Question

I was playing around with call backs and deferred functions in jQuery and was wondering if anyone could tell me why this works

http://jsfiddle.net/austinbv/QVujr/

  get_each_total = function(callback) {
    var requests;
    requests = [];
      var url;
        url = "http://otter.topsy.com/search.js?callback=?&apikey=38A260E9D12A4908B1AF9184B691131&q=justin+bieber&window=d";
        return requests.push($.getJSON(url, function(data) {
        }));
    return $.when.apply($, requests).then(function() {
      callback();
    }, function() {
      return alert("There was an error communicating with a remote library, try again in a few");
    });
  };



  get_each_total_broken = function(callback) {
    var requests;
    requests = [];
      var url;
        url = "http://otter.topsy.com/hjhkl/sehjkhhkjhkarch.js?callback=?&apikey=38A260E9D12A4908B1AF9184B691131&q=justin+bieber&window=d";
        return requests.push($.getJSON(url, function(data) {
        }));
    return $.when.apply($, requests).then(function() {
      callback();
    }, function() {
      return alert("There was an error communicating with a remote library, try again in a few");
    });
  };

$(function () {
    get_each_total(alert("success"));
    get_each_total_broken(alert("fail"));
});

and this does not

http://jsfiddle.net/austinbv/wzve6/

  get_each_total = function(callback) {
    var requests;
    requests = [];
      var url;
        url = "http://otter.topsy.com/search.js?callback=?&apikey=38A260E9D12A4908B1AF9184B691131&q=justin+bieber&window=d";
        return requests.push($.getJSON(url, function(data) {
        }));
    return $.when.apply($, requests).then(function() {
      callback();
    }, function() {
      return alert("There was an error communicating with a remote library, try again in a few");
    });
  };



  get_each_total_broken = function(callback) {
    var requests;
    requests = [];
      var url;
        url = "http://otter.topsy.com/hjhkl/sehjkhhkjhkarch.js?callback=?&apikey=38A260E9D12A4908B1AF9184B691131&q=justin+bieber&window=d";
        return requests.push($.getJSON(url, function(data) {
        }));
    return $.when.apply($, requests).then(function() {
      callback();
    }, function() {
      return alert("There was an error communicating with a remote library, try again in a few");
    });
  };

$(function () {
    get_each_total(function () { alert("success")});
    get_each_total_broken(function () {alert("fail")});
});

as you can see the only difference is in the last two lines, where an anonymous function wraps the callback. Any insight would be nice.

Answer 1

This piece of code:

return requests.push($.getJSON(url, function(data) {
}));

exits out of your function. The callback is never called.

P.S. When you’re saying the only difference is an anonymous function wrapping the callback, you imply that you’re also passing a function in the first version of your code. That is not true; you’re trying to pass in whatever alert('whatever'); is returning, which is undefined!

Further explanation:

Both of your functions (get_each_total, & get_each_total_broken) expect the parameter to be a function. This is evident by you trying to call it as a function later on in your code (callback()). However, this line:

get_each_total(alert("success"));

does not pass a function to get_each_total. It is equivalent to the following:

var returnedFromAlert = alert("success");
get_each_total(returnedFromAlert);

So, basically, you’re not passing anything into your get_each_total function. You get a success alert right away, before get_each_total has been called.