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to-callback

Converts result-first callbacks to classic (node.js-style) error-first callbacks with 3 lines of code. Useful when you want to promisify result-first APIs (like emitter.on). Inspired by rfpify and probably used in it.

Table of Contents

• Install
• What is this for?
• Why it exists?
• How to promisify "result-first" functions
• Usage
• API
  • toCallback
• Related
• Contributing

Install

Install with npm

$ npm i to-callback --save

What is this for?

Some APIs do not use an error-first callback approach, which is convention in NodeJS land. This library converts those APIs with "result-first" callback to have "error-first" callback.

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Why it exists?

Inspiration for this was rfpify and especially the wanted feature issue#3 at pify library. The @SamVerschueren take on that forks the original pify code base and andjust the needed changes. But I'm curious why? Isn't it easier to convert simply "result-first" to "error-first" and than just pass to original pify.

So that's what this package does. :)

I hope to merge it into rfpify or directly to pify so we can resolve the issue#3.

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How to promisify "result-first" functions

This is example how rfpify can look like in future

var pify = require('pify')
module.exports = (fn, opts) => pify(toCallback(fn), opts)

then we can promisify "result-first" functions:

const rfpify = require('rfpify')
const EventEmitter3 = require('eventemitter3')
const emitter = new EventEmitter3()

// classic "result-first" case
emitter.on('foo', (a, b) => {
  console.log(a, b) // => 33, 55
})

// good idea is to `bind` the given function
const onPromisified = rfpify(emitter.on.bind(emitter))

onPromisified('foo')
  .then((res) => {
    console.log(res[0], res[1])
    // twice
    // => 33, 55
  })
  .catch(console.error) // => Error if something fails

emitter.emit('foo', 33, 55)
emitter.emit('foo', 33, 55)

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Usage

For more use-cases see the tests

const toCallback = require('to-callback')

API

toCallback

Gets a fn function, that has "result-first" callback and return same function, but with "error-first" callback, like is the convention at node.js land. Such APIs that has "result-first" callbacks are for example the eventemitter's (streams too) .on method, which should have two arguments eventName and "result-first" callback, that gets as many arguments as .emit sends.

Params

• <fn> {Function}: function that has "result-first" callback
• returns {Function}: function with same arguments, but instead of "result-first" callback it has "error-first" callback

Example

var EventEmitter3 = require('eventemitter3')
var toCallback = require('to-callback')
var emitter = new EventEmitter3()

// classic "result-first" case
emitter.on('foo', function (a, b) {
  console.log(a, b) // => 1, 2
})

var onWithCallback = toCallback(emitter.on.bind(emitter))

// node.js's "error-first" style
onWithCallback('foo', function (err, a, b) {
  console.log(err, a, b) // => null, 1, 2
})

emitter.emit('foo', 1, 2)

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Related

• always-done: Handle completion and errors with elegance! Support for streams, callbacks, promises, child processes, async/await and sync functions. A drop-in replacement… more | homepage
• pify: Promisify a callback-style function | homepage
• rfpify: Promisify a result-first callback-style function. | homepage
• to-promise: Convert to promise. | homepage
• try-catch-callback: try/catch block with a callback, used in try-catch-core. Use it when you don't care about asyncness so much and don't… more | homepage
• try-catch-core: Low-level package to handle completion and errors of sync or asynchronous functions, using once and dezalgo libs. Useful for and… more | homepage

Contributing

Pull requests and stars are always welcome. For bugs and feature requests, please create an issue.
But before doing anything, please read the CONTRIBUTING.md guidelines.

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