util.callbackify

A backport of util.callbackify for Node.js 6.0 and up.

Installation

npm install --save util-callbackify

Usage

const callbackify = require('util-callbackify')

async function fn () {
  return 'Hello, World!'
}

const callbackFunction = callbackify(fn)

fn((err, result) => {
  if (err) throw err

  console.log(result)
  //=> Hello, World!
})

API

callbackify(original)

• original <Function> An async function
• Returns: <Function> a callback style function

Takes an async function (or a function that returns a Promise) and returns a function following the error-first callback style, i.e. taking an (err, value) => ... callback as the last argument. In the callback, the first argument will be the rejection reason (or null if the Promise resolved), and the second argument will be the resolved value.

The callback is executed asynchronously, and will have a limited stack trace. If the callback throws, the process will emit an 'uncaughtException' event, and if not handled will exit.

Since null has a special meaning as the first argument to a callback, if a wrapped function rejects a Promise with a falsy value as a reason, the value is wrapped in an Error with the original value stored in a field named reason.

function fn() {
  return Promise.reject(null)
}
const callbackFunction = util.callbackify(fn)

callbackFunction((err, ret) => {
  // When the Promise was rejected with `null` it is wrapped with an Error and
  // the original value is stored in `reason`.
  err && err.hasOwnProperty('reason') && err.reason === null // true
})