jquery deferred in .each loop

​​jquery deferred in .each loop​​

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This should be a simple one. I have a function that is called and I need to wait for all the async operations to complete. what I want is something like this...

self.processSchema(data).done(function(results){ //do stuff}); 

The processSchema function loops using $.each and calls an async method.

var processSchema = function(data) {      var def = new $.Deferred();      $.each(table, function()      {          //calls an async SQLitePlugin method          db.executeSql(sql, data, function(tx, results){             def.resolve(results);          }      }       return(def.promise()); } 

This does not seem to work, I am new to $.Deferred so any guidance would be helpful

​​javascript​​​​jquery​​​​loops​​​​deferred​​​ ​​Share​​​ Edit Follow asked Dec 19 '13 at 18:14 ​​​gdex​​ 45511 gold badge44 silver badges99 bronze badges

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• I think you would need a new deferred for every single db.executeSql, and then do a large $.when(deferred1, deferred2...defferedN).then(function(data1, data2...dataN) { }); – ​​Robin Giltner​​​ ​​Dec 19 '13 at 18:17​​
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2 Answers

​​Active​​​​Oldest​​​​Votes​​ 36

You'll need a promise for each iteration

var processSchema = function(data) {      var promises = [];       $.each(table, function() {          var def = new $.Deferred();          db.executeSql(sql, data, function(tx, results){             def.resolve(results);          });          promises.push(def);      });       return $.when.apply(undefined, promises).promise(); } 

​​Share​​​ Edit Follow ​​edited Dec 19 '13 at 18:38​​​ answered Dec 19 '13 at 18:19 ​​​adeneo​​ 291k2626 gold badges358358 silver badges360360 bronze badges

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• adeneo, should that return defcon.promise() ? – ​​gdex​​​ ​​Dec 19 '13 at 18:24​​
• 1 Yes it should! One could return the array of promises, but to keep the syntax of how the OP want's to call the function, I figure one master promise that is returned and then resolved when all the other promises are resolved would be easier, and then pass all the returned data as the arguments array directly ? – ​​adeneo​​​ ​​Dec 19 '13 at 18:28​​
• @Jan Dvorak, so the def.apply will work inside of loop? – ​​gdex​​​ ​​Dec 19 '13 at 18:31​​
• 2 @adeneo actually you can simply return result of ​​$.when​​​, without creating "master" promise? – ​​hawk​​​ ​​Dec 19 '13 at 18:35​​
• 2 @Tomalak ​​$.when.apply(...).promise()​​​ – ​​John Dvorak​​​ ​​Dec 19 '13 at 18:39​​
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​​Show 6 more comments​​ 8

For Functional Programming fiends (like myself), here's a single-expression version of ​​adeneo's answer​​:

var processSchema = function(data) {     return $.when.apply($, $.map(table, function() {         var def = new $.Deferred();         db.executeSql(sql, data, function(tx, results){             def.resolve(results);         });         return def;     })).promise(); }; 

Also I'd like to note that you are iterating over ​​table​​​, but aren't doing anything with each item in the iteration (i.e. the callback in your ​​each​​ has no arguments.) Now, I'm not sure what your goal is, but this doesn't seem right to me :P

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