) && $this->post_id > 0 ) {
			$menus = wp_get_post_terms(
				$this->post_id,
				WP_Customize_Nav_Menu_Setting::TAXONOMY,
				array(
					'fields' => 'ids',
				)
			);
			if ( ! empty( $menus ) ) {
				$this->value['nav_menu_term_id'] = array_shift( $menus );
			} else {
				$this->value['nav_menu_term_id'] = 0;
			}
		}

		foreach ( array( 'object_id', 'menu_item_parent', 'nav_menu_term_id' ) as $key ) {
			if ( ! is_int( $this->value[ $key ] ) ) {
				$this->value[ $key ] = intval( $this->value[ $key ] );
			}
		}
		foreach ( array( 'classes', 'xfn' ) as $key ) {
			if ( is_array( $this->value[ $key ] ) ) {
				$this->value[ $key ] = implode( ' ', $this->value[ $key ] );
			}
		}

		if ( ! isset( $this->value['title'] ) ) {
			$this->value['title'] = '';
		}

		if ( ! isset( $this->value['_invalid'] ) ) {
			$this->value['_invalid'] = false;
			$is_known_invalid        = (
				( ( 'post_type' === $this->value['type'] || 'post_type_archive' === $this->value['type'] ) && ! post_type_exists( $this->value['object'] ) )
				||
				( 'taxonomy' === $this->value['type'] && ! taxonomy_exists( $this->value['object'] ) )
			);
			if ( $is_known_invalid ) {
				$this->value['_invalid'] = true;
			}
		}

		// Remove remaining properties available on a setup nav_menu_item post object which aren't relevant to the setting value.
		$irrelevant_properties = array(
			'ID',
			'comment_count',
			'comment_status',
			'db_id',
			'filter',
			'guid',
			'ping_status',
			'pinged',
			'post_author',
			'post_content',
			'post_content_filtered',
			'post_date',
			'post_date_gmt',
			'post_excerpt',
			'post_mime_type',
			'post_modified',
			'post_modified_gmt',
			'post_name',
			'post_parent',
			'post_password',
			'post_title',
			'post_type',
			'to_ping',
		);
		foreach ( $irrelevant_properties as $property ) {
			unset( $this->value[ $property ] );
		}
	}