Aren t promises just callbacks

Question

I ve been developing JavaScript for a few years and I don t understand the fuss about promises at all   It seems like all I do is change   api function result       api2 function result2           api3 function result3                   do work                           Which I could use a library like async for anyway  with something like   api   then function result        api2   then function result2             api3   then function result3                     do work                              Which is more code and less readable  I didn t gain anything here  it s not suddenly magically  flat  either  Not to mention having to convert things to promises   So  what s the big fuss about promises here

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No  Not at all   Callbacks are simply Functions In JavaScript which are to be called and then executed after the execution of another function has finished  So how it happens    Actually  In JavaScript  functions are itself considered as objects and hence as all other objects  even functions can be sent as arguments to other functions  The most common and generic use case one can think of is setTimeout   function in JavaScript   Promises are nothing but a much more improvised approach of handling and structuring asynchronous code in comparison to doing the same with callbacks    The Promise receives two Callbacks in constructor function   resolve and reject  These callbacks inside promises provide us with fine-grained control over error handling and success cases  The resolve callback is used when the execution of promise performed successfully and the reject callback is used to handle the error cases

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Yes  Promises are asynchronous callbacks  They can t do anything that callbacks can t do  and you face the same problems with asynchrony as with plain callbacks   However  Promises are more than just callbacks  They are a very mighty abstraction  allow cleaner and better  functional code with less error-prone boilerplate      So what s the main idea    Promises are objects representing the result of a single  asynchronous  computation  They resolve to that result only once  There s a few things what this means   Promises implement an observer pattern    You don t need to know the callbacks that will use the value before the task completes   Instead of expecting callbacks as arguments to your functions  you can easily return a Promise object The promise will store the value  and you can transparently add a callback whenever you want  It will be called when the result is available   Transparency  implies that when you have a promise and add a callback to it  it doesn t make a difference to your code whether the result has arrived yet - the API and contracts are the same  simplifying caching memoisation a lot  You can add multiple callbacks easily   Promises are chainable  monadic  if you want     If you need to transform the value that a promise represents  you map a transform function over the promise and get back a new promise that represents the transformed result  You cannot synchronously get the value to use it somehow  but you can easily lift the transformation in the promise context  No boilerplate callbacks  If you want to chain two asynchronous tasks  you can use the  then   method  It will take a callback to be called with the first result  and returns a promise for the result of the promise that the callback returns    Sounds complicated  Time for a code example   var p1   api1       returning a promise var p3   p1 then function api1Result        var p2   api2       returning a promise     return p2     The result of p2                becomes the result of p3     So it does not make a difference whether you write api1   then function api1Result        return api2   then console log        or the flattened version api1   then function api1Result        return api2       then console log    Flattening does not come magically  but you can easily do it  For your heavily nested example  the  near  equivalent would be  api1   then api2  then api3  then    do-work-callback        If seeing the code of these methods helps understanding  here s a most basic promise lib in a few lines      What s the big fuss about promises    The Promise abstraction allows much better composability of functions  For example  next to then for chaining  the all function creates a promise for the combined result of multiple parallel-waiting promises   Last but not least Promises come with integrated error handling  The result of the computation might be that either the promise is fulfilled with a value  or it is rejected with a reason  All the composition functions handle this automatically and propagate errors in promise chains  so that you don t need to care about it explicitly everywhere - in contrast to a plain-callback implementation  In the end  you can add a dedicated error callback for all occurred exceptions      Not to mention having to convert things to promises    That s quite trivial actually with good promise libraries  see How do I convert an existing callback API to promises

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Promises are not callbacks  A promise represents the future result of an asynchronous operation  Of course  writing them the way you do  you get little benefit  But if you write them the way they are meant to be used  you can write asynchronous code in a way that resembles synchronous code and is much more easy to follow   api   then function result       return api2       then function result2       return api3       then function result3           do work       Certainly  not much less code  but much more readable    But this is not the end  Let s discover the true benefits  What if you wanted to check for any error in any of the steps  It would be hell to do it with callbacks  but with promises  is a piece of cake   api   then function result       return api2       then function result2       return api3       then function result3           do work    catch function error           handle any error that may occur before this point       Pretty much the same as a try         catch block    Even better   api   then function result       return api2       then function result2       return api3       then function result3           do work    catch function error           handle any error that may occur before this point    then function            do something whether there was an error or not        like hiding an spinner if you were performing an AJAX request        And even better  What if those 3 calls to api  api2  api3 could run simultaneously  e g  if they were AJAX calls  but you needed to wait for the three  Without promises  you should have to create some sort of counter  With promises  using the ES6 notation  is another piece of cake and pretty neat   Promise all  api    api2    api3     then function result          do work  result is an array contains the values of the three fulfilled promises     catch function error          handle the error  At least one of the promises rejected        Hope you see Promises in a new light now

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No promises are just wrapper on callbacks  example     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  amp  amp  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

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In addition to the awesome answers above  2 more points may be added   1  Semantic difference   Promises may be already resolved upon creation  This means they guarantee conditions rather than events  If they are resolved already  the resolved function passed to it is still called   Conversely  callbacks handle events  So  if the event you are interested in has happened before the callback has been registered  the callback is not called   2  Inversion of control  Callbacks involve inversion of control  When you register a callback function with any API  the Javascript runtime stores the callback function and calls it from the event loop once it is ready to be run    Refer The Javascript Event loop for an explanation   With Promises  control resides with the calling program  The  then   method may be called at any time if we store the promise object

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Promises overview   In JS we can wrap asynchronous operations  e g database calls  AJAX calls  in promises   Usually we want to run some additional logic on the retrieved data  JS promises have handler functions which process the result of the asynchronous operations  The handler functions can even have other asynchronous operations within them which could rely on the value of the previous asynchronous operations   A promise always has of the 3 following states    pending  starting state of every promise  neither fulfilled nor rejected  fulfilled  The operation completed successfully  rejected  The operation failed    A pending promise can be resolved fullfilled or rejected with a value  Then the following handler methods which take callbacks as arguments are called    Promise prototype then     When the promise is resolved the callback argument of this function will be called  Promise prototype catch     When the promise is rejected the callback argument of this function will be called    Although the above methods skill get callback arguments they are far superior than using  only callbacks here is an example that will clarify a lot   Example   x000D   x000D  function createProm resolveVal  rejectVal    x000D      return new Promise  resolve  reject    gt    x000D          setTimeout      gt    x000D              if  Math random    gt  0 5    x000D                  console log  Resolved    x000D                  resolve resolveVal   x000D                else   x000D                  console log  Rejected    x000D                  reject rejectVal   x000D                x000D             1000   x000D          x000D    x000D   x000D  createProm 1  2  x000D       then  resVal    gt    x000D          console log resVal   x000D          return resVal   1  x000D         x000D       then  resVal    gt    x000D          console log resVal   x000D          return resVal   2  x000D         x000D       catch  rejectVal    gt    x000D          console log rejectVal   x000D          return rejectVal   1  x000D         x000D       then  resVal    gt    x000D          console log resVal   x000D         x000D       finally      gt    x000D          console log  Promise done    x000D          x000D   x000D   x000D     The createProm function creates a promises which is resolved or rejected based on a random Nr after 1 second If the promise is resolved the first then method is called and the resolved value is passed in as an argument of the callback If the promise is rejected the first catch method is called and the rejected value is passed in as an argument The catch and then methods return promises that s why we can chain them  They wrap any returned value in Promise resolve and any thrown value  using the throw keyword  in Promise reject  So any value returned is transformed into a promise and on this promise we can again call a handler function  Promise chains give us more fine tuned control and better overview than nested callbacks  For example the catch method handles all the errors which have occurred before the catch handler

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Promises are not callbacks  both are programming idioms that facilitate async programming  Using an async await-style of programming using coroutines or generators that return promises could be considered a 3rd such idiom  A comparison of these idioms across different programming languages  including Javascript  is here  https   github com KjellSchubert promise-future-task

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In addition to the other answers  the ES2015 syntax blends seamlessly with promises  reducing even more boilerplate code      Sequentially  api1      then r1   gt  api2 r1      then r2   gt  api3 r2      then r3   gt             Done           Parallel  Promise all       api1        api2        api3      then   r1  r2  r3     gt           Done

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JavaScript Promises actually use callback functions to determine what to do after a Promise has been resolved or rejected  therefore both are not fundamentally different  The main idea behind Promises is to take callbacks - especially nested callbacks where you want to perform a sort of actions  but it would be more readable

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In addition to the already established answers  with ES6 arrow functions Promises turn from a modestly shining small blue dwarf straight into a red giant  That is about to collapse into a supernova   api   then result   gt  api2    then result2   gt  api3    then result3   gt  console log result3     As oligofren pointed out  without arguments between api calls you don t need the anonymous wrapper functions at all   api   then api2  then api3  then r3   gt  console log r3     And finally  if you want to reach a supermassive black hole level  Promises can be awaited   async function callApis         let api1Result   await api        let api2Result   await api2 api1Result       let api3Result   await api3 api2Result        return api3Result

[javascript] Aren't promises just callbacks?

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