capture client-side javascript errors and log to server

gistfile1.js

// for support before ECMA-262 5th edition ...
if (!Date.now) {
  Date.now = function now() {
    return +(new Date);
  };
}

// keep track of how many errors have been logged to server
var num_errors = 0;
var max_errors = 10;
var error_batch = 1;

// keep track of when last error occured
var last_error = 0;

// throttle error reports occuring in quick succession
var single_throttle = new Number(1000);

// allow another batch of 'max_errors' if 5 seconds have elapsed
// since the last error
var batch_throttle = new Number(5000);


try {
    window.onerror = function(msg, url, line){
        // do not send errors that have occured in the last 'single_throttle' seconds.
        // use absolute value of time difference in case browser clock is wonky
        if (Math.abs(Date.now() - last_error) > single_throttle){
            // if error limit has been reached and a new error occurs
            // more than 'batch_throttle' seconds later than previous error,
            // increment 'error_batch' to allow another batch of 'max_errors'
            if ((num_errors >= (max_errors*error_batch)) && (Math.abs(Date.now() - last_error) >= batch_throttle)) {
              error_batch = error_batch + 1;
            }
            // increment error count
            num_errors = num_errors + 1;
            // reset time of last_error
            last_error = Date.now();

            // only report a finite number of errors so the browser
            // isn't overwhelmed loading so many gifs, and so we don't
            // ddos our server (stop after first 'max_errors' worth of errors)
            if (num_errors < (max_errors*error_batch)){
                var sid;
                if (document.getElementById('session-id')){
                    sid = document.getElementById('session-id').innerHTML;
                }

                // prepare payload, encode everything except numbers
                var payload = '/jserrors?msg='+encodeURIComponent(msg)+
                '&url='+encodeURIComponent(url)+'&line='+line+
                '&ref='+encodeURIComponent(document.referrer)+'&ua='+encodeURIComponent(navigator.userAgent)+
                '&proto='+encodeURIComponent(window.location.protocol)+'&e='+num_errors+
                '&last='+last_error;
                // if session id was found on the page, append it to payload
                if (!(typeof sid === undefined)){
                    payload = payload + '&sid='+encodeURIComponent(sid);
                };
                // fetch image from payload url
                (new Image).src = payload;
            };
        };
        // suppress browser error messages by returning true
        return true;
    };
} catch(e) {}