Is it possible with javascript to find file's last modified time?

Question

Is it possible? Now, I have done live chat, where with jquery's help I connect to .php file and check last modified time and if it is not as before, I retrieve messages. If it were possible in javascript I probably would save a lot of resources.

Thanks.

Answer 1

It's definitely possible if the server is sending an accurate Last-Modified header for that particular file:

var getMTime = function(url, callback) {
  var xhr = XMLHttpRequest();
  xhr.open('HEAD', url, true); // use HEAD - we only need the headers
  xhr.onreadystatechange = function() {
    if (xhr.readyState === 4 && xhr.status === 200) {
      var mtime = new Date(xhr.getResponseHeader('Last-Modified'));
      if (mtime.toString() === 'Invalid Date') {
        callback(); // dont want to return a bad date
      } else {
        callback(mtime);
      }
    }
  }
  xhr.send();
};

getMTime('url here', function(mtime) {
  if (mtime) console.log('the mtime is:' + mtime.toISOString());
});

Answer 2

Short answer: there's no way but AJAX + a server-side script (in your case, jQuery + php)

Being a client-side script, javascript gets run on the client's computer, so if the file whose m-time you want to check is on the server, then you are correct to use AJAX and a server-side script. No other way will work.

If the file whose m-time you want to check is on the client's computer, then you're out of luck. Javascript is intentionally designed to be prevented from accessing the client's files. (It can only access cookies, which are on the client's computer, however, because the browser (not any javascript) loads those into its work environment.)