Question

I am trying to create a javascript function which returns the last modified date of a remote url which it recieves. I have tried many different methods, none of which seem to work. The following code seems like it may be close, but unfortunately doesnt work.

function getlastmod(url) {
    var req = new XMLHttpRequest();
    req.open("GET", url, false);
    req.getResponseHeader("Last-Modified");
    req.send("");
    return req.responseText;
}

The function will be used in a chrome extension that I am making.
Thanks for any help-Josh

Answer 1

Some issues:

• You are trying to get the headers before a connection has even been made. You can only read the headers after the server has responded.
• You are not doing anything with the result

Try this:

function getlastmod(url, cb) {
    var req = new XMLHttpRequest();
    req.open("GET", url);
    req.send(null);
    req.addEventListener("load", function() {
        cb(req.getResponseHeader("Last-Modified"));
    }, false);
}

getlastmod("/", function(v) {
    console.log(v); //"Wed, 01 Aug 2012 14:13:22 GMT"
});

This requires your extension to ask for the right permissions (because you are doing cross-origin xhr) as well as the server to send the header.