CODEWITHSUNDEEP
javascriptajax
lang2
lang2

Reputation: 11966

javascript function return value confusion

I'm unclear about return value of the following dummy code:

function foo()
  var ret = 0;
  var xhr=send_request( "bla", function() {
      // do something with the AJAX response
      // based on the value of response, var ret get set
  } );
  return ret;
}

What I would like to achieve is that: based on the AJAX response, I might decide to try the request again. But the function above always returns 0 regardlessly.

Apparently I can make the foo() function decide to call send_request() twice when needed but it's a bit ugly. Is there a easy and nice way to do this?

Thanks

Upvotes: 4

Views: 2279

Answers (4)

riwalk
riwalk

Reputation: 14223

You are trying to make the ajax call synchronously, but you are a making an asynchronous call.

It is important to understand that the way you have it written, the code does not wait for the AJAX call to finish before moving on to the next line. Therefore, it always returns the initial value of ret.

Do several things to fix this:

  • Use jQuery (if you are not already)
  • Use jQuery's ajax() function, and set async to false.

Should look something like this:

function foo()
    var ret = $.ajax({ url: "blah",
                       async: false
                     }).responseText;

    // do your stuff here

    return ret;
}

EDIT: It is possible to do this with an asynchronous call, but you have to adjust the way you think about the problem. Rather than thinking about return values, you have to think about callback functions.

For the sake of example, lets say I'm trying to get the user's name and put it on the page. My code would look something like this:

function GetUsername() {
    $.ajax( { url: "blah",
              success: PopulateUsername    // Specify a callback
            });
    // I don't do anything else. Execution will continue when the
    // callback gets called from the AJAX call.
}

function PopulateUsername(data) {
    alert(data);
    // Anything else I want to do, I do here, because it is only 
    // here that I have access to the result.
}

GetUsername();  // I call GetUsername() here, and that's it. Any
                // further actions that need to happen are going to
                // occur in the callback function

Upvotes: 7

g.d.d.c
g.d.d.c

Reputation: 47988

You don't want to return a value from a function that's going to make an AJAX call because the AJAX Request will not have completed before the function returns (and personally, I disagree with the answers that say you should set async to false). You want to do something like this instead:

function retFunction(val) {
  // Do something here for various values of val
  if (val == 0) {
    // Something
  } else if (val == 1) {
    // Something else
  }
}

function foo()
  var xhr=send_request( "bla", function() {
    var myResult = 0; // Something here based on return values.
    retFunction(myResult);
  });
}

Upvotes: 0

Nathan
Nathan

Reputation: 6216

If you want to keep things synchronous, then use Stargazer712's suggestion.

You could try keeping things asynchronous with something like this:

function foo(callback)
  var xhr=send_request( "bla", function(result) {
     callback(result)
  } );
}


function test(result) {
  // test result here 
  if(result != "what I want")
     foo(test);   // repeat the ajax call under certain conditions
  else
     alert("got it");
}


$(function() {
  foo(test);
});

This will repeat the ajax request until the response matches a certain value.

Upvotes: 0

sv_in
sv_in

Reputation: 14039

the variable ret has a local scope within the function. Therefore, each time you call it, the variable gets initialized to 0.

Moreover, when the function is returning the variable ret, the send_request function (which set something else to ret) has not run yet due to which the value returned is always 0. It must be after the function got returned that the ajax request gets completed and the send_request function set a new value to ret.

Upvotes: 0

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