Function to retrieve value from AJAX request

How do you write a function that returns a value fetched from server via $.get?

This is what I have tried, which does not work:

function getMessage(key) {
    $.get("/messages.json", function(data) {
        return data.me开发者_如何学Gossages[key];
    }, "json");
}

Any ideas?

---

Because Ajax requests are asynchronous. That is why you have to pass a callback to $.get, to handle the data once it is available. But the getMessage function returns before the $.get callback is executed.
You have to pass a callback that is doing something with the return value. E.g.:

function getMessage(key, cb) {
  $.get("/messages.json", function(data) {
    cb(data.messages[key]);
  }, "json");
}

getMessage('foo', function(data) {
    alert(data);
});

Of course you can also pass the callack directly to $.get and handle the data extraction there:

function getMessage(cb) {
  $.get("/messages.json", cb);
}

---

There are two ways to handle this: use a synchronmous call via $.ajax or pass in a callback to your function instead of having it return a value. The latter is the canonical way to deal with AJAX since it retains the asynchronous nature of the call.

Asynchronous

function processMessage(key,elem,cb) {
     $.get('/messages.json', function(data) {
           if (cb && typeof(cb) === 'function') {
               cb.apply(elem,data.messages[key]);
           }
     }
}


$('.something').each( function() {
      processMessage('somekey', this, function(msg) {
            $(this).append(msg);
      });
});

Synchronous - try not to do it this way, since you'll lock your browser until it's done.

function getMessage(key)
{
   var result = '';
   $.ajax({
       url: '/messages.json',
       aSync: false,
       type: 'get',
       dataType: 'json',
       success: function(data) {
                     result = data.messages[key];
                }
   });
   return result;
}

$('.something').each( function() {
    var msg = getMessage('somekey');
    $(this).append(msg);
});

Note: these are untested.