express-promise-router

What is express-promise-router?

express-promise-router is an npm package that provides a drop-in replacement for Express's Router, adding support for handling promises. This allows you to write route handlers that return promises and automatically handle errors, making your code cleaner and more readable.

What are express-promise-router's main functionalities?

Promise-based Route Handling

This feature allows you to define routes that return promises. The router will automatically catch any errors and pass them to the next middleware.

const express = require('express');
const router = require('express-promise-router')();

router.get('/example', async (req, res) => {
  const data = await someAsyncFunction();
  res.json(data);
});

const app = express();
app.use(router);
app.listen(3000, () => console.log('Server running on port 3000'));

Error Handling

This feature demonstrates how express-promise-router handles errors thrown in async route handlers. The error is automatically caught and passed to the error-handling middleware.

const express = require('express');
const router = require('express-promise-router')();

router.get('/error', async (req, res) => {
  throw new Error('Something went wrong');
});

const app = express();
app.use(router);
app.use((err, req, res, next) => {
  res.status(500).json({ error: err.message });
});
app.listen(3000, () => console.log('Server running on port 3000'));

Middleware Support

This feature shows how you can use async middleware with express-promise-router. The middleware can perform asynchronous operations and call next() to proceed to the next middleware or route handler.

const express = require('express');
const router = require('express-promise-router')();

const asyncMiddleware = async (req, res, next) => {
  await someAsyncFunction();
  next();
};

router.use(asyncMiddleware);
router.get('/middleware', (req, res) => {
  res.send('Middleware executed');
});

const app = express();
app.use(router);
app.listen(3000, () => console.log('Server running on port 3000'));

Other packages similar to express-promise-router

express-async-handler

express-async-handler is a middleware for handling exceptions inside of async express routes and passing them to your express error handlers. Unlike express-promise-router, it is not a router replacement but a middleware that you wrap around your async route handlers.

async-express-router

async-express-router is another package that provides a router with built-in support for async route handlers. It is similar to express-promise-router but offers additional features like route grouping and middleware chaining.

express-async-router

express-async-router is a lightweight alternative to express-promise-router that also supports async route handlers. It focuses on simplicity and minimalism, providing a straightforward way to handle promises in Express routes.

README

express-promise-router

A simple wrapper for Express 4's Router that allows middleware to return promises. This package makes it simpler to write route handlers for Express when dealing with promises by reducing duplicate code.

Getting Started

Install the module with npm

npm install express-promise-router --save

or yarn.

yarn add express-promise-router

express-promise-router is a drop-in replacement for Express 4's Router.

Documentation

Middleware and route handlers can simply return a promise. If the promise is rejected, express-promise-router will call next with the reason. This functionality removes the need to explicitly define a rejection handler.

// With Express 4's router
var router = require("express").Router();

router.use("/url", function (req, res, next) {
  Promise.reject().catch(next);
});

// With express-promise-router
var router = require("express-promise-router")();

router.use("/url", function (req, res) {
  return Promise.reject();
});

Calling next() and next("route") is supported by resolving a promise with either "next" or "route". No action is taken if the promise is resolved with any other value.

router.use("/url", function (req, res) {
  // equivalent to calling next()
  return Promise.resolve("next");
});

router.use("/url", function (req, res) {
  // equivalent to calling next('route')
  return Promise.resolve("route");
});

This package still allows calling next directly.

router = require("express-promise-router")();

// still works as expected
router.use("/url", function (req, res, next) {
  next();
});

ES6 Imports

express-promise-router can be imported via ES6 imports. The Router constructor is the default export.

import Router from "express-promise-router";
const router = Router();

Async / Await

Using async / await can dramatically improve code readability.

router.get('/url', async (req, res) {
    const user = await User.fetch(req.user.id);

    if (user.permission !== "ADMIN") {
      throw new Error("You must be an admin to view this page.");
    }

    res.send(`Hi ${user.name}!`);
})

Error handling

Just like with regular express.Router you can define custom error handlers.

router.use((err, req, res, next) => {
  res.status(403).send(err.message);
});

Frequently Asked Questions

Cannot read property '0' of undefined

This error may indicate that you call a method that needs a path, without one. Calling router.get (or post, all or any other verb) without a path is not valid. You should always specify a path like this:

// DO:
router.get("/", function (req, res) {
  res.send("Test");
});

// DON'T:
router.get(function (req, res) {
  res.send("Test");
});

For more information take a look at this comment.

Can i use this on app?

We currently don't support promisifying the app object. To use promises with the top-level router we recommend mounting a Router on the app object, like this:

import express from "express";
import Router from "express-promise-router";

const app = express();
const router = Router();
app.use(router);

router.get("/", function (req, res) {
  res.send("Test");
});

Why aren't promise values sent to the client

We don't send values at the end of the promise chain to the client, because this could easily lead to the unintended leak of secrets or internal state. If you intend to send the result of your chain as JSON, please add an explicit .then(data => res.send(data)) to the end of your chain or send it in the last promise handler.

Contributing

Add unit tests for any new or changed functionality. Lint and test your code using npm test.

Unit tests use mocha and chai.

We use eslint, but styling is controlled mostly by prettier which reformats your code before you commit. You can manually trigger a reformat using npm run-script format.

Release History

Unreleased

See CHANGELOG

Attribution

Licensed under the MIT license.

Initial implementation by Alex Whitney
Maintained by Moritz Mahringer
Contributed to by awesome people