jquery deferred in .each loop

Question

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

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
– Robin Giltner, Commented Dec 19, 2013 at 18:17

adeneo · Accepted Answer · 2013-12-19 18:38:07Z

37

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();
}

edited Dec 19, 2013 at 18:38

answered Dec 19, 2013 at 18:19

adeneo

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11 Comments

gdex Over a year ago

adeneo, should that return defcon.promise() ?

adeneo Over a year ago

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 ?

gdex Over a year ago

@Jan Dvorak, so the def.apply will work inside of loop?

hawk Over a year ago

@adeneo actually you can simply return result of $.when, without creating "master" promise?

John Dvorak Over a year ago

@Tomalak $.when.apply(...).promise()

|

Community · Accepted Answer · 2017-05-23 12:25:34Z

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

edited May 23, 2017 at 12:25

CommunityBot

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answered Dec 19, 2013 at 18:57

Milosz

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1 Comment

gdex Over a year ago

There is a lot more to the function then I posted, for brevity I did not include some of the non essential stuff...

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