Javascript understanding callback functions and execution

Question

I have script1 and script2. Here is some code from script1:

var pos;

function foo(param){pos = param;}

bar(foo);

Now in script2:

function bar(callback){
    //prints 'function'
    console.log(typeof callback);

    navigator.geolocation.getCurrentPosition(
        success,
        fail
    );
}

function success(position, callback){
    //prints 'undefined'
    console.log(typeof callback);
    var pos= {
        lat: position.coords.latitude, 
        long: position.coords.longitude
    };
    callback(pos);
}

How do I correctly pass foo as a callback function to bar, and chain it to navigator.geolocation.getCurrentPosition? How can I correctly get pos back to script1? Why can't I just return the values instead??

Due to the asynchronous behavior of navigator.geolocation.getCurrentPosition, I recommend using promises for what you're after. Here's an answer I submitted for this use case, earlier today: stackoverflow.com/questions/38287407/… — jmealy
– jmealy, Commented Jul 10, 2016 at 3:50

Steve · Accepted Answer · 2016-07-10 03:29:17Z

3

You need to re-pass the callback to the success function

function bar(callback){
    //prints 'function'
    console.log(typeof callback);

    navigator.geolocation.getCurrentPosition(
        function(pos){
            // repass callback to success method
            success(pos, callback);
        },
        fail
    );
}

answered Jul 10, 2016 at 3:29

Steve

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2 Comments

REAL O G Over a year ago

this works, but as someone from a Java background, i just don't understand - even though i know that functions are objects in javascript

Steve Over a year ago

getCurrentPostion(success, fail) passes two function pointers to invoke async and those methods don't know anything about the scope where it is invoked - same applies to Java. in parameters are private scope - just like Java

Yasin Yaqoobi · Accepted Answer · 2016-07-10 03:39:55Z

var pos;

function foo(param){pos=param; console.log(param);}

bar(foo);

function bar(callback){
    //prints 'function'
    //console.log(callback);

    navigator.geolocation.getCurrentPosition(
         success.bind(null, callback), 
        fail
    );

}

function success(callback, position){
    //prints 'function'

    var pos= {
        lat: position.coords.latitude, 
        long: position.coords.longitude
    };

   callback(pos);
}

function fail(){
  console.log('failed');
}

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