Source

File: wp-includes/rest-api/class-wp-rest-request.php

public function has_param( $key ) {
		$order = $this->get_parameter_order();

		foreach ( $order as $type ) {
			if ( is_array( $this->params[ $type ] ) && array_key_exists( $key, $this->params[ $type ] ) ) {
				return true;
			}
		}

		return false;
	}