Imagine i have a simple javascript function:

  function someFunction(integer)
  {
    data = integer + 1;
    return data;
  }

I need to call this from inside another function and use the returned value:

  function anotherFunction(integer)
  {
    int_plus_one = someFunction(integer);
    //Do something with the returned data...
    int_plus_two = int_plus_one + 1;
    return int_plus_two;
  }

How can i ensure that the return of anotherFunction return is only returned after someFunction pletes? It actually seems to work ok with very fast functions like these. However if someFunction has to do some ajax lookups, the return of aotherFunction fails.

Thanks, Steve

Imagine i have a simple javascript function:

  function someFunction(integer)
  {
    data = integer + 1;
    return data;
  }

I need to call this from inside another function and use the returned value:

  function anotherFunction(integer)
  {
    int_plus_one = someFunction(integer);
    //Do something with the returned data...
    int_plus_two = int_plus_one + 1;
    return int_plus_two;
  }

How can i ensure that the return of anotherFunction return is only returned after someFunction pletes? It actually seems to work ok with very fast functions like these. However if someFunction has to do some ajax lookups, the return of aotherFunction fails.

Thanks, Steve

• javascript
• callback

• 1 You can't return an asynchronous operation. You would have to make your async call, then do whatever you had to do with the data you received in the callback. – Daniel Lizik Commented Sep 24, 2015 at 12:56
• Thanks Daniel_L - how do i do that with the example? I think i need to use a call back - but i can't seem to figure out how to do that and still use the return from someFunction? – user8262 Commented Sep 24, 2015 at 12:59

5 Answers 5

You do not know when or even if an asynchronous function will plete. The only way to handle this is to use a callback function, a function that gets executed after the async operation has pleted.

This was my "aha!" moment: How to return the response from an asynchronous call?

As far as your code is sync, the approach above is fine.

Once you start introducing async parts, the one below involving callbacks is a mon used approach:

function fn (v, cb) {
    doSomethingAsyncWithV(function (err, _v) {
        if(err) return cb(err);
        cb(null, _v);
    })
}

function yourFirstFn () {
    var v = 0;
    fn(v, function (err, _v) {
        // do here whatever you want with the asynchronously puted value
    });
}

How about promise? With that in mind, there's no need to worry about callback. It's one of the cool things in AngularJS.

var q = require('q');

var myPromise =function() {
    var deferred = q.defer();
    setTimeout(function(){  
        var output = anotherFunction(1);
        deferred.resolve(output)
    }, 10000);  // take times to pute!!! 
    return deferred.promise;
}

var objPromise = myPromise();
objPromise.then(function(outputVal){
    console.log(outputVal) ; // your output value from anotherFunction
}).catch(function(reason) {
    console.log('Error: ' + reason);
})

then is ONLY exeucted after promise has been resolved. If an exception or error is caught, the catch function executes.

how about this?

   function someFunction(integer, callback)
   {
       data = integer + 1;
       return callback(data);
   }



  function anotherFunction(integer)
  {
     int_plus_one = someFunction(integer, function(data){
         int_plus_two = int_plus_one + 1;
         return int_plus_two;
     });
    //Do something with the returned data...

  }

You could use promises:

new Promise(function someFunction(resolve, reject) {
    ajaxLib.get(url, options, function (data) {
        resolve(data);
    });
}).then(function anotherFunction(integer)
  {
    int_plus_one = integer;
    //Do something with the returned data...
    int_plus_two = int_plus_one + 1;
    return int_plus_two;
});

If you use jQuery, $.ajax returns a thenable :

$.ajax(url, options).then(function processDataFromXHR(data) {
     return data.integer;
}).then(function anotherFunction(integer){
    int_plus_one = integer;
    //Do something with the returned data...
    int_plus_two = int_plus_one + 1;
    return int_plus_two;
});