Use the N/scriptTypes/restlet module to create custom HTTP responses for your RESTlet script. This module is available only to RESTlet script type.
N/scriptTypes/restlet Module Members

restlet.Response :-

restlet.createResponse(options): –

Parameters:- 
Errors:-
Response Object Members
The following members are called on the restlet.Response object.

Response.content:-

Response.contentType:-

Syntax:
/**
* @NApiVersion 2.1
* @NScriptType RESTlet
*/
define(['N/scriptTypes/restlet', 'N/query'],
function(restlet, query) {
const get = function (requestParams) {
const htmlResponse = restlet.createResponse({
content: '<h1>Hello World</h1>',
contentType: 'text/html'
});
log.debug({
title: 'Response Content: ',
details: htmlResponse.content
});
return htmlResponse;
};
return { get: get };
});