responsax

Topics: api · envelope · error · express · mern-packages · merndev · middleware · nodejs · npm-pm · observability · pagination · responsax · response · standard · trace · typescript

Standardize every API response your Express app sends — typed envelopes, paginated lists, structured errors, and a per-request trace ID — in one tiny middleware.

Why

Every team reinvents { success, data, error, meta, pagination }. responsax ships a battle-tested envelope, an ApiError class with HTTP helpers, and an error handler that catches everything (including native throws) and emits consistent payloads.

Install

npm install responsax

Usage

import express from "express";
import { responsax, errorHandler, ApiError, notFound } from "responsax";

const app = express();
app.use(express.json());
app.use(responsax());

app.get("/users/:id", async (req, res, next) => {
  const user = await db.users.findById(req.params.id);
  if (!user) return next(notFound("User not found"));
  res.success(user);
});

app.get("/users", async (req, res) => {
  const page = Number(req.query.page ?? 1);
  const pageSize = 20;
  const [items, total] = await Promise.all([
    db.users.list({ page, pageSize }),
    db.users.count(),
  ]);
  res.paginated(items, { page, pageSize, total });
});

app.post("/users", (req, res, next) => {
  if (!req.body.email) return next(new ApiError("email required", { status: 422 }));
  res.created({ id: "u_1", ...req.body });
});

app.use(errorHandler());

Success envelope

{
  "success": true,
  "data": { "id": 1 },
  "meta": { "traceId": "tr_…", "timestamp": "2026-…", "durationMs": 8 }
}

Paginated envelope

{
  "success": true,
  "data": [ … ],
  "pagination": { "page": 2, "pageSize": 20, "total": 100, "totalPages": 5, "hasNext": true, "hasPrev": true },
  "meta": { "traceId": "tr_…", "timestamp": "2026-…", "durationMs": 12 }
}

Error envelope

{
  "success": false,
  "error": { "code": "NOT_FOUND", "message": "User not found", "status": 404 },
  "meta": { "traceId": "tr_…", "timestamp": "2026-…", "durationMs": 2 }
}

Response helpers (typed)

| Method | What it does | | --- | --- | | res.success(data) | 200 + success envelope | | res.created(data) | 201 + success envelope | | res.noContent() | 204 + empty body | | res.paginated(items, { page, pageSize, total }) | 200 + paginated envelope | | res.error(message, { status, code, details }) | error envelope | | res.fail(err) | normalize any error into the error envelope |

Error helpers

import {
  ApiError, badRequest, unauthorized, forbidden,
  notFound, conflict, unprocessable, tooManyRequests, internal,
} from "responsax";

Each helper returns an ApiError with the right status, default error code, and exposure flag.

Options

responsax({
  traceIdHeader: "x-trace-id",
  exposeTraceIdHeader: true,
  generateTraceId: () => crypto.randomUUID(),
  defaultErrorStatus: 500,
  defaultErrorCode: "INTERNAL_ERROR",
  includeStack: false,
  errorMapper: (err) =>
    err && typeof err === "object" && "code" in err && err.code === "P2002"
      ? { status: 409, code: "CONFLICT", message: "Duplicate" }
      : undefined,
});

errorMapper lets you map Prisma/Mongo/library-specific errors to the standard envelope.

TypeScript

responsax augments Express' Response with success, created, paginated, error, fail, and traceId. Import the package once anywhere in your project and the methods become typed everywhere.

License

MIT