var mp4check_xhr = function (weburls, url, callback) {
        var url_mp4 = url.replace(".webm", ".mp4");
        var xhr = new XMLHttpRequest;
        xhr.open('head', url_mp4);
        xhr.onload = function () {
            var length = xhr.getResponseHeader('content-length');
            if(length > 100){
                var weburl = weburls[url];
                for(var id in weburl) {
                    weburl[id].recordFile = url_mp4;
                }
                delete weburls[url];
                if(Object.keys(weburls).length <= 0) {
                    callback();
                }
            }else{
                delete weburls[url];
                if(Object.keys(weburls).length <= 0) {
                    callback();
                }
            }
        };
        xhr.onerror = function () {
            delete weburls[url];
            if(Object.keys(weburls).length <= 0) {
                callback();
            }
        }
        xhr.send();
    };

Audit JavaScript execution errors by logging the errors.

window.addEventListener('error', function(e) {
    var errorText = [
        e.message,
        'URL: ' + e.filename,
        'Line: ' + e.lineno + ', Column: ' + e.colno,
        'Stack: ' + (e.error && e.error.stack || '(no stack trace)')
    ].join('\n');

    // Example: log errors as visual output into the host page.
    // Note: you probably don’t want to show such errors to users, or
    //       have the errors get indexed by Googlebot; however, it may
    //       be a useful feature while actively debugging the page.
    var DOM_ID = 'rendering-debug-pre';
    if (!document.getElementById(DOM_ID)) {
        var log = document.createElement('pre');
        log.id = DOM_ID;
        log.style.whiteSpace = 'pre-wrap';
        log.textContent = errorText;
        if (!document.body) document.body = document.createElement('body');
        document.body.insertBefore(log, document.body.firstChild);
    } else {
        document.getElementById(DOM_ID).textContent += '\n\n' + errorText;
    }

    // Example: log the error to remote service.
    // Note: you can log errors to a remote service, to understand
    //       and monitor the types of errors encountered by regular users,
    //       Googlebot, and other crawlers.
    var client = new XMLHttpRequest();
    client.open('POST', 'https://example.com/logError');
    client.setRequestHeader('Content-Type', 'text/plain;charset=UTF-8');
    client.send(errorText);
});

http://pajhome.org.uk/crypt/index.html
http://pajhome.org.uk/crypt/md5/sha1.html
http://pajhome.org.uk/crypt/md5/md5.html