How can jQuery deferred be used

Question

jQuery 1 5 brings the new Deferred object and the attached methods  when   Deferred and   Deferred   For those who haven t used  Deferred before  I ve annotated the source for it   What are the possible usages of these new methods  how do we go about fitting them into patterns   I have already read the API and the source  so I know what it does  My question is how can we use these new features in everyday code   I have a simple example of a buffer class that calls AJAX request in order   Next one start after previous one finishes       Class  Buffer     methods  append        Constructor  takes a function which will be the task handler to be called         append appends a task to the buffer  Buffer will only call a task when the      previous task has finished     var Buffer   function handler        var tasks              empty resolved deferred object     var deferred     when            handle the next object     function handleNextTask                if the current deferred task has resolved and there are more tasks         if  deferred isResolved    amp  amp  tasks length  gt  0                   grab a task             var task   tasks shift                   set the deferred to be deferred returned from the handler             deferred   handler task                  if its not a deferred object then set it to be an empty deferred object             if    deferred  amp  amp  deferred promise                     deferred     when                                 if we have tasks left then handle the next one when the current one                 is done              if  tasks length  gt  0                    deferred done handleNextTask                                         appends a task      this append   function task               add to the array         tasks push task              handle the next task         handleNextTask                I m looking for demonstrations and possible uses of  Deferred and  when   It would also be lovely to see examples of   Deferred    Linking to the new jQuery ajax source for examples is cheating   I am particularly interested in what techniques are available when we abstract away whether an operation is synchronously or asynchronously done

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This is a self-promotional answer  but I spent a few months researching this and presented the results at jQuery Conference San Francisco 2012    Here is a free video of the talk   https   www youtube com watch v juRtEEsHI9E

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1  Use it to ensure an ordered execution of callbacks   var step1   new Deferred    var step2   new Deferred   done function     return step1     var step3   new Deferred   done function     return step2      step1 done function     alert  Step 1       step2 done function     alert  Step 2       step3 done function     alert  All done         now the 3 alerts will also be fired in order of 1 2 3   no matter which Deferred gets resolved first   step2 resolve    step3 resolve    step1 resolve      2  Use it to verify the status of the app   var loggedIn   logUserInNow      deferred var databaseReady   openDatabaseNow      deferred  jQuery when loggedIn  databaseReady  then function         do something

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Here is a slightly different implementation of an AJAX cache as in ehynd s answer   As noted in fortuneRice s follow-up question  ehynd s implementation didn t actually prevent multiple identical requests if the requests were performed before one of them had returned  That is   for  var i 0  i lt 3  i          getData  xxx        will most likely result in 3 AJAX requests if the result for  xxx  has not already been cached before   This can be solved by caching the request s Deferreds instead of the result   var cache        function getData  val            Return a promise from the cache  if available         or create a new one  a jqXHR object  and store it in the cache      var promise   cache val       if   promise            promise     ajax   foo                  data    value  val                dataType   json                      cache val    promise            return promise       when getData  foo    then function resp          do something with the response  which may        or may not have been retreived using an        XHR request

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You can use a deferred object to make a fluid design that works well in webkit browsers  Webkit browsers will fire resize event for each pixel the window is resized  unlike FF and IE which fire the event only once for each resize  As a result  you have no control over the order in which the functions bound to your window resize event will execute  Something like this solves the problem   var resizeQueue   new   Deferred      new is optional but it sure is descriptive resizeQueue resolve     function resizeAlgorithm       some resize code here      window  resize function         resizeQueue done resizeAlgorithm         This will serialize the execution of your code so that it executes as you intended it to  Beware of pitfalls when passing object methods as callbacks to a deferred  Once such method is executed as a callback to deferred  the  this  reference will be overwritten with reference to the deferred object and will no longer refer to the object the method belongs to

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Another example using Deferreds to implement a cache for any kind of computation  typically some performance-intensive or long-running tasks    var ResultsCache   function computationFunction  cacheKeyGenerator        this  cache           this  computationFunction   computationFunction      if  cacheKeyGenerator          this  cacheKeyGenerator   cacheKeyGenerator      ResultsCache prototype compute   function            try to retrieve computation from cache     var cacheKey   this  cacheKeyGenerator apply this  arguments       var promise   this  cache cacheKey           if not yet cached  start computation and store promise in cache      if   promise            var deferred     Deferred            promise   deferred promise            this  cache cacheKey    promise              perform the computation         var args   Array prototype slice call arguments           args push deferred resolve           this  computationFunction apply null  args              return promise         Default cache key generator  works with Booleans  Strings  Numbers and Dates     You will need to create your own key generator if you work with Arrays etc  ResultsCache prototype  cacheKeyGenerator   function args        return Array prototype slice call arguments  join            Here is an example of using this class to perform some  simulated heavy  calculation      The addingMachine will add two numbers var addingMachine   new ResultsCache function a  b  resultHandler        console log  Performing computation  adding     a     and     b          simulate rather long calculation time by using a 1s timeout     setTimeout function             var result   a   b          resultHandler result          1000        addingMachine compute 2  4  then function result        console log  result      result        addingMachine compute 1  1  then function result        console log  result      result           cached result will be used addingMachine compute 2  4  then function result        console log  result      result         The same underlying cache could be used to cache Ajax requests   var ajaxCache   new ResultsCache function id  resultHandler        console log  Performing Ajax request for id      id               getJSON  http   jsfiddle net echo jsonp  callback      value  id   function data            resultHandler data value                ajaxCache compute  anID   then function result        console log  result      result        ajaxCache compute  anotherID   then function result        console log  result      result           cached result will be used ajaxCache compute  anID   then function result        console log  result      result         You can play with the above code in this jsFiddle

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The best use case I can think of is in caching AJAX responses   Here s a modified example from Rebecca Murphey s intro post on the topic   var cache        function getData  val            return either the cached value or jqXHR object wrapped Promise     return   when          cache  val                 ajax   foo                  data    value  val                dataType   json               success  function  resp                    cache  val     resp                                     getData  foo   then function resp          do something with the response  which may        or may not have been retrieved using an        XHR request        Basically  if the value has already been requested once before it s returned immediately from the cache   Otherwise  an AJAX request fetches the data and adds it to the cache   The   when  then doesn t care about any of this  all you need to be concerned about is using the response  which is passed to the  then   handler in both cases  jQuery when   handles a non-Promise Deferred as a Completed one  immediately executing any  done   or  then   on the chain   Deferreds are perfect for when the task may or may not operate asynchronously  and you want to abstract that condition out of the code   Another real world example using the   when helper     when   getJSON   some data       get  template tpl    then function  data  tmpl           tmpl     create a jQuery object out of the template      tmpl data     compile it      appendTo   target       insert it into the DOM

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I ve just used Deferred in real code  In project jQuery Terminal I have function exec that call commands defined by user  like he was entering it and pressing enter   I ve added Deferreds to the API and call exec with arrays  like this   terminal exec  command   then function        terminal echo  command finished          or   terminal exec   command 1    command 2    command 3    then function        terminal echo  all commands finished          the commands can run async code  and exec need to call user code in order  My first api use pair of pause resume calls and in new API I call those automatic when user return promise  So user code can just use  return   get   some url      or  var d   new   Deferred    setTimeout function         d resolve  Hello Deferred       resolve value will be echoed    500   return d promise      I use code like this   exec  function command  silent  deferred        var d      if    isArray command             return   when apply      map command  function command                return self exec command  silent                             both commands executed here  resume will call Term  exec      if  paused               delay command multiple time         d   deferred    new   Deferred            dalyed commands push  command  silent  d            return d promise          else              commands may return promise from user code            it will resolve exec promise when user promise            is resolved         var ret   commands command  silent  true  deferred           if   ret                if  deferred                    deferred resolve self                   return deferred promise                  else                   d   new   Deferred                    ret   d promise                    ret resolve                                    return ret             dalyed commands is used in resume function that call exec again with all dalyed commands   and part of the commands function  I ve stripped not related parts   function commands command  silent  exec  deferred         var position   lines length-1         Call user interpreter function     var result   interpreter interpreter command  self          user code can return a promise     if  result    undefined               new API - auto pause resume when using promises         self pause            return   when result  then function result                   don t echo result if user echo something             if  result  amp  amp  position     lines length-1                    display object result                                resolve promise from exec  This will fire                code if used terminal  exec  command   then             if  deferred                    deferred resolve                              self resume                             this is old API        if command call pause - wait until resume     if  paused            self bind  resume command   function                    exec with resume pause in user code             if  deferred                    deferred resolve                              self unbind  resume command                      else              this should not happen         if  deferred                deferred resolve

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The answer by ehynds will not work  because it caches the responses data  It should cache the jqXHR which is also a Promise  Here is the correct code   var cache        function getData  val            return either the cached value or an        jqXHR object  which contains a promise      return cache  val        ajax   foo              data    value  val            dataType   json           success  function data  textStatus  jqXHR               cache  val     jqXHR                       getData  foo   then function resp          do something with the response  which may        or may not have been retreived using an        XHR request        The answer by Julian D  will work correct and is a better solution

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Another use that I ve been putting to good purpose is fetching data from multiple sources   In the example below  I m fetching multiple  independent JSON schema objects used in an existing application for validation between a client and a REST server   In this case  I don t want the browser-side application to start loading data before it has all the schemas loaded     when apply   then   is perfect for this   Thank to Raynos for pointers on using then fn1  fn2  to monitor for error conditions     fetch sources   function  schema urls        var fetch one   function  url                return   ajax                   url  url                  data                      contentType   application json  charset utf-8                   dataType   json                                 return   map schema urls  fetch one      var promises   fetch sources data  schemas       when apply null  promises  then   function          var schemas     map arguments  function  a            return a 0              start application schemas      function          console log  FAIL   this  arguments

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A deferred can be used in place of a mutex  This is essentially the same as the multiple ajax usage scenarios   MUTEX  var mutex   2   setTimeout function      callback       800    setTimeout function      callback       500    function callback      if  --mutex     0        run code        DEFERRED  function timeout x     var dfd   jQuery Deferred     setTimeout function       dfd resolve        x    return dfd promise       jQuery when  timeout 800   timeout 500   done function         run code       When using a Deferred as a mutex only  watch out for performance impacts  http   jsperf com deferred-vs-mutex 2   Though the convenience  as well as additional benefits supplied by a Deferred is well worth it  and in actual  user driven event based  usage the performance impact should not be noticeable

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You can also integrate it with any 3rd-party libraries which makes use of JQuery   One such library is Backbone  which is actually going to support Deferred in their next version

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