Source

File: wp-includes/rest-api/endpoints/class-wp-rest-posts-controller.php

protected function prepare_items_query( $prepared_args = array(), $request = null ) {
		$query_args = array();

		foreach ( $prepared_args as $key => $value ) {
			/**
			 * Filters the query_vars used in get_items() for the constructed query.
			 *
			 * The dynamic portion of the hook name, `$key`, refers to the query_var key.
			 *
			 * @since 4.7.0
			 *
			 * @param string $value The query_var value.
			 */
			$query_args[ $key ] = apply_filters( "rest_query_var-{$key}", $value ); // phpcs:ignore WordPress.NamingConventions.ValidHookName.UseUnderscores
		}

		if ( 'post' !== $this->post_type || ! isset( $query_args['ignore_sticky_posts'] ) ) {
			$query_args['ignore_sticky_posts'] = true;
		}

		// Map to proper WP_Query orderby param.
		if ( isset( $query_args['orderby'] ) && isset( $request['orderby'] ) ) {
			$orderby_mappings = array(
				'id'            => 'ID',
				'include'       => 'post__in',
				'slug'          => 'post_name',
				'include_slugs' => 'post_name__in',
			);

			if ( isset( $orderby_mappings[ $request['orderby'] ] ) ) {
				$query_args['orderby'] = $orderby_mappings[ $request['orderby'] ];
			}
		}

		return $query_args;
	}