EZ Promise

A lightweight way to convert error-first Node.js style callbacks into promises.

Simply take a function that follows the Node.js callback convention, pass it to ezpromise and pass any arguments afterwards.

ezpromise(rmdir, './path/to/dir')

Partial application / Currying / Thunks

Comes with built in support for currying, simply import ezThunk and you are ready to go. Pass the function you wish to convert into a promise first, with any arguments in the curried function.

import { ezThunk } from 'ez-promise'

const removeDir = ezThunk(rmdir)

removeDir('./path/to/dir').then(() => { console.log('Success!') })

Batteries Included

The library comes with everything required to run. If no window.Promise is detected, a polyfill is automatically added.

import/require

const ezpromise = require('ez-promise').default
import ezpromise from 'ez-promise'

const { ezThunk } = require('ez-promise')
import { ezThunk } from 'ez-promise'

Resolving Multiple Values

Since a promise can only resolve a single value, if a function's callback has an arity of 3 or greater an array containing the values is returned.

ezpromise(someAsyncFn, './some/path')
    .then(([arg1, arg2, ...rest]) => {
        // ...
    })

Example

We have an environment script that takes an environment config object from a set of available configs and dumps it into an environment folder inside our source code.

Libraries like fs, ncp, and rmdir all make use of async callbacks, and since they follow the error-first callback convention (AKA Node.js-style callbacks), we can easily convert them into promises.

Let's start with the callback pattern:

import ncp from 'ncp'

function copyFile (from, to) {
    ncp(from, to, err => {
        if (err) {
            console.log(`Error copying file: ${err}`)
        } else {
            console.log('Successfully copied file!')
        }
    )
}

copyFile('./env/production.js', './src/env.js')

Now, let's convert our copyFile function into a promise:

import ncp from 'ncp'
import ezpromise from 'ez-promise'

const copyFile = ({ from, to }) => ezpromise(ncp, from, to)

Or if you prefer to use the curried version:

import ncp from 'ncp'
import { ezThunk } from 'ez-promise'

const copyFile = ezThunk(ncp)

That's it! Now we are free to use our newly-converted promises anyway we like.

Chaining promise methods:

copyFile({ from: './env/production.js', to: './src/env.js' })
    .then(() => { console.log('Success!') })
    .catch(err => { console.log(`Error: ${err}`) })

Async/await:

async function multipleTasks () {
    const result = await copyFile({ to: './env/production.js', to: './src/env.js' })
    // ...
}

Note that when currying, the arguments will be in the original format of the function we are converting into a promise, but with the callback at the end omitted:

copyFile('./env/production.js', './src/env.js')
ncp('./env/production.js', './src/env.js', () => {
    // ...
})

Dependencies

The only dependency is a Promise polyfill!