# make-promises-safe [](https://travis-ci.org/mcollina/make-promises-safe)
A node.js module to make the use of promises safe.
It implements the deprecation [DEP0018][unhandled] 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:
```js
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
```js
'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
}
```
## License
MIT
[unhandled]: https://nodejs.org/dist/latest-v8.x/docs/api/deprecations.html#deprecations_dep0018_unhandled_promise_rejections
