Log JavaScript errors in a server-side log file

log.js

/**
 * Logs JavaScript errors via an AJAX request. 
 * Written in pure JavaScript so it does not have any dependencies. 
 */
window.onerror = function(errorMessage, url, lineNumber){
  
	var ajaxUrl = "/log-js-errors?t=" + (new Date()).getTime();
	
	var data = {
		errorMessage: errorMessage,
		file: url,
		lineNumber: lineNumber,
		referrer: document.referrer,
		userAgent: navigator.userAgent
	};
	
	var dataString = "";
	
	for(var key in data){
		dataString += "&" + key + "=" + escape(data[key]);
	}
	
	var httpRequest;
	
	//https://developer.mozilla.org/en-US/docs/AJAX/Getting_Started
	if (window.XMLHttpRequest) { // Mozilla, Safari, ...
      httpRequest = new XMLHttpRequest();
    } else if (window.ActiveXObject) { // IE
      try {
        httpRequest = new ActiveXObject("Msxml2.XMLHTTP");
      } 
      catch (e) {
        try {
          httpRequest = new ActiveXObject("Microsoft.XMLHTTP");
        } 
        catch (e) {}
      }
    }
	
	httpRequest.onreadystatechange = function(){ /* get data back */ };
	httpRequest.open("POST", ajaxUrl);
	httpRequest.setRequestHeader('Content-Type', 'application/x-www-form-urlencoded');
	httpRequest.send(dataString);
};