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 Dec 19 '13 at 18:17 — Robin Giltner, Dec 19 '13 at 18:17

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

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();
}

edited Dec 19 '13 at 18:38

answered Dec 19 '13 at 18:19

adeneo

291k2626 gold badges358358 silver badges360360 bronze badges

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

Show 6 more comments

score 8 · Answer 2

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

jquery deferred in .each loop

jquery deferred in .each loop

2 Answers