make-promises-safe

A node.js module to make the use of promises safe. It implements the deprecation DEP0018 of Node.js in versions 6+. Using Promises without this module might cause file descriptor and memory leaks.

It is important that this module is only used in top-level program code, not in reusable modules!

The Problem

Node.js crashes if there is an uncaught exception, while it does not crash if there is an 'unhandledRejection', i.e. a Promise without a .catch() handler.

If you are using promises, you should attach a .catch() handler synchronously.

As an example, the following server will leak a file descriptor because of a missing .catch() handler:

const http = require('http')
const server = http.createServer(handle)

server.listen(3000)

function handle (req, res) {
  doStuff()
    .then((body) => {
      res.end(body)
    })
}

function doStuff () {
  if (Math.random() < 0.5) {
    return Promise.reject(new Error('kaboom'))
  }

  return Promise.resolve('hello world')
}

The Solution

make-promises-safe installs an process.on('unhandledRejection') handler that prints the stacktrace and exits the process with an exit code of 1, just like any uncaught exception.

Install

npm install make-promises-safe --save

Usage

'use strict'

require('make-promises-safe') // installs an 'unhandledRejection' handler
const http = require('http')
const server = http.createServer(handle)

server.listen(3000)

function handle (req, res) {
  doStuff()
    .then((body) => {
      res.end(body)
    })
}

function doStuff () {
  if (Math.random() < 0.5) {
    return Promise.reject(new Error('kaboom'))
  }

  return Promise.resolve('hello world')
}

as a preloader

You can add this behavior to any Node.js application by using it as a preloader:

node -r make-promises-safe server.js

with core dumps

You can also create a core dump when an unhandled rejection occurs:

require('make-promises-safe').abort = true

With custom logging

You can add a custom logger to log errors in your own format. To do this override the logError property with a function that takes a single Error parameter. This defaults to console.error.

const makePromisesSafe = require('make-promises-safe');
makePromisesSafe.logError = function(err) {
  // log the err object
}