gistfile1.js

var Q = require("promise"); // http://gist.github.com/312929

function xhrcall() {
    var deferred = Q.Deferred();
    return promise(function(P){
        var xhr = new XMLHttpRequest();
        xhr.open("GET","/");
        xhr.onreadystatechange = function(){
            if (xhr.readyState == 4) {
                if (xhr.responseText.match(/success/))
                    deferred.resolve(xhr.responseText);
                else
                    deferred.resolve(Q.Rejection(xhr.responseText));
            }
        });
        xhr.send();
    });
    return deferred.promise;
}

Q.when(xhrcall(), function (content) {
    alert('success: ' + content);
}, function (error) {
    alert('failure: ' + error);
});

// but the error can propagate backward:

var promise = Q.when(xhrcall(), function (content) {
    alert('success: ' + content);
});

Q.when(promise, function () {
    // undefined
}, function (error) {
    alert('failure: ' + error);
});

// or the error can be resolved:
var promise = Q.when(xhrcall(), function (content) {
    alert('success: ' + content);
}, function (error) {
    var deferred = Q.Deferred();
    setTimeout(function () {
        // yeah, you can resolve with an eventual value too:
        deferred.resolve(xhrcall());
    }, 1000); 
    return deferred.promise;
});