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

Solution 1

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();
}

Solution 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

Comments

[Robin Giltner]

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) { });

[gdex]

adeneo, should that return defcon.promise() ?

[adeneo]

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]

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

[hawk]

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

[Tomalak]

@hawk Because you actually want to return a Promise, not a Deferred. Because Promises are read-only.

[John Dvorak]

@gdex apply takes an array and uses it as a list of arguments. $.when takes a list of promises and returns a promise that resolves when all arguments resolve

[John Dvorak]

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

[adeneo]

@hawk - indeed you could, as chaining done on the function call would be the same as chaining then on the $.when, and as Jan points out, just adding promise() returns the promise and would be the same as the master deferred.

[Milosz]

For what it's worth, it would be safer to pass $ as the context to apply. We don't know if $.when uses this internally, but if it does we want it pointing to $, hence $.when.apply($, promises).promise().

[gdex]

Using the apply().then gives me an argument array of each deferred result which may be helpful in my scenario. The .promise() only returns the last result which is fine in some cases. I appreciate the help!

[adeneo]

@gdex - that's actually true, then() would return an array of results, which could be useful.

[gdex]

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