How to get the response of XMLHttpRequest?

You can get it by XMLHttpRequest.responseText in XMLHttpRequest.onreadystatechange when XMLHttpRequest.readyState equals to XMLHttpRequest.DONE.

Here’s an example (not compatible with IE6/7).

var xhr = new XMLHttpRequest();
xhr.onreadystatechange = function() {
    if (xhr.readyState == XMLHttpRequest.DONE) {
        alert(xhr.responseText);
    }
}
xhr.open('GET', 'http://example.com', true);
xhr.send(null);

For better crossbrowser compatibility, not only with IE6/7, but also to cover some browser-specific memory leaks or bugs, and also for less verbosity with firing ajaxical requests, you could use jQuery.

$.get('http://example.com', function(responseText) {
    alert(responseText);
});

Note that you’ve to take the Same origin policy for JavaScript into account when not running at localhost. You may want to consider to create a proxy script at your domain.