How do I convert an existing callback API to promises?

Promises have state, they start as pending and can settle to:

fulfilled meaning that the computation completed successfully.

rejected meaning that the computation failed.

Promise returning functions should never throw, they should return rejections instead. Throwing from a promise returning function will force you to use both a } catch { and a .catch. People using promisified APIs do not expect promises to throw. If you're not sure how async APIs work in JS - please see this answer first.

1. DOM load or other one time event:

So, creating promises generally means specifying when they settle - that means when they move to the fulfilled or rejected phase to indicate the data is available (and can be accessed with .then).

With modern promise implementations that support the Promise constructor like native ES6 promises:

function load() {
    return new Promise(function(resolve, reject) {
        window.onload = resolve;
    });
}

You would then use the resulting promise like so:

load().then(function() {
    // Do things after onload
});

With libraries that support deferred (Let's use $q for this example here, but we'll also use jQuery later):

function load() {
    var d = $q.defer();
    window.onload = function() { d.resolve(); };
    return d.promise;
}

Or with a jQuery like API, hooking on an event happening once:

function done() {
    var d = $.Deferred();
    $("#myObject").once("click",function() {
        d.resolve();
    });
    return d.promise();
}

2. Plain callback:

These APIs are rather common since well… callbacks are common in JS. Let's look at the common case of having onSuccess and onFail:

function getUserData(userId, onLoad, onFail) { …

With modern promise implementations that support the Promise constructor like native ES6 promises:

function getUserDataAsync(userId) {
    return new Promise(function(resolve, reject) {
        getUserData(userId, resolve, reject);
    });
}

With libraries that support deferred (Let's use jQuery for this example here, but we've also used $q above):

function getUserDataAsync(userId) {
    var d = $.Deferred();
    getUserData(userId, function(res){ d.resolve(res); }, function(err){ d.reject(err); });
    return d.promise();
}

jQuery also offers a $.Deferred(fn) form, which has the advantage of allowing us to write an expression that emulates very closely the new Promise(fn) form, as follows:

function getUserDataAsync(userId) {
    return $.Deferred(function(dfrd) {
        getUserData(userId, dfrd.resolve, dfrd.reject);
    }).promise();
}

Note: Here we exploit the fact that a jQuery deferred's resolve and reject methods are "detachable"; ie. they are bound to the instance of a jQuery.Deferred(). Not all libs offer this feature.

3. Node style callback ("nodeback"):

Node style callbacks (nodebacks) have a particular format where the callbacks is always the last argument and its first parameter is an error. Let's first promisify one manually:

getStuff("dataParam", function(err, data) { …

To:

function getStuffAsync(param) {
    return new Promise(function(resolve, reject) {
        getStuff(param, function(err, data) {
            if (err !== null) reject(err);
            else resolve(data);
        });
    });
}

With deferreds you can do the following (let's use Q for this example, although Q now supports the new syntax which you should prefer):

function getStuffAsync(param) {
    var d = Q.defer();
    getStuff(param, function(err, data) {
        if (err !== null) d.reject(err);
        else d.resolve(data);
    });
    return d.promise;   
}

In general, you should not promisify things manually too much, most promise libraries that were designed with Node in mind as well as native promises in Node 8+ have a built in method for promisifying nodebacks. For example

var getStuffAsync = Promise.promisify(getStuff); // Bluebird
var getStuffAsync = Q.denodeify(getStuff); // Q
var getStuffAsync = util.promisify(getStuff); // Native promises, node only

4. A whole library with node style callbacks:

There is no golden rule here, you promisify them one by one. However, some promise implementations allow you to do this in bulk, for example in Bluebird, converting a nodeback API to a promise API is as simple as:

Promise.promisifyAll(API);

Or with native promises in Node:

const { promisify } = require('util');
const promiseAPI = Object.entries(API).map(([key, v]) => ({key, fn: promisify(v)}))
                         .reduce((o, p) => Object.assign(o, {[p.key]: p.fn}), {});

Notes:

Of course, when you are in a .then handler you do not need to promisify things. Returning a promise from a .then handler will resolve or reject with that promise's value. Throwing from a .then handler is also good practice and will reject the promise - this is the famous promise throw safety.

In an actual onload case, you should use addEventListener rather than onX.

---

Today, I can use Promise in Node.js as a plain Javascript method.

A simple and basic example to Promise (with KISS way):

Plain Javascript Async API code:

function divisionAPI (number, divider, successCallback, errorCallback) {

    if (divider == 0) {
        return errorCallback( new Error("Division by zero") )
    }

    successCallback( number / divider )

}

Promise Javascript Async API code:

function divisionAPI (number, divider) {

    return new Promise(function (fulfilled, rejected) {

        if (divider == 0) {
            return rejected( new Error("Division by zero") )
        }

        fulfilled( number / divider )

     })

}

(I recommend visiting this beautiful source)

Also Promise can be used with together async\await in ES7 to make the program flow wait for a fullfiled result like the following:

function getName () {

    return new Promise(function (fulfilled, rejected) {

        var name = "John Doe";

        // wait 3000 milliseconds before calling fulfilled() method
        setTimeout ( 
            function() {
                fulfilled( name )
            }, 
            3000
        )

    })

}


async function foo () {

    var name = await getName(); // awaits for a fulfilled result!

    console.log(name); // the console writes "John Doe" after 3000 milliseconds

}


foo() // calling the foo() method to run the code

Another usage with the same code by using .then() method

function getName () {

    return new Promise(function (fulfilled, rejected) {

        var name = "John Doe";

        // wait 3000 milliseconds before calling fulfilled() method
        setTimeout ( 
            function() {
                fulfilled( name )
            }, 
            3000
        )

    })

}


// the console writes "John Doe" after 3000 milliseconds
getName().then(function(name){ console.log(name) })

Promise can also be used on any platform that is based on Node.js like react-native.

Bonus: An hybrid method (The callback method is assumed to have two parameters as error and result)

function divisionAPI (number, divider, callback) {

    return new Promise(function (fulfilled, rejected) {

        if (divider == 0) {
            let error = new Error("Division by zero")
            callback && callback( error )
            return rejected( error )
        }

        let result = number / divider
        callback && callback( null, result )
        fulfilled( result )

     })

}

The above method can respond result for old fashion callback and Promise usages.

Hope this helps.

---

Before converting a function as promise In Node.JS

var request = require('request'); //http wrapped module

function requestWrapper(url, callback) {
    request.get(url, function (err, response) {
      if (err) {
        callback(err);
      }else{
        callback(null, response);             
      }      
    })
}


requestWrapper(url, function (err, response) {
    console.log(err, response)
})

After Converting It

var request = require('request');

function requestWrapper(url) {
  return new Promise(function (resolve, reject) { //returning promise
    request.get(url, function (err, response) {
      if (err) {
        reject(err); //promise reject
      }else{
        resolve(response); //promise resolve
      }
    })
  })
}


requestWrapper('http://localhost:8080/promise_request/1').then(function(response){
    console.log(response) //resolve callback(success)
}).catch(function(error){
    console.log(error) //reject callback(failure)
})

Incase you need to handle multiple request

var allRequests = [];
allRequests.push(requestWrapper('http://localhost:8080/promise_request/1')) 
allRequests.push(requestWrapper('http://localhost:8080/promise_request/2'))
allRequests.push(requestWrapper('http://localhost:8080/promise_request/5'))    

Promise.all(allRequests).then(function (results) {
  console.log(results);//result will be array which contains each promise response
}).catch(function (err) {
  console.log(err)
});

---

I don't think the window.onload suggestion by @Benjamin will work all the time, as it doesn't detect whether it is called after the load. I have been bitten by that many times. Here is a version which should always work:

function promiseDOMready() {
    return new Promise(function(resolve) {
        if (document.readyState === "complete") return resolve();
        document.addEventListener("DOMContentLoaded", resolve);
    });
}
promiseDOMready().then(initOnLoad);

---

Node.js 8.0.0 includes a new util.promisify() API that allows standard Node.js callback style APIs to be wrapped in a function that returns a Promise. An example use of util.promisify() is shown below.

const fs = require('fs');
const util = require('util');

const readFile = util.promisify(fs.readFile);

readFile('/some/file')
  .then((data) => { /* ... */ })
  .catch((err) => { /* ... */ });

See Improved support for Promises

---

A simple generic function I normally use.

const promisify = (fn, ...args) => {
  return new Promise((resolve, reject) => {
    fn(...args, (err, data) => {
      if (err) {
        return reject(err);
      }
      resolve(data);
    });
  });
};

How to use it

The function promisify accepts a function with a callback:

   const cb = (result) => `The result is ${result}`;

   const sum = (a, b, cb) => {
    const result = a + b;
    cb(result); // passing args to the callback function
   }


  // using the util
  promise = promisify(sum, 3, 1, cb);
  promise.then(x => console.log(x)) // 4

You are probably not looking for this answer, but this will help understand the inner workings of the available utils

---

In release candidate for Node.js 8.0.0, there's a new utility, util.promisify (I've written about util.promisify), that encapsulates the capacity of promisifying whatever function.

It is not much different from the approaches suggested in the other answers, but has the advantage of being a core method, and not requiring additional dependencies.

const fs = require('fs');
const util = require('util');

const readFile = util.promisify(fs.readFile);

Then you've a readFile method that returns a native Promise.

readFile('./notes.txt')
  .then(txt => console.log(txt))
  .catch(...);

---

You can use JavaScript native promises with Node JS.

My Cloud 9 code link: https://ide.c9.io/adx2803/native-promises-in-node

/**
* Created by dixit-lab on 20/6/16.
*/

var express = require('express');
var request = require('request');   //Simplified HTTP request client.


var app = express();

function promisify(url) {
    return new Promise(function (resolve, reject) {
        request.get(url, function (error, response, body) {
            if (!error && response.statusCode == 200) {
                resolve(body);
            }
            else {
                reject(error);
            }
        })
    });
}

//get all the albums of a user who have posted post 100
app.get('/listAlbums', function (req, res) {
    //get the post with post id 100
    promisify('http://jsonplaceholder.typicode.com/posts/100').then(function (result) {
        var obj = JSON.parse(result);
        return promisify('http://jsonplaceholder.typicode.com/users/' + obj.userId + '/albums')
    })
    .catch(function (e) {
        console.log(e);
    })
    .then(function (result) {
        res.end(result);
    })
})

var server = app.listen(8081, function () {
    var host = server.address().address
    var port = server.address().port

    console.log("Example app listening at http://%s:%s", host, port)
})

//run webservice on browser : http://localhost:8081/listAlbums

---

With plain old vanilla javaScript, here's a solution to promisify an api callback.

function get(url, callback) {
        var xhr = new XMLHttpRequest();
        xhr.open('get', url);
        xhr.addEventListener('readystatechange', function () {
            if (xhr.readyState === 4) {
                if (xhr.status === 200) {
                    console.log('successful ... should call callback ... ');
                    callback(null, JSON.parse(xhr.responseText));
                } else {
                    console.log('error ... callback with error data ... ');
                    callback(xhr, null);
                }
            }
        });
        xhr.send();
    }

/**
     * @function promisify: convert api based callbacks to promises
     * @description takes in a factory function and promisifies it
     * @params {function} input function to promisify
     * @params {array} an array of inputs to the function to be promisified
     * @return {function} promisified function
     * */
    function promisify(fn) {
        return function () {
            var args = Array.prototype.slice.call(arguments);
            return new Promise(function(resolve, reject) {
                fn.apply(null, args.concat(function (err, result) {
                    if (err) reject(err);
                    else resolve(result);
                }));
            });
        }
    }

var get_promisified = promisify(get);
var promise = get_promisified('some_url');
promise.then(function (data) {
        // corresponds to the resolve function
        console.log('successful operation: ', data);
}, function (error) {
        console.log(error);
});

---

The Q library by kriskowal includes callback-to-promise functions. A method like this:

obj.prototype.dosomething(params, cb) {
  ...blah blah...
  cb(error, results);
}

can be converted with Q.ninvoke

Q.ninvoke(obj,"dosomething",params).
then(function(results) {
});

---

When you have a few functions that take a callback and you want them to return a promise instead you can use this function to do the conversion.

function callbackToPromise(func){

    return function(){

        // change this to use what ever promise lib you are using
        // In this case i'm using angular $q that I exposed on a util module

        var defered = util.$q.defer();

        var cb = (val) => {
            defered.resolve(val);
        }

        var args = Array.prototype.slice.call(arguments);
        args.push(cb);    
        func.apply(this, args);

        return defered.promise;
    }
}

---

Under node v7.6+ which has built in promises and async:

// promisify.js
let promisify = fn => (...args) =>
    new Promise((resolve, reject) =>
        fn(...args, (err, result) => {
            if (err) return reject(err);
            return resolve(result);
        })
    );

module.exports = promisify;

How to use:

let readdir = require('fs').readdir;
let promisify = require('./promisify');
let readdirP = promisify(readdir);

async function myAsyncFn(path) {
    let entries = await readdirP(path);
    return entries;
}

---

In Node.js 8 you can promisify object methods on the fly using this npm module:

https://www.npmjs.com/package/doasync

It uses util.promisify and Proxies so that your objects stay unchanged. Memoization is also done with the use of WeakMaps). Here are some examples:

With objects:

const fs = require('fs');
const doAsync = require('doasync');

doAsync(fs).readFile('package.json', 'utf8')
  .then(result => {
    console.dir(JSON.parse(result), {colors: true});
  });

With functions:

doAsync(request)('http://www.google.com')
  .then(({body}) => {
    console.log(body);
    // ...
  });

You can even use native call and apply to bind some context:

doAsync(myFunc).apply(context, params)
  .then(result => { /*...*/ });

---

You can use native Promise in ES6, for exemple dealing with setTimeout:

enqueue(data) {

    const queue = this;
    // returns the Promise
    return new Promise(function (resolve, reject) {
        setTimeout(()=> {
                queue.source.push(data);
                resolve(queue); //call native resolve when finish
            }
            , 10); // resolve() will be called in 10 ms
    });

}

In this exemple, the Promise has no reason to fail, so reject() is never called.

---

The callback style function always like this(almost all function in node.js is this style):

//fs.readdir(path[, options], callback)
fs.readdir('mypath',(err,files)=>console.log(files))

This style has same feature:

the callback function is passed by last argument. the callback function always accept the error object as it's first argument.

So, you could write a function for convert a function with this style like this:

const R =require('ramda')

/**
 * A convenient function for handle error in callback function.
 * Accept two function res(resolve) and rej(reject) ,
 * return a wrap function that accept a list arguments,
 * the first argument as error, if error is null,
 * the res function will call,else the rej function.
 * @param {function} res the function which will call when no error throw
 * @param {function} rej the function which will call when  error occur
 * @return {function} return a function that accept a list arguments,
 * the first argument as error, if error is null, the res function
 * will call,else the rej function
 **/
const checkErr = (res, rej) => (err, ...data) => R.ifElse(
    R.propEq('err', null),
    R.compose(
        res,
        R.prop('data')
    ),
    R.compose(
        rej,
        R.prop('err')
    )
)({err, data})

/**
 * wrap the callback style function to Promise style function,
 * the callback style function must restrict by convention:
 * 1. the function must put the callback function where the last of arguments,
 * such as (arg1,arg2,arg3,arg...,callback)
 * 2. the callback function must call as callback(err,arg1，arg2,arg...)
 * @param {function} fun the callback style function to transform
 * @return {function} return the new function that will return a Promise,
 * while the origin function throw a error, the Promise will be Promise.reject(error),
 * while the origin function work fine, the Promise will be Promise.resolve(args: array),
 * the args is which callback function accept
 * */
 const toPromise = (fun) => (...args) => new Promise(
    (res, rej) => R.apply(
        fun,
        R.append(
            checkErr(res, rej),
            args
        )
    )
)

For more concise, above example used ramda.js. Ramda.js is a excellent library for functional programming. In above code, we used it's apply(like javascript function.prototype.apply) and append(like javascript function.prototype.push ). So, we could convert the a callback style function to promise style function now:

const {readdir} = require('fs')
const readdirP = toPromise(readdir)
readdir(Path)
    .then(
        (files) => console.log(files),
        (err) => console.log(err)
    )

toPromise and checkErr function is own by berserk library, it's a functional programming library fork by ramda.js(create by me).

Hope this answer is useful for you.

---

es6-promisify converts callback-based functions to Promise-based functions.

const promisify = require('es6-promisify');

const promisedFn = promisify(callbackedFn, args);

Ref: https://www.npmjs.com/package/es6-promisify

---

You can do something like this

// @flow

const toPromise = (f: (any) => void) => {
  return new Promise<any>((resolve, reject) => {
    try {
      f((result) => {
        resolve(result)
      })
    } catch (e) {
      reject(e)
    }
  })
}

export default toPromise

Then use it

async loadData() {
  const friends = await toPromise(FriendsManager.loadFriends)

  console.log(friends)
}

---

My promisify version of a callback function is the P function:

var P = function() { var self = this; var method = arguments[0]; var params = Array.prototype.slice.call(arguments, 1); return new Promise((resolve, reject) => { if (method && typeof(method) == 'function') { params.push(function(err, state) { if (!err) return resolve(state) else return reject(err); }); method.apply(self, params); } else return reject(new Error('not a function')); }); } var callback = function(par, callback) { var rnd = Math.floor(Math.random() * 2) + 1; return rnd > 1 ? callback(null, par) : callback(new Error("trap")); } callback("callback", (err, state) => err ? console.error(err) : console.log(state)) callback("callback", (err, state) => err ? console.error(err) : console.log(state)) callback("callback", (err, state) => err ? console.error(err) : console.log(state)) callback("callback", (err, state) => err ? console.error(err) : console.log(state)) P(callback, "promise").then(v => console.log(v)).catch(e => console.error(e)) P(callback, "promise").then(v => console.log(v)).catch(e => console.error(e)) P(callback, "promise").then(v => console.log(v)).catch(e => console.error(e)) P(callback, "promise").then(v => console.log(v)).catch(e => console.error(e))

The P function requires that the callback signature must be callback(error,result).

---

Below is the implementation of how a function (callback API) can be converted to a promise.

function promisify(functionToExec) {
  return function() {
    var array = Object.values(arguments);
    return new Promise((resolve, reject) => {
      array.push(resolve)
      try {
         functionToExec.apply(null, array);
      } catch (error) {
         reject(error)
      }
    })
  }
}

// USE SCENARIO

function apiFunction (path, callback) { // Not a promise
  // Logic
}

var promisedFunction = promisify(apiFunction);

promisedFunction('path').then(()=>{
  // Receive the result here (callback)
})

// Or use it with await like this
let result = await promisedFunction('path');

---

Perhaps already answered, but this is how I do it typically:

// given you've defined this `Future` fn somewhere:
const Future = fn => {return new Promise((r,t) => fn(r,t))}

// define an eventFn that takes a promise `resolver`
const eventFn = resolve => {
  // do event related closure actions here. When finally done, call `resolve()`
  something.oneventfired = e => {resolve(e)}
}

// invoke eventFn in an `async` workflowFn using `Future`
// to obtain a `promise` wrapper
const workflowFn = async () => {await Future(eventFn)}

Especially for things like indexedDb event wrappers to simplify usage.

Or you might find this variation of Future to be more general purpose

class PromiseEx extends Promise {
  resolve(v,...a) {
    this.settled = true; this.settledValue = v;
    return(this.resolve_(v,...a))
  }
  reject(v,...a) {
    this.settled = false; this.settledValue = v;
    return(this.reject_(v,...a))
  }
  static Future(fn,...args) {
    let r,t,ft = new PromiseEx((r_,t_) => {r=r_;t=t_})
    ft.resolve_ = r; ft.reject_ = t; fn(ft,...args);
    return(ft)
  }
}

---

Necromancing a little, bit this link may be useful....

TLDR; look at the snippet example at the end of this answer

write/convert functions that can be called expecting

a cb(error,result) or new Promise (...) format

promiseToCB converts and exports an existing function that's been previously coded to return a promise

cbToPromise converts and exports an existing function that's been previously coded to call the last argument with (error,result) if wrapped function supplies more than 1 result, the result will be an array of results eg cb(undefined,path,stat) ---> resolve([path,stat]) / cb(undefined,[path,stat])

if wrapped function supplies more than 1 result, the result will be an array of results

eg cb(undefined,path,stat) ---> resolve([path,stat]) / cb(undefined,[path,stat])

asPromise lets you code a new function to return a promise, but it can be called either way

asCallback lets you code a new function to call cb(err,result), but it can be invoked either way

sample functions

each sample takes 2 arguments, and resolves/rejects/errors based on random number.

arg2 can be also be used to force pass or fail. (looks for "-pass" or "-fail").

wrap existing functions

exports the function to current "this" (or use promiseToCB(function myFunc(){},newThis); )

    promiseToCB(function sampleFunc1(arg1,arg2) {
        console.log("deciding:",arg1,arg2);
        return new Promise(function(resolve,reject){
       
           const timer = setTimeout(function(){reject([arg1,arg2,"ouch"].join("-"));},5000);
           
           setTimeout(function(){
               if (arg2.endsWith("-pass") || (!arg2.endsWith("-fail") && Math.random()<0.5)) {
    
                   console.log("complete:",arg1,arg2);
                   clearTimeout(timer);
                   resolve([arg1,arg2,"all good"].join("-"));
               }
           },2000);
        
        });
    });
    
    cbToPromise('sampleFunc2',function someOtherName(arg1,arg2,cb) {
       console.log("deciding:",arg1,arg2);
       const timer = setTimeout(function(){cb([arg1,arg2,"ouch"].join("-"));},5000);
       
       setTimeout(function(){
           if (arg2.endsWith("-pass") || (!arg2.endsWith("-fail") && Math.random()<0.5)) {
               console.log("complete:",arg1,arg2);
               clearTimeout(timer);
               cb(undefined,[arg1,arg2,"all good"].join("-"));
           }
       },2000);
        
    },local);
    

or code new functions, which embed a wrapper.

     function sampleFunc3(arg1,arg2) {return asPromise(arguments,function(resolve,reject){
       console.log("deciding:",arg1,arg2);
       const timer = setTimeout(function(){reject([arg1,arg2,"ouch"].join("-"));},5000);
       
       setTimeout(function(){
           if (arg2.endsWith("-pass") || (!arg2.endsWith("-fail") && Math.random()<0.5)) {
               console.log("complete:",arg1,arg2);
               clearTimeout(timer);
               resolve([arg1,arg2,"all good"].join("-"));
           }
       },2000);
        
    });}
    
    function sampleFunc4(arg1,arg2) {return asCallback(arguments,function(cb){
       console.log("deciding:",arg1,arg2);
       const timer = setTimeout(function(){cb([arg1,arg2,"ouch"].join("-"));},5000);
       
       setTimeout(function(){
           if (arg2.endsWith("-pass") || (!arg2.endsWith("-fail") && Math.random()<0.5)) {
               console.log("complete:",arg1,arg2);
               clearTimeout(timer);
               cb(undefined,[arg1,arg2,"all good"].join("-"));
           }
       },2000);
        
    });}

test scipt for above functions

    const local = {}; 
    promiseToCB(function sampleFunc1(arg1,arg2) {
        console.log("deciding:",arg1,arg2);
        return new Promise(function(resolve,reject){
       
           const timer = setTimeout(function(){reject([arg1,arg2,"ouch"].join("-"));},5000);
           
           setTimeout(function(){
               if (arg2.endsWith("-pass") || (!arg2.endsWith("-fail") && Math.random()<0.5)) {
    
                   console.log("complete:",arg1,arg2);
                   clearTimeout(timer);
                   resolve([arg1,arg2,"all good"].join("-"));
               }
           },2000);
        
        });
    });
    
    cbToPromise('sampleFunc2',function someOtherName(arg1,arg2,cb) {
       console.log("deciding:",arg1,arg2);
       const timer = setTimeout(function(){cb([arg1,arg2,"ouch"].join("-"));},5000);
       
       setTimeout(function(){
           if (arg2.endsWith("-pass") || (!arg2.endsWith("-fail") && Math.random()<0.5)) {
               console.log("complete:",arg1,arg2);
               clearTimeout(timer);
               cb(undefined,[arg1,arg2,"all good"].join("-"));
           }
       },2000);
        
    },local);
    
    function sampleFunc3(arg1,arg2) {return asPromise(arguments,function(resolve,reject){
       console.log("deciding:",arg1,arg2);
       const timer = setTimeout(function(){reject([arg1,arg2,"ouch"].join("-"));},5000);
       
       setTimeout(function(){
           if (arg2.endsWith("-pass") || (!arg2.endsWith("-fail") && Math.random()<0.5)) {
               console.log("complete:",arg1,arg2);
               clearTimeout(timer);
               resolve([arg1,arg2,"all good"].join("-"));
           }
       },2000);
        
    });}
    
    function sampleFunc4(arg1,arg2) {return asCallback(arguments,function(cb){
       console.log("deciding:",arg1,arg2);
       const timer = setTimeout(function(){cb([arg1,arg2,"ouch"].join("-"));},5000);
       
       setTimeout(function(){
           if (arg2.endsWith("-pass") || (!arg2.endsWith("-fail") && Math.random()<0.5)) {
               console.log("complete:",arg1,arg2);
               clearTimeout(timer);
               cb(undefined,[arg1,arg2,"all good"].join("-"));
           }
       },2000);
        
    });}
    
    const log=console.log.bind(console),info=console.info.bind(console),error=console.error.bind(console);
    
    sampleFunc1("sample1","promise").then (log).catch(error);
    local.sampleFunc2("sample2","promise").then (log).catch(error);
    sampleFunc3("sample3","promise").then (log).catch(error);
    sampleFunc4("sample4","promise").then (log).catch(error);

    sampleFunc1("sample1","callback",info);
    local.sampleFunc2("sample2","callback",info);
    sampleFunc3("sample3","callback",info);
    sampleFunc4("sample4","callback",info);
    
    sampleFunc1("sample1","promise-pass").then (log).catch(error);
    local.sampleFunc2("sample2","promise-pass").then (log).catch(error);
    sampleFunc3("sample3","promise-pass").then (log).catch(error);
    sampleFunc4("sample4","promise-pass").then (log).catch(error);

    sampleFunc1("sample1","callback-pass",info);
    local.sampleFunc2("sample2","callback-pass",info);
    sampleFunc3("sample3","callback-pass",info);
    sampleFunc4("sample4","callback-pass",info);
    
    
    sampleFunc1("sample1","promise-fail").then (log).catch(error);
    local.sampleFunc2("sample2","promise-fail").then (log).catch(error);
    sampleFunc3("sample3","promise-fail").then (log).catch(error);
    sampleFunc4("sample4","promise-fail").then (log).catch(error);
    
    sampleFunc1("sample1","callback-fail",info);
    local.sampleFunc2("sample2","callback-fail",info);
    sampleFunc3("sample3","callback-fail",info);
    sampleFunc4("sample4","callback-fail",info);
 

var cpArgs = Array.prototype.slice.call.bind(Array.prototype.slice); function promiseToCB (nm,fn,THIS) { if (typeof nm==='function') { THIS=fn;fn=nm;nm=fn.name; } THIS=THIS||this; const func = function () { let args = cpArgs(arguments); if (typeof args[args.length-1]==='function') { const cb = args.pop(); return fn.apply(THIS,args).then(function(r){ cb (undefined,r); }).catch(cb); } else { return fn.apply(THIS,args); } }; Object.defineProperty(func,'name',{value:nm,enumerable:false,configurable: true}); if (THIS[nm]) delete THIS[nm]; Object.defineProperty(THIS,nm,{value:func,enumerable:false,configurable: true}); return func; } function cbToPromise (nm,fn,THIS) { if (typeof nm==='function') { THIS=fn;fn=nm;nm=fn.name; } THIS=THIS||this; const func = function () { let args = cpArgs(arguments); if (typeof args[args.length-1]==='function') { return fn.apply(THIS,args); } else { return new Promise(function(resolve,reject){ args.push(function(err,result){ if (err) return reject(err); if (arguments.length==2) { return resolve(result); } return resolve(cpArgs(arguments,1)); }); fn.apply(THIS,args); }); } }; Object.defineProperty(func,'name',{value:nm,enumerable:false,configurable: true}); if (THIS[nm]) delete THIS[nm]; Object.defineProperty(THIS,nm,{value:func,enumerable:false,configurable: true}); return func; } function asPromise (args,resolver,no_err) { const cb = args[args.length-1], promise = new Promise(resolver); return (typeof cb==='function') ? promise.then(function(result){return cb(no_err,result)}).catch(cb) : promise; } function asCallback (args,wrap,no_err) { const cb = args[args.length-1], promise=new Promise(function resolver(resolve,reject) { return wrap (function (err,result) { if (err) return reject(err); resolve(result); }); }); return (typeof cb==='function') ? promise.then(function(result){return cb(no_err,result)}).catch(cb) : promise; } function cbPromiseTest(){ /*global sampleFunc1,sampleFunc2*/ const local = {}; promiseToCB(function sampleFunc1(arg1,arg2) { console.log("deciding:",arg1,arg2); return new Promise(function(resolve,reject){ const timer = setTimeout(function(){reject([arg1,arg2,"ouch"].join("-"));},5000); setTimeout(function(){ if (arg2.endsWith("-pass") || (!arg2.endsWith("-fail") && Math.random()<0.5)) { console.log("complete:",arg1,arg2); clearTimeout(timer); resolve([arg1,arg2,"all good"].join("-")); } },2000); }); }); cbToPromise('sampleFunc2',function someOtherName(arg1,arg2,cb) { console.log("deciding:",arg1,arg2); const timer = setTimeout(function(){cb([arg1,arg2,"ouch"].join("-"));},5000); setTimeout(function(){ if (arg2.endsWith("-pass") || (!arg2.endsWith("-fail") && Math.random()<0.5)) { console.log("complete:",arg1,arg2); clearTimeout(timer); cb(undefined,[arg1,arg2,"all good"].join("-")); } },2000); },local); function sampleFunc3(arg1,arg2) {return asPromise(arguments,function(resolve,reject){ console.log("deciding:",arg1,arg2); const timer = setTimeout(function(){reject([arg1,arg2,"ouch"].join("-"));},5000); setTimeout(function(){ if (arg2.endsWith("-pass") || (!arg2.endsWith("-fail") && Math.random()<0.5)) { console.log("complete:",arg1,arg2); clearTimeout(timer); resolve([arg1,arg2,"all good"].join("-")); } },2000); });} function sampleFunc4(arg1,arg2) {return asCallback(arguments,function(cb){ console.log("deciding:",arg1,arg2); const timer = setTimeout(function(){cb([arg1,arg2,"ouch"].join("-"));},5000); setTimeout(function(){ if (arg2.endsWith("-pass") || (!arg2.endsWith("-fail") && Math.random()<0.5)) { console.log("complete:",arg1,arg2); clearTimeout(timer); cb(undefined,[arg1,arg2,"all good"].join("-")); } },2000); });} const log=console.log.bind(console),info=console.info.bind(console),error=console.error.bind(console); sampleFunc1("sample1","promise").then (log).catch(error); local.sampleFunc2("sample2","promise").then (log).catch(error); sampleFunc3("sample3","promise").then (log).catch(error); sampleFunc4("sample4","promise").then (log).catch(error); sampleFunc1("sample1","callback",info); local.sampleFunc2("sample2","callback",info); sampleFunc3("sample3","callback",info); sampleFunc4("sample4","callback",info); sampleFunc1("sample1","promise-pass").then (log).catch(error); local.sampleFunc2("sample2","promise-pass").then (log).catch(error); sampleFunc3("sample3","promise-pass").then (log).catch(error); sampleFunc4("sample4","promise-pass").then (log).catch(error); sampleFunc1("sample1","callback-pass",info); local.sampleFunc2("sample2","callback-pass",info); sampleFunc3("sample3","callback-pass",info); sampleFunc4("sample4","callback-pass",info); sampleFunc1("sample1","promise-fail").then (log).catch(error); local.sampleFunc2("sample2","promise-fail").then (log).catch(error); sampleFunc3("sample3","promise-fail").then (log).catch(error); sampleFunc4("sample4","promise-fail").then (log).catch(error); sampleFunc1("sample1","callback-fail",info); local.sampleFunc2("sample2","callback-fail",info); sampleFunc3("sample3","callback-fail",info); sampleFunc4("sample4","callback-fail",info); } cbPromiseTest();

---

Promises always have a resolve and a reject. When you write an async wrapper, just call the resolve and there you go.

You can write a wrapper function for pretty much any function taking a callback like so:

const myAsyncWrapper = (...params) =>
  new Promise((resolve, reject) => 
    someFunctionWithCallback(...params, (error, response) =>
      error ? reject(error) : resolve(response)
    )
  );

You can take this further to write a conversion function of callbacks to promises:

const promisify =
  (functionWithCallback) =>
  (...params) =>
    new Promise((resolve, reject) =>
      functionWithCallback(...params, (error, response) =>
        error ? reject(error) : resolve(response)
      )
    );

This concept of wrapper functions is especially useful when using older libraries or SDKs. For example, consider the Facebook Graph API's JavaScript SDK, which uses a similar callback structure for making API requests.

FB.api(apiURL, options, function (request) {
  if (request.error || !request) return;
  // handle request
});

In modern applications, it is much more useful to use a promise-based API. If you use a function only once or twice, it may be better to promisify the response individually:

// in an async function
const response = await new Promise((resolve, reject) =>
  FB.api(apiURL, (res) => (res?.error ? reject(res?.error) : resolve(res)))
);

If you use the function a lot, you can use the same wrapper concept to write a function like so:

const apiWrapper = (...params) =>
  new Promise((resolve, reject) => 
    FB.api(...params, (res) => (res?.error ? reject(res?.error) : resolve(res)))
  );

Although promisifiers are great sometimes, they won't work for specific instances like this. In times like these, look on Github for a modern wrapper, or write your own like this.

---

since we know the characteristics of a callback-based function in advance, we can create a function that transforms a callback-based function into an equivalent function returning a Promise.

The callback is the last argument of the function

If there is an error, it always the first argument passed to the callback

Any return value is passed after the error to the callback function promisify(yourCallbackApi) { return function promisified(...args) { return new Promise((resolve, reject) => { // newArgs=[..args,callback] const newArgs = [ ...args, function (err, result) { if (err) { return reject(err); } resolve(result); }, ]; // invoke yourCallbackApi with the new list of arguments yourCallbackApi(...newArgs); }); }; }

---

It is like 5 years late, but I wanted to post here my promesify version which takes functions from callbacks API and turns them into promises

const promesify = fn => {
  return (...params) => ({
    then: cbThen => ({
      catch: cbCatch => {
        fn(...params, cbThen, cbCatch);
      }
    })
  });
};

Take a look to this very simple version here: https://gist.github.com/jdtorregrosas/aeee96dd07558a5d18db1ff02f31e21a

---