How to use Promise to return value from assync Angular function into calling method?

Question

I have list which is filled in for cycle where I calling assync function by thos way:

In For cycle I'm calling

  row.SUCCES_RATE_SINCE = $scope.computeSuccessRateSinceStart(row);

Called function

  // Calculate percentage of a whole from since
    $scope.computeSuccessRateSinceStart = function(row) {

        db = window.sqlitePlugin.openDatabase({name:"callplanner"});
        // GET APPT COUNT
        db.transaction(function(tx) {
        tx.executeSql(sqlQuery, [], function(tx,results){
            // init empty array for results
            for (var i=0; i < results.rows.length; i++){
                row = results.rows.item(i);
                //Udpate date for writeout
                //row.DATE = moment(row.DATE).format('ddd DD.M');
                console.log("row APPT count is " + JSON.stringify(row));
                apptCnt = row.APPT_CNT;
                convCnt = row.CONVERS_CNT;
                dailySuccessRateSince = apptCnt / convCnt * 100;
                console.log("Success rate since is " +dailySuccessRateSince);
                // THIS IS NOT WORKING
                return Math.round(dailySuccessRateSince);

            }
            });
        },function (e) {
                console.log("ERROR: " + e.message);
                $ionicLoading.show({
                    template: $translate.instant('ERROR_DATABASE'),
                    duration:1000
                });
        });

    };

Problem is that computed value is always returned null (return function is executed before value is available in scope).

I'm quite new in Angular but i found that this issue could be solved using promises. Could somebody give me the example how to return value properly?

Many thanks for any help.

EDIT:

Called method is now triggered, but i cannot pass returned value into variable like this:

var test = $scope.computeSuccessRateSinceStart(row).then(function(result){
     //ALERT WITH VALUE WORKS FINE
     alert("Result " + result);
     return result;
    });
    // THIS GIVES ME EMPTY ARRAY {}
    alert("Result " + JSON.stringify(test));

Not even promises allow you to return the value - it still would be asynchronous. You could return a promise for the value, though. — Bergi
– Bergi, Commented Oct 8, 2014 at 12:00
Could You please add some example based on my code? I tried a few tutorials but without luck. — redrom
– redrom, Commented Oct 8, 2014 at 12:03
Mohammad's answer covers it quite well (I've edited it for a bit clarity). And NO, it is outright impossible to pass the "returned value" into a variable, because your code is asynchronous. Do everything in the callback! — Bergi
– Bergi, Commented Oct 8, 2014 at 13:53
Could You add some example of "Do Everything in callback" please? Because in for cycle are called some other functions which are assync too. — redrom
– redrom, Commented Oct 8, 2014 at 16:14
What for cycle? Yes, make that loop a recursive function and call the next step from the callback. — Bergi
– Bergi, Commented Oct 8, 2014 at 16:16

Bergi · Accepted Answer · 2014-10-08 13:51:35Z

2

Why don't you just make your method such that it always returns a promise, and then extract the result from the promise?

$scope.computeSuccessRateSinceStart = function(row) {
    var deferred = $q.defer();

    db = window.sqlitePlugin.openDatabase({name:"callplanner"});
    // GET APPT COUNT
    db.transaction(function(tx) {
        tx.executeSql(sqlQuery, [], function(tx,results){
            // init empty array for results
            for (var i=0; i < results.rows.length; i++){
                row = results.rows.item(i);
                //Udpate date for writeout
                //row.DATE = moment(row.DATE).format('ddd DD.M');
                console.log("row APPT count is " + JSON.stringify(row));
                apptCnt = row.APPT_CNT;
                convCnt = row.CONVERS_CNT;
                dailySuccessRateSince = apptCnt / convCnt * 100;
                console.log("Success rate since is " +dailySuccessRateSince);
                // THIS IS NOW WORKING:
                deferred.resolve(Math.round(dailySuccessRateSince));
            }
        });
    }, function(e) {
        console.log("ERROR: " + e.message);
        deferred.reject(e);
    });

    return deferred.promise;
};

Usage:

$scope.computeSuccessRateSinceStart(row).then(function(result){
    // THIS GIVES THE VALUE:
    alert("Result " + JSON.stringify(test));
    return result;
}, function(e) 
    $ionicLoading.show({
        template: $translate.instant('ERROR_DATABASE'),
        duration:1000
    });
});

edited Oct 8, 2014 at 13:51

Bergi

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answered Oct 8, 2014 at 12:04

ms87

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4 Comments

Bergi Over a year ago

Is it intentional that you didn't fix the problem in your code?

ms87 Over a year ago

If you mean the line below:// THIS IS CALLED WHEN DATA ARE NOT AVAILABLE Where The promise is being resolved regardless of what happened before it, Yes. I thought it's not relevant to the main issue of handling async tasks, but yea, that line shouldn't be there.

ms87 Over a year ago

@redrom The idea is to return a promise or a callback if you're performing an async task. Tbh I don't know how many async calls there are in the code you posted, Try the code in my answer now, I removed that problematic line Bergi mentioned.

redrom Over a year ago

Seems to be triggered right now, but i still cannot pass returned value into variable, please see my updated question.

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