callbackify

backwards compatibilify your callback functions while migrating apis to promises

usage

var callbackify = require('callbackify')
 
var getUserById = callbackify(function (id) {
  // in this example, we're using [minq](https://npm.im/minq) 
  return db.users.byId(id).first()
})
 
// later in your code, we can use a callback 
 
getUserById(23, function (err, user) {
  if (err) { /* give up */ return }
  console.log('hello, ', user.name)
})
 
// but for newer code, we can consume it as a promise 
 
getUserById(23).then(function (user) {
  console.log('hello, ', user.name)
}, function (err) {
  console.error(err)
})

callbackify will also preserve the this context of your functions:

getUserById.call({foo:true}, 12)
// the underlying promise-returning function is called with the supplied context argument

Normally, callbackify will only work with fixed-parameter-length functions, and will use the declared parameter length to determine if the extra callback argument is present. If you need to use callbackify with variadic functions, or functions that don't declare their full argument list, you can use:

// options argument is optional 
var getUserById = callbackify.variadic(function (id, options) {
  if (options === undefined) { options = {} }
  if (options.select) {
    return db.users.byId(id).select(options.select).first()
  } else {
    return db.users.byId(id).first()
  }
})
 
// we can do either of these 
getUserById(23, function (err, user) { })
getUserById(23, { select: [ 'name' ] }, function (err, user) {} )

Note that this will not work if the last argument your function can take is a function, as that last argument will always be detected as a callback function.

api

`callbackify : (fn: (...args) => Promise<T> ) => (...args, Callback<T>) => Promise<T>`

Takes a Promise-returning function fn and returns a new function which can return a Promise or take a callback as the last parameter. If a callback is supplied, the function returns void. If no callback is supplied, the promise is returned.