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问题:

Can jQuery provide a fallback for failed AJAX calls? This is my try:

function update() {
    var requestOK = false;

    $.getJSON(url, function(){
        alert('request successful');
        requestOK = true;
    });

    if (!requestOK) {
        alert('request failed');
    }
}

Unfortunately, even if the callback function of the $.getJSON() method is called, i get the message 'request failed', before the callback function has the opportunity to set the requestOK variable. I think it's because the code runs in parallel. Is there a way to handle such situations? I thought about chaining or some way of waiting for the AJAX request, including its callback function. But how? Does anyone know how to do that?

回答1:

You will need to either use the lower level $.ajax call, or the ajaxError function. Here it is with the $.ajax method:

function update() {
  $.ajax({
    type: 'GET',
    dataType: 'json',
    url: url,
    timeout: 5000,
    success: function(data, textStatus ){
       alert('request successful');
    },
    fail: function(xhr, textStatus, errorThrown){
       alert('request failed');
    }
  });
}

EDIT I added a timeout to the $.ajax call and set it to five seconds.

回答2:

Dougs answer is correct, but you actually can use $.getJSON and catch errors (not having to use $.ajax). Just chain the getJSON call with a call to the fail function:

$.getJSON('/foo/bar.json')
    .done(function() { alert('request successful'); })
    .fail(function() { alert('request failed'); });

Live demo: http://jsfiddle.net/NLDYf/5/

This behavior is part of the jQuery.Deferred interface.
Basically it allows you to attach events to an asynchronous action after you call that action, which means you don't have to pass the event function to the action.

Read more about jQuery.Deferred here: http://api.jquery.com/category/deferred-object/

回答3:

Yes, it's built in to jQuery. See the docs at jquery documentation.

ajaxError may be what you want.

回答4:

I prefer to this approach because you can return the promise and use .then(successFunction, failFunction); anywhere you need to.

var promise = $.ajax({
    type: 'GET',
    dataType: 'json',
    url: url,
    timeout: 5000
  }).then(function( data, textStatus, jqXHR ) {
    alert('request successful');
  }, function( jqXHR, textStatus, errorThrown ) {
    alert('request failed');
});

//also access the success and fail using variable
promise.then(successFunction, failFunction);