Promisify anything preserving this

Simple promisify that works on object instances preserving this

Why?

Many existing libraries loose the this binding, others would wrap the object and loose typeof and instanceof functionality, yet others would modify the object in place breaking it all together. This library uses proxies to lazily intercept calls to instance methods and functions.

NOTE: There might be some edge cases where it still breaks, so far I'm not aware of any, if you run into one, please open an issue.

API

• promisify(instance [, this], ) - promisify a function, object literal or a class instance, optionally using an alternative this. Instance methods are promisified lazily, which allows promisifying large objects without much initial overhead.
  • instance - the instance to promisify, can be object, function or a class instance.
    • All functions and methods are assumed to be in standard callback style, e.g. function(...args, callback) where callback takes an error as its first argument.
  • this optional - this to call the methods with
  • promisifyFn optional (boolean, default false) - if true will treat the passed instance as a callable function.

Usage

Typescript

import { promisify } from './src'

(async () => {
  class MyClass {
    d: string
    constructor(d: string) {
      this.d = d
    }
    fn(p1, p2, cb) {
      return cb(null, `called with ${p1} ${p2} ${this.d}`)
    }
  }

  const mP = promisify(new MyClass('d'))
  const res = await mP.fn('a', 'b')
  console.log(res)
  // prints 'called with a b d'
})()

Javascript

(async () => {
  const { promisify } = require('promisify-this')

  class MyClass {
    constructor (d) {
      this.d = d
    }
    fn (p1, p2, cb) {
      return cb(null, `called with ${p1} ${p2} ${this.d}`)
    }
  }

  const mP = promisify(new MyClass('d'))
  const res = await mP.fn('a', 'b')
  console.log(res)
  // prints 'called with a b d'
})()

VERSIONS 2.1.0 & 2.2.0 broke CommonJs compatibility

TL;DR

Either upgrade to version 3.0.0 and use a named export, as in:

import { promisify } from 'promisify-this'

or

const { promisify } = require('promisify-this')

or downgrade to version 2.0.2 to continue using the previous CommonJs default export pattern of:

const promisify = require('promisify-this)

Alternatively, you can continue using versions 2.1.0 & 2.2.0 (note however, that this versions have been deprecated) with the following syntax:

const {default: promisify} = require('promisify-this')

Typescript migration and breaking changes

This module has been rewritten in Typescript beginning with version 2.1.0. The previous pure js implementation relied on the CommonJs pattern of module.exports = function(){}. This pattern doesn't work well under the new ES6 modules spec, nor is it fully supported by Typescript as such. Versions 2.1.0 and 2.2.0 relied on the default export syntax which doesn't simply map to module.exports = function(){}, but adds a new entry - default in the exports object, this in turn, ends up mapping to module.exports = { default: function(){} } in CommonJs, which is not the intent and breaks compatibility.

Beginning with version 3.0.0 the default export has been removed in favor on named exports.

Enjoy!

Licence

MIT