How do I convert an existing callback API to promises?

Question

I want to work with promises but I have a callback API in a format like:

1. DOM load or other one time event:

window.onload; // set to callback
...
window.onload = function() {

};

2. Plain callback:

function request(onChangeHandler) {
    ...
}
request(function() {
    // change happened
    ...
});

3. Node style callback ("nodeback"):

function getStuff(dat, callback) {
    ...
}
getStuff("dataParam", function(err, data) {
    ...
})

4. A whole library with node style callbacks:

API;
API.one(function(err, data) {
    API.two(function(err, data2) {
        API.three(function(err, data3) {
            ...
        });
    });
});

How do I work with the API in promises, how do I "promisify" it?

Solution 1

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.

Solution 2

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.

Solution 3

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)
});

Solution 4

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);

Solution 5

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

View more solutions

Comments

[Roamer-1888]

Benjamin, I accepted your invitation to edit and added a further jQuery example to case 2. It will need peer reviewing before it appears. Hope you like it.

[Benjamin Gruenbaum]

@Roamer-1888 it got rejected since I didn't see and accept it in time. For what it's worth I don't think the addition is too relevant although useful.

[Roamer-1888]

Benjamin, whether or not resolve() and reject() are written to be reusable, I venture that my suggested edit is relevant because it offers a jQuery example of the form $.Deferred(fn), which is otherwise lacking. If only one jQuery example is included, then I suggest that it should be of this form rather than var d = $.Deferred(); etc. as people should be encouraged to use the oft neglected $.Deferred(fn) form, plus, in an answer like this, it puts jQuery more on a par with libs that use the Revealing Constructor Pattern.

[Benjamin Gruenbaum]

Heh, to be 100% fair I didn't know jQuery let you do $.Deferred(fn), if you edit that in instead of the existing example in the next 15 minutes I'm sure I can try to approve it on time :)

[Roamer-1888]

Benjamin, sorry I missed your 15 minute deadline but will try to catch you online soon. I'm sort of pleased my earlier edit wasn't accepted as I've had a chance to amend it to something better. It will need to be an addition, not a replacement.

[fraxture]

What are you referring to when you write "modern promise implementation"? What are you including to be able to use the Promise constructor?

[Benjamin Gruenbaum]

@fraxture pretty much every implementation written after the Promises/A+ spec and the consteuctor spec was born or was altered because of it to match it - these are most promises we know : Q, Bluebird, When, RSVP, $q (since 1.3), native promises, jQuery starting with the next version and so on.

[fraxture]

@BenjaminGruenbaum: got ya. But what do I need to include to use the Promise constructor? Installing Q with npm and then requring it did not work...

[Benjamin Gruenbaum]

With Q it's called Q.Promise - the API docs of Q are good and cover this.

[fraxture]

The answer was that I needed to npm install promise --save and then var Promise = require('promise') in order to be able to new Promise.

[Benjamin Gruenbaum]

Don't use that library - use Bluebied instead.

[Bergi]

The canonical answer already mentions Q.denodeify. Do we need to emphasize library helpers?

[Alnitak]

shouldn't the "already complete" branch use setTimeout(resolve, 0) (or setImmediate, if available) to ensure that it's called asynchronously?

[Ed Sykes]

i found this useful as a google about promisifying in Q leads here

[velop]

What happens if my onX event can happen multiple times. Can this also promisified?

[Benjamin Gruenbaum]

No, if a callback happens multiple times it should not be promisified, promises are for one time things. For things that happen multiple times the easiest thing would be to use an event emitter - more advanced patterns like async iterators (iterators of promises) or observables (push object streams) exist. Promises are only for one time things.

[user1164937]

I have a question about number 3 with nested callbacks. How come I can't simply call reject/resolve? It seems that at least in my case, I have to return them.

[Jeff Bowman]

@Alnitak Calling resolve synchronously is fine. The Promise's then handlers are guaranteed by the framework to be called asynchronously, regardless of whether resolve is called synchronously.

[Akansh]

@BenjaminGruenbaum I want to ask regarding the 3rd heading promisify method, though it converts a callback to promise but is it possible to handle error exactly where I am making it a promise?

[hitautodestruct]

It seems that this answer included everything except the actual usage of a promisified function. I added an example of the load functions use after it was promisified.

[Bruno]

This is a great answer. You may want to update it by mentioning also util.promisify, that Node.js is going to add to its core starting from RC 8.0.0. Its working it's not much different from Bluebird Promise.promisify, but has the advantage of not requiring additional dependencies, in case you just want native Promise. I've written a blog post about util.promisify for anyone who want read more on the topic.

[Benjamin Gruenbaum]

Hey, I (OP) actually suggested util.promisify twice (back in 2014 when this question was written, and a few months ago - which I pushed for as a core member of Node and is the current version we have in Node). Since it is not yet publicly available - I did not add it to this answer yet. We would deeply appreciate usage feedback though and getting to know what some pitfalls are in order to have better docs for the release :)

[Benjamin Gruenbaum]

In addition, you might want to discuss the custom flag for promisifying with util.promisify in your blog post :)

[Bruno]

@BenjaminGruenbaum Do you mean the fact that using the util.promisify.custom symbol it is possible to override the result of util.promisify? To be honest this was an intentional miss, cause I'm not yet able to find a useful use case. Perhaps you can give me some inputs?

[Benjamin Gruenbaum]

Sure, consider APIs like fs.exists or APIs that do not follow the Node convention - a bluebird Promise.promisify would get them wrong, but util.promisify gets them right.

[Benjamin Gruenbaum]

There are already two answers describing this, why post a third one?

[Gian Marco]

Just because that version of node is now released, and I've reported "official" feature description and link.

[Benjamin Gruenbaum]

What advantage does this have over native promisify or over the answers above?

[loretoparisi]

What do you mean for native promisify?

[Benjamin Gruenbaum]

util.promisify(fn)

[loretoparisi]

ah yes of course :). Just and example to show the basic idea. In fact you can see how even the native one requires that the function signature must some defined like (err, value) => ... or you must define a custom one (see Custom promisified functions). Thank you good catcha.

[Dmitri Zaitsev]

These do not seem to show how to convert to promises.

[robe007]

@BenjaminGruenbaum A few days ago, I sent an edit propose to you last code: const promiseAPI = Object.keys(API).map(key => {return {key, fn:util.promisify(API[key])} }).reduce((o, p) => Object.assign(o, {[p.key] : p.fn}), {}); Why did you rejected?

[Benjamin Gruenbaum]

@robe007 probably just butter fingers - sorry! I edited it in now.

[robe007]

@BenjaminGruenbaum Please, see it again, your code still wrong. Look on my edit proposal few days ago.

[Benjamin Gruenbaum]

@robe007 I did - I think the way I fixed it works - there are two ways to return objects from arrow functions: either adding a return to the block body x => { return {}} or to mark it as an expression x => ({}) - both are valid and work. Maybe I'm still missing your point?

[robe007]

@BenjaminGruenbaum Yes I know my friend, but you are missing to close the curly bracket in the returned object {key, fn: util.promisify(API[key])} from your map. You see it?

[robe007]

@BenjaminGruenbaum I have sent another edit proposal again to your last modification.

[Benjamin Gruenbaum]

Hey, I'm not sure what this adds to existing answers (maybe clarify?). Also, there is no need for the try/catch inside the promise constructor (it does this automatically for you). It's also unclear what functions this works for (that call the callback with a single argument on success? How are errors handled?)

[Benjamin Gruenbaum]

That's not a promise, it doesn't chain, deal with errors thrown in the callback or accept a second parameter in then...

[Patrick Roberts]

@loretoparisi FYI, var P = function (fn, ...args) { return new Promise((resolve, reject) => fn.call(this, ...args, (error, result) => error ? reject(error) : resolve(result))); }; would do the same thing as yours and it's a lot simpler.

[jameshfisher]

That load implementation looks unreliable: (1) it overrides onload instead of using addEventListener; (2) it will never resolve if the load event has already happened

[Benjamin Gruenbaum]

Yes that has been discussed see stackoverflow.com/a/27935772/1348195 :)

[Lucio Mollinedo]

@BenjaminGruenbaum I upvoted this because it was the less 'cluttered' and effective one. The one at the top has so many other things that the answer gets lost.

[Philip Stratford]

I'm trying to use this, but if I call promisify(fn, arg1, arg2).then(() => { alert("Done!"); }); the alert is never fired. Would you expect this to work?

[Josiah Nyarega]

Thanks, @Philip Stratford for the question. The promisify is used to convert a function with a callback into a promise. I will update my answer to explain this.

[Josiah Nyarega]

I will be happy to hear any suggestions on this solution, cc @Philip Stratford. Thank you