Middleware for Express to add promise support
Last updated 2 years ago by hash-bang .
MIT · Repository · Bugs · Original npm · Tarball · package.json
$ cnpm install @momsfriendlydevco/express-middleware-promise 
SYNC missed versions from official npm registry.


A simple wrapper around Express to make it promise compatible.

  • Completely Promise library agnostic - Use native promises, Q, Bluebird or whatever library you want
  • Backwards compatible - Mix and match callbacks and promises as needed
  • Low overhead - While this library does have to wrap all middleware calls the codebase is minimal to prevent drag
var express = require('express');
var expressPromises = require('@momsfriendlydevco/express-middleware-promise');

var app = express();

// Any simple return value gets sent via res.send() automatically
app.get('/api/hello-world', ()=> 'Hello World!'); 

// Use setTimeout to pause for 100 inside a promise
app.get('/api/pause', ()=> new Promise(resolve => setTimeout(()=> resolve('I waited!'), 100)));

// Make a middleware chain of promises
	()=> BigComplexPromise(),
	()=> AnotherComplexPromise(),
	()=> YetAnotherPromise()

// Normal callback style express functions are still fully supported
app.get('/api/classic', (req, res) => res.send('Old school!'));

// Mix and match approches
	(req, res, next) => next(),
	(req, res, next) => setTimeout(()=> next(), 10),
	(req, res, next) => new Promise(resolve => setTimeout(()=> resolve(), 20)),
	(req, res) => res.send('Done!'),


Express-As-Promised exports one function which mutates the Express app object, replacing some built in functions so that promises are supported.

The function can either be called with the entire Express object:

var app = express();

... or with options:

var app = express();
	app: app,
	setting1: settingValue,

Supported options are listed below.

Option Type Default Description
app Express App none The Express app object you want to mutate
isPromise function(obj) (complex, see code) How to identify a promise return. By default this function checks that the passed object is an object and contains a then property
isResolvable function(obj) (complex, see code) How to identify what responses should be resolved. Default is to identify a scalar result that looks like a JS native
resolve function(req, res, data) (see code) When isResolvable passes, this function indicates how the response is output. Default is to use res.send() with some minor wrapping
resolveEmpty function(req, res, data) res.sendStatus(200) How to resolve the last item in a middleware chain if no result has been provided
error function(req, res, data) res.sendStatus(400).send(data) How to output caught errors
wrapFunctions array ['delete', 'get', 'post', 'put'] What Express functions should be wrapped to add promise support

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