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Same core call as everywhere else in this section — render() wrapped in try/catch, with RenderError.code deciding the response status.

Installation

npm install @maildeno/renderer

A render plugin

// plugins/render.ts
import type { FastifyInstance, FastifyRequest, FastifyReply } from "fastify";
import { render, RenderError } from "@maildeno/renderer";

interface RenderBody {
  templateId: string;
  mergeTags?: Record<string, Record<string, string>>;
  context?: Record<string, string | number | boolean>;
}

const STATUS_BY_CODE: Record<string, number> = {
  TEMPLATE_NOT_FOUND: 404,
  INVALID_TEMPLATE: 400,
  RENDER_ERROR: 500,
};

export async function renderRoutes(app: FastifyInstance) {
  app.post(
    "/render",
    async (request: FastifyRequest<{ Body: RenderBody }>, reply: FastifyReply) => {
      const { templateId, mergeTags, context } = request.body;

      try {
        const html = await render(`${templateId}.json`, {
          baseDir: process.env.TEMPLATE_DIR,
          mergeTags,
          context,
        });

        return { html };
      } catch (err) {
        if (err instanceof RenderError) {
          request.log.warn({ code: err.code }, err.message);
          return reply.code(STATUS_BY_CODE[err.code] ?? 500).send({
            error: err.code,
            message: err.message,
          });
        }

        request.log.error(err);
        return reply.code(500).send({ error: "RENDER_FAILED" });
      }
    },
  );
}

Registering it

// app.ts
import Fastify from "fastify";
import { renderRoutes } from "./plugins/render";

const app = Fastify({ logger: true });
app.register(renderRoutes);

app.listen({ port: Number(process.env.PORT ?? 3000) });

Where to go next