Javascript callbacks - passing variables to them

Question

I can't get the following code to work. Assume that the ajax call works, and msg['username'] is preset to 'john'. I think I am getting confused with how to pass variables to my callback. Edit: I think my main confusion was how to get the 'msg' variable out of Ajax. It looks like Ajax has a 'hard wired' success method which is pretty much compulsory to use if you want to subsequently use the data from the ajax query- and that method is the only one that can access the result of the ajax call. Here's the code:

<script>
   $(document).ready(function(){
        function freedome(horace){
           $.ajax({
               url: "RESPONDERdetails.php",
               type: "GET",           
               dataType: "json",
               data:{thing:31}
           });    
           horace(msg);
        } 

        function callbacker(msg){
            alert("I am callback");
            name = msg["username"];
            alert(name);
        }

        freedome(callbacker(msg));
        });
</script>

Possible duplicate of How do I return the response from an asynchronous call? — nem035
– nem035, Commented Oct 21, 2016 at 14:31
$.ajax has a success option for running a callback. Your horace needs to be there. Also, you need to just pass callbacker to freedome. Right now, you're attempting to call callbacker by passing it some non-existent msg value then pass it's return value to freedome. — Mike Cluck
– Mike Cluck, Commented Oct 21, 2016 at 14:33

gen_Eric · Accepted Answer · 2016-10-21 14:33:56Z

1

You just want to use freedome(callbacker);. In JavaScript, functions can be treated like variables. So, all we need to do is pass the function itself as a parameter.

This makes horace into a function, so that when you do horace(msg);, callbacker will be called with msg as a parameter.

Also, msg is never declared anywhere in your example and your horace(msg); will run before the AJAX call is done. You need use the AJAX call's callback here.

$(document).ready(function(){
    function freedome(horace){
        $.ajax({
            url: "RESPONDERdetails.php",
            type: "GET",           
            dataType: "json",
            data:{thing:31},
            success: function(msg){
                horace(msg);
            }
        });
    } 

    function callbacker(msg){
        alert("I am callback");
        name = msg["username"];
        alert(name);
    }

    freedome(callbacker);
});

answered Oct 21, 2016 at 14:33

gen_Eric

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5 Comments

John Over a year ago

Ok to clarify further...I think I have two confusions. The first is that when you pass a callback to a function, the callback should always be executed last.

John Over a year ago

Ok to clarify further...I think I have two confusions. The first is that when you pass a callback to a function, the callback should always be executed last.

John Over a year ago

Ok to clarify further...I think I have two confusions. (1) The first is that when you pass a callback to a function, the callback should always be executed last. However I note the comment above that in my original code, horace would be executed before the ajax call is finished. So it seems that in the ajax function callbacks dont automatically run last after the ajax is finished. (2) It seems that to get the return value of the ajax call you have to run either 'success' or the 'done' method - and only then will you get access to the result variable.

John Over a year ago

Also apologies for the double post above, kept pressing the return key.

gen_Eric Over a year ago

@John: AJAX runs in the background. It downloads the data while the rest of your code runs. Once the download is done, then the success function runs with the downloaded data. The callback is an event that is triggered by the browser when its ready.

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