Ajax – Get size of file before downloading

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You can get XHR response header data manually:

http://www.w3.org/TR/XMLHttpRequest/#the-getresponseheader()-method

This function will get the filesize of the requested URL:

function get_filesize(url, callback) {
    var xhr = new XMLHttpRequest();
    xhr.open("HEAD", url, true); // Notice "HEAD" instead of "GET",
                                 //  to get only the header
    xhr.onreadystatechange = function() {
        if (this.readyState == this.DONE) {
            callback(parseInt(xhr.getResponseHeader("Content-Length")));
        }
    };
    xhr.send();
}

get_filesize("http://example.com/foo.exe", function(size) {
    alert("The size of foo.exe is: " + size + " bytes.");
});

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