take-oath

"Promisify" that works

Requirements

• ES6 (Node >= 6)

Installation

npm i take-oath

Examples

Promisify a module

const takeOath = require('take-oath')
const {readdir} = takeOath('fs')

readdir('/path/to/directory').then(console.log)

The module can be a function or an object. take-oath is immutable in nature and will not modify the original module in any way.

Promisify a function

const takeOath = require('take-oath')

const inc = (number, callback) => {
  callback(null, number + 1)
}

takeOath(inc)(2).then(console.log) // 3

You can also bind the function to a context...

const takeOath = require('take-oath')

const obj = {number: 1}

const inc = function (callback) {
  callback(null, this.number + 1)
}

takeOath(inc, obj)().then(console.log) // 2

Promisify an array of functions

const promisify = require('take-oath')

const [fn1, fn2] = promisify([
  cb => cb(null, 'foo'),
  cb => cb(null, 'bar')
])

fn1().then(console.log) // foo

Promisify all functions in an Object

const takeOath = require('take-oath')

const obj = {
  fn1: cb => cb(null, 'foo'),
  fn2: cb => cb(new Error('bar'))
}

takeOath(obj).fn1().then(console.log) // foo
takeOath(obj).fn2().catch(console.error) // Error: bar

Note: take-oath is immutable in nature and will not modify the original object.

Custom promisify function

Node < 8

const promisify = require('take-oath')

const inc = promisify((number, callback) => {
  callback(number + 1)
}, null, {
  promisifyFunction: () => number =>
    new Promise(resolve => resolve(number + 1))
})

inc(2).then(console.log) // 3

Node >= 8

const promisify = require('take-oath')
const util = require('util')

const inc = (number, callback) => {
  callback(null, number + 1)
}

inc[util.promisify.custom] = number =>
  new Promise(resolve => resolve(number + 1))

promisify(inc)(2).then(console.log) // 3

Cookbook

Separating callback and promise methods

take-oath is fairly light weight and doesn't use any complex decisions on whether the function should or should not be promisified. It is also immutable by nature and will not modify the object. Therefore we recommend separating the callback and promise style methods. Here's an example:

When using the nedb module to construct a datastore we have several callback style functions on the datastore instance. However, when you don't pass the callback it will return a chainable query instance. We'll need this to construct more complex queries, but we may also want simple promise style methods for simpler calls.

// db.js

const promisify = require('take-oath')
const Datastore = require('nedb')
const db = new Datastore('./data.db')

db.promise = promisify(db)
module.exports = db

We've now got a datastore object as a module ./db.js. The instance has both the usual callback style methods and promise methods available on the promise object.

const db = require('./db')
const promisify = require('take-oath')

exports.getLatest = schemaVersion => {
  const query = db.findOne({schema: schemaVersion}).sort({insertedAt: -1})
  return promisify(query.exec, query)
}

exports.getAll = () => db.promise.findAll()

Now the above module can use the datastore in either way when appropriate.