how to make Javascript setTimeout returns value in a function

Question

I have a function that calls some service and returns the response. If the response is FALSE, it waits 1 second to ask the service again (which then probably returns TRUE).

How can I do to call my function "checkService()" once, and get the real value? (the first or second try, decided by the function) I set the RET value inside the function, but the functions always return the first RET, because the setTimeout is asynchronous.

In other words, I need some "sleep" trick, or any solution (may be jQuery too).

function checkService() {

  //this may return TRUE or FALSE
  var RET = someServiceResponse(); 

  // here waits 1 second, then ask the service again
  if( RET == true ) {
    return true;
  } else {

    setTimeout(
        function() {
            //it must return the second response of the service
            RET = someServiceResponse();  
        }, 
        1000
    );

    // I want the checkService() return the response after de timeout
    return RET;  
  }
}

function alertResponse() {
    alert( checkService() );
}

Answer 1

You should use a callback function when you expect a result from the service.

Like this :

function checkService(callback) {

    //this may return TRUE or FALSE
    var RET = someServiceResponse();

    // here waits 1 second, then ask the service again
    if( RET == true ) {
        callback(RET);
    } else {

        setTimeout(
                function() {
                    //it must return the second response of the service
                    RET = someServiceResponse();
                    callback(RET);
                },
                1000
        );

        // I want the checkService() return the response after de timeout
        return RET;
    }
}

So when you want to call the service, you just need to do :

checkService(function(status){
    alert(status);

    // Here some code after the webservice response
});

Answer 2

Googling 'javascript setTimeout callback' here's a handy jsFiddle about 3 results down:

getData('http://fakedomain1234.com/userlist', writeData);

document.getElementById('output').innerHTML += "show this before data ...";

function getData(dataURI, callback) {
    // Normally you would actually connect to a server here.
    // We're just going to simulate a 3-second delay.
    var timer = setTimeout(function () {
        var dataArray = [123, 456, 789, 012, 345, 678];
        callback(dataArray);
    }, 3000);
}

function writeData(myData) {
    console.log(myData);
}

http://jsfiddle.net/cwbuecheler/Y9Ca8/