Question

I’m looking for a way through AJAX (not via a JS framework!) to real time monitor a file for changes. If changes where made to that file, I need it to give an alert message. I’m a total AJAX noob, so please be gentle. 😉

Edit: let me explain the purpose a bit more in detail. I’m using a chat script I’ve written in PHP for a webhop, and what I want is from an admin module monitor the chat requests. The chats are stored in text files, and if someone starts a chat session a new file is created. If that’s the case, in the admin module I want to see that in real time.

Makes sense?

Answer 1

To monitor a file for changes with AJAX you could do something like this.

var previous = "";

setInterval(function() {
    var ajax = new XMLHttpRequest();
    ajax.onreadystatechange = function() {
        if (ajax.readyState == 4) {
            if (ajax.responseText != previous) {
                alert("file changed!");
                previous = ajax.responseText;
            }
        }
    };
    ajax.open("POST", "foo.txt", true); //Use POST to avoid caching
    ajax.send();
}, 1000);

I just tested it, and it works pretty well, but I still maintain that AJAX is not the way to go here. Comparing file contents will be slow for big files. Also, you mentionned no framework, but you should use one for AJAX, just to handle the cross-browser inconsistencies.