return a boolean after promise has been resolved/rejected?

Question

I have a function that works absolutely fine. I just want to return true or false depending on the promise.

 //I want this function to return a simple true or false!!!
function isAppOnline() {
            var is_connected = connectivityMonitor.isInternetConnected();
            is_connected.then(function(result) {
                console.log('INTERNET_CHECK_API : app online');//works fine
                return true;//this is not being returned
            }, function(error) {
                console.log('INTERNET_CHECK_API : app offline');//works fine
                return false;//this is not being returned
            });
        }

But when I call this function,

is_online = isAppOnline();

is_online is always undefined . Why is the function not able to return a simple boolean value?

Update :

This is what I am trying to do : I simply want to open a popup which notifies the user that he is offline. I am calling the function isAppOnline periodically after 10secs. This function is already using a promise in my factories . I dont want to overcomplicate things but its important to me that this function returns a boolean so based on this,I can take my actions accordingly.

Answer 1

EDIT: For ES2017 If your one of the lucky souls who gets to use ES2017 then you can use the new await/async keywords. They are pretty brilliant and allow you to write async code that reads synchronous. (It is still promises under the hood, just unboxing of them).

function isOnline() {
    return Promise.resolve(true);
}

async function Main() {
    const online = await isOnline();

  console.log(online);
}

Main();

fidle

---

Because it is asynchronous. Your isAppOnline method returns before your promise has resolved.

I presume is making some form of AJAX call to check network connectivity so there it will have to wait for it to respond. JavaScript is single threaded, if that thread locked up waiting for that request to respond so it could be synchronous your whole JavaScript would pause until it returns. Not good.

So if you want the caller of isAppOnline to know the result you have to options. Either passing it a call back or return the promise (better option)

function isAppOnline(cb) {
    var is_connected = connectivityMonitor.isInternetConnected();
    is_connected.then(function(result) {
        console.log('INTERNET_CHECK_API : app online');//works fine
        cb(true);
    }, function(error) {
        console.log('INTERNET_CHECK_API : app offline');//works fine
        cb(false);
    });
}

//better option
function isAppOnline() {
    return connectivityMonitor.isInternetConnected().then(function(result) {
        console.log('INTERNET_CHECK_API : app online');//works fine
        return true;
    }, function(error) {
        console.log('INTERNET_CHECK_API : app offline');//works fine
        return false;
    });
}

//used as
isAppOnline().then(function (isOnline) {
    console.log('Is it online?', isOnline);
});

Answer 2

Promise works differently with simple statements because they might return later, so you might need to rethink the flow of your program to handle different initial and final states.

function isAppOnline() {
    var is_connected = connectivityMonitor.isInternetConnected();
    return is_connected.then(function(result) { // add return here to return the whole promise
        console.log('INTERNET_CHECK_API : app online');//works fine
        return; //this will resolve the promise returned
    }, function(error) {
        console.log('INTERNET_CHECK_API : app offline');//works fine
        $q.reject(); // this will reject the promise returned
    });
}

You will have to handle changing states in your calling controller (it might be false for a short while then turned to true, etc). You might consider using a loading state to prevent misdirecting users.

is_online = false;
isLoading = true;

isAppOnline().then(function() {
    is_online = true;
}, function() {
    is_online = false;
})
.finally(function(){
    isLoading = false;
});

Answer 3

Your question indicates a pretty deep need for further exploration of Javascript's asynchronous processing. I recommend reading: How do I return the response from an asynchronous call?

There are hacks you can implement to force a method to return synchronously, but angular is built in a way that honors promises almost everywhere. If you could give us some more context on what you're trying to accomplish we may be able to help you write your code in a way that takes advantage of the promise itself.

Answer 4

You could try the following (this is untested code since you didn't provide a working script):

function isAppOnline() {
    var defer = $q.defer();

    var is_connected = connectivityMonitor.isInternetConnected();
    is_connected.then(function(result) {
        console.log('INTERNET_CHECK_API : app online');//works fine
        defer.resolve(true);
    }, function(error) {
        console.log('INTERNET_CHECK_API : app offline');//works fine
        defer.resolve(false);
    });

    return defer.promise;
}

And call it like this:

var is_online = false;
isAppOnline().then(function(data){
    is_online = data;
});

Or, pass the is_connected object directly, than you do better error handling:

function isAppOnline() {
    var defer = $q.defer();

    var is_connected = connectivityMonitor.isInternetConnected();
    is_connected.then(function(result) {
        console.log('INTERNET_CHECK_API : app online');//works fine
        defer.resolve(is_connected);
    }, function(error) {
        console.log('INTERNET_CHECK_API : app offline');//works fine
        defer.reject();
    });

    return defer.promise;
}