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Posted 28 Nov 2012


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JSNLog - JavaScript logging integrated with ASP.NET and MVC

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4 Apr 2014CPOL
JSNLog lets you put loggers in your JavaScript code, configure them in your web.config, and capture their output on your server.

Know what your JavaScript application is up to after it has gone into production.


A JavaScript application is like your favorite child. You shower it with love, prepare it for life the best way you can, and then send it into production where it faces users that do things you never imagined and servers slower than you thought possible.

The issue is that now your application is live, there will be users. If the users are not happy, they will complain to your manager (or your client). This in turn has the potential to make you unhappy. You don't want unhappy users.

To keep your users happy, you want to know whether their experience is good and if not, fix it. In restaurants, the waiter comes to your table and asks if everything is ok. You could call your users, but unless you have only a few, it is easier to ask your application instead. After all, it can talk to you in a way a restaurant meal can't.

Specifically, you will want to know about:

  1. Exceptions. Any syntax error or undefined variable will cause one of these. Yes, they will happen no matter how well you craft and test your code.
  2. Failed AJAX requests. You'll want to know about unreliable servers.
  3. Lengthy AJAX requests. A user still looking at a spinner after 10 seconds is not a happy user. When a request takes too long, you want to know about it, so you know how often it happens, which server is involved, etc.
  4. Debug data. Users are reporting bugs that you can't reproduce in your dev environment. So you add code to tell you what's going on in production. You want to control this from a central point, without adding and removing lines deep inside your code.

Ok, so you need client side logging. However:

  1. You don't want to be swamped with log data. You'll want filters to give you only the good stuff.
  2. The log data needs to go into your server side logs. That way, you can get at it.
  3. You'll want to correlate client side and server side log data, so you know what log data refers to the same request.
  4. Ideally, you could control everything via a configuration file on the server, without changing your JavaScript code.

There are many logging packages for JavaScript, including JSLog, Log4js and log4javascript. In this article, you'll meet JSNLog, a relatively new JavaScript package that ticks all these boxes. It is open source, and at 1.5kb about the size of a small image. It is also very well integrated with ASP.NET MVC and Webforms, and has integration with PHP as well.

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Contents of this series

Let's do some logging

First Download and install JSNLog.

Then to log something to the server, in your JavaScript write something like this:

JL().info("log message");

This calls function JL(), which returns a logger. It then calls function info on that logger to log an info message.

JSNLog provides lots of options, named loggers, etc. But if you simply want to log something, this is all it takes.

You can pass in strings, objects and functions. If you pass in a function, it will execute the function and then log whatever the function returns.

It not only sends the log data, but also the time, browser type, etc.

How it works

The JavaScript logger takes the message and hands it off to the AJAX appender, a piece of code inside JSNLog that sends the log message to the server running your MVC or Webforms site or your PHP site. The JSNLog server side component then stores the log message in your server side log, using whatever logging package you already use (for .Net supports Log4Net, NLog, Elmah and Common.Logging).

Image 1

There are JSNLog editions for .Net and for PHP.

If you use Java, Python or some other technology for which there is no server side component, you can use the stand alone JSNLog.js library. The JSNLog documentation has the format of the log messages, making it easy to create a server side component yourself.

If terms such as "appender" and "info" have a familiar ring, that makes sense. JSNLog gets its inspiration from Log4J and its offshoots, but then adds its own sauce in the form of a shortened and simplified syntax and features to make it work better on the client.

The remainder of this article assumes you have an ASP.NET MVC or Webforms web site. It shows how to control your JavaScript loggers from your server side web.config file, where all JSNLog definitions sit inside the <jsnlog> element.

Note however that you can just as easily configure your loggers in your JavaScript (how). Also, JSNLog comes with sensible configuration defaults, so for simple logging you don't need to do configuration at all.

Console Appender

During development it can be useful to simply send log items to the console provided by the F12 developer tools in Chrome, Firefox and Internet Explorer. To enable this, JSNLog includes a Console Appender.

Image 2

Real world scenarios

To log any exceptions so you know there is a bug in your code, you can use windows.onerror. This handler is invoked by the browser for each uncaught exception:

window.onerror = function myErrorHandler(errorMsg, url, lineNumber) {
    // Send message with severity fatal
    JL().fatal("Exception - " + errorMsg + ", url: " + url + ", line number: " + lineNumber);
    return false;

If instead of all exceptions you are only interested in exceptions in some specific piece of code, you could use try catch:

try {
    // Code that may throw exception
catch (e) {
    // e contain a string or object with exception data
    JL().fatalException("message", e);
    // Handle the exception, ignore it, or throw it again using throw e

fatalException takes both a message and an exception. These will both be logged in the same log message. On Chrome, Firefox and IE10 and higher, a stack trace will be logged as well.

To get a log message if an AJAX request has failed, or has taken too long:

var msBeforeAjaxCall = new Date().getTime();
// Send AJAX request and receive response
<a href="">$.ajax</a>( "url" )
.done(function() {
    alert( "success" );
.fail(function() {
    // Send a message with severity error if AJAX request failed
    JL().error("Request to ..... failed.");
.always(function() {
    // Send error message if response took longer than 10 seconds to arrive
    var msAfterAjaxCall = new Date().getTime();
    var timeTakenInMs = msAfterAjaxCall - msBeforeAjaxCall;
    if (timeTakenInMs > 10000) {
        JL().error("Request to ..... took " + timeTakenInMs + " ms.");

Reducing generated log data

One problem with loggers is that they can generate a lot of useless data. JSNLog includes many options to get the data you actually want.

Switching JavaScript loggers on and off

When you log something, you can give it severity:

JL().trace("message with severity trace");
JL().debug("message with severity debug");
JL().info("message with severity info");
JL().warn("message with severity warn");
JL().error("message with severity error");
JL().fatal("message with severity fatal");

You've already seen info(), error() and fatal() in the examples above.

To filter out all messages below a given severity:

<!--Only send error and fatal log messages-->
<logger name="" level="ERROR" />

To stop all messages:

<!--Stop all messages-->
<logger name="" level="OFF" />

Named loggers

If you do logging in multiple places, having just one logger quickly gets limiting. What if you want to have debug logging in some functions, but only error logging everywhere else?

The solution is to use named loggers:

<!--Set options on logger1. If it does not exist, it will be created.-->
<logger name="logger1" level="DEBUG" />
<!--Set other options on logger2. This does not affect logger1.-->
<logger name="logger2" level="ERROR" />

In your JavaScript code, using named loggers looks like this:

JL("logger1").error("message from logger1");

In applications with dozens of loggers, setting options for each one individually can get boring. Because of this, JSNLog uses the same inheritance mechanism as Log4J and Log4Net. Here is how this works:

  1. Logger names can have multiple parts separated by periods, such as "abc.def" and "abc.def.ghi".
  2. If the parts of a logger name match the first parts of another logger name, than the first logger is called the parent of the other logger. For example:
    • The parent of logger "abc.def.ghi" is logger "abc.def".
    • The parent of logger "abc.def" is logger "abc".
    • The parent of logger "abc" is the root logger.

Loggers inherit their options from their ancestors. So if you set options on logger "abc", then "abc.def", "abc.def.ghi" and all other descendents of "abc" get the same options, unless you specifically override them.

The root logger is the only logger without a name. As you saw in the beginning of this article, you access it by simply leaving out the name:

<!--Set options on root logger-->
<logger name="" level="ERROR" />
<!--Or leave out the name attribute altogether-->
<logger level="ERROR" />

Now that you've seen how to filter messages by severity and how to configure multiple JavaScript loggers through inheritance, let's have a look at the other options.

Filter by regular expression

Sometimes it's just not worth the time to fix some rare exception. Or there is a third party component throwing exceptions and there is nothing you can do about it.

The disallow option suppresses all messages that match a given regular expression. If you're logging an object, it will be converted to JSON and then matched:

<!--Suppress messages that match regular expression [Ss]uppress\s+me-->
<logger name="" disallow="[Ss]uppress\\s+me" />

Filter by browser type

Imagine trying to solve a bug that only happens with everybody's favorite browsers, IE 7 and 8. You want to switch on the JavaScript logger, but only for those browsers.

The userAgentRegex option is your friend here. It disables the logger, except if the client's user agent string matches a given regular expression:

<!--Disable logger, except for IE 7 and IE 8-->
<logger name="" userAgentRegex="MSIE 7|MSIE 8" />

Filter out (near) duplicates

Imagine having a loop that executes 50 times, and somewhere in that loop a logger decides to send a message. You would get the same message 50 times, like this. Ouch!

Something is wrong - x = 1
Something is wrong - x = 2
Something is wrong - x = 3
Something is wrong - x = 49
Something is wrong - x = 50

The onceOnly option will get rid of the near duplicates for you. It takes one or more regular expressions. When it sees a message that matches one of those, it remembers that it has seen a message matching that particular regular expression. If it gets another message matching that regular expression, it will suppress it.

<!--Send only one message that matches regular expression "Something is wrong - x ="-->
<logger name="" onceOnly="Something is wrong - x =" />

With the onceOnly option in place, you get only the first message:

Something is wrong - x = 1

Limit number of messages

You can set the maxMessages option to limit the total number of messages sent by the client. This is a blunt instrument, because it stops sending messages after the limit is reached, regardless of which JavaScript loggers are trying to send or the severity of the messages.

<!--Ensure that the client sends no more than 3 log messages-->
<jsnlog maxmessages="3" >

Note that this option is applied to the <jsnlog> element itself. That's because this option applies to the entire JSNLog library, not to an individual logger.

Only send debug messages if an exception happens

When an exception happens, it is not always clear why it happened. So you may need to generate debug messages in several places to find the cause:

window.onerror = function myErrorHandler(errorMsg, url, lineNumber) {
    JL().fatal("Exception - " + errorMsg + ", url: " + url + ", line number: " + lineNumber);
    return false;
function abc(x) {
    JL().debug("abc: x=" + x);
function def(y) {
    JL().debug("def: y=" + y);

The issue is that you want the debug information, but only if there is an exception. If there is no exception, the debug information is of no value.

To solve this, you can store the debug messages in browser memory, and only send them to the server if there is a fatal message. Because this may involve multiple named JavaScript loggers, this is configured on the AJAX appender - the JSNLog component that receives messages from the JavaScript loggers and sends them to the server.

The following code will make this all work. For details, see the JSNLog documentation.

<!--Create new appender-->
<!--Tell the root logger to send all messages to the new appender-->
<logger appenders="appender1"/>

What's the result of this magic?

  1. A new appender is created.
  2. The root logger is told to send all its messages to the new appender. Because the root logger is the ancestor of all loggers, any other loggers you have will now also send their messages to the new appender.
  3. When the appender receives from a logger a message with severity debug or higher, it is stored in a memory buffer.
  4. When it receives a message with severity error or higher, it is not buffered but sent to the server. Everything else in the buffer stays in the buffer though.
  5. When it receives a message with severity fatal, it is sent to the server along with all messages in the buffer.
  6. The buffer holds no more than 20 messages. If another message needs to be stored, the oldest message in the buffer is removed.

This way, you can generate as much debug data as you want without worrying about being swamped, because it will only be sent when you actually need it.

Correlating all messages related to a request

In a complicated application, you may get several log messages for a single request, some generated on the server, some generated on the client. You will want to tag each log message with a request id. That way, you can sort your log by request id and time stamp to get a unified picture of what happened for each request, first on the server and later on the client.

The JSNLog version for .Net generates a unique id for each request. When a JavaScript logger sends a log message, that message includes the request id, so it can be logged in your server side log.

It also provides a server side method RequestId, which gives you the same id. That way, you can include the same request id when a server side logger logs a message.

JSNLog uses the same request id that is used internally by IIS. This way, you can correlate with tracing done using Event Tracing for Windows (ETW). If you do not have ETW tracing switched on, JSNLog generates a random GUID.

Advanced options

JSNLog supports more advanced options that are beyond the scope of this article. These include batching of multiple log messages in one request to improve efficiencies, and filtering by client ip address to investigate problems reported by a particular user.


Once your JavaScript has gone live, it is prone to unexpected events, such as exceptions, failed AJAX requests and AJAX requests that take too long. To find out about these issues so you can fix them quickly, you need to log them to your server side log.

JSNLog is a relatively new client side logging package. Heavily optimized for life on the client, it sends log messages to the server running your MVC or Webforms or PHP site and includes many options to reduce the volume of log messages.

The next part, will show in detail all JavaScript functions provided by JSNLog.


This article, along with any associated source code and files, is licensed under The Code Project Open License (CPOL)


About the Author

Matt Perdeck
Australia Australia
Twitter: @MattPerdeck
Current project: JSNLog JavaScript Logging Package

Matt has over 9 years .NET and SQL Server development experience. Before getting into .Net, he worked on a number of systems, ranging from the largest ATM network in The Netherlands to embedded software in advanced Wide Area Networks and the largest ticketing web site in Australia. He has lived and worked in Australia, The Netherlands, Slovakia and Thailand.

He is the author of the book ASP.NET Performance Secrets ( in which he shows in clear and practical terms how to quickly find the biggest bottlenecks holding back the performance of your web site, and how to then remove those bottlenecks. The book deals with all environments affecting a web site - the web server, the database server and the browser.

Matt currently lives in Sydney, Australia. He recently worked at Readify and the global professional services company PwC. He now works at SP Health, a global provider of weight loss web sites such at CSIRO's and

Comments and Discussions

Questionno log record is appended Pin
Member 1146337018-Feb-15 22:58
MemberMember 1146337018-Feb-15 22:58 
I write some example code to throw an exception.
If the exception is throw in the server-side code it is written to a log table in the database with no problems.
If I try to throw a client-side Javascript exception it is NOT written to a log table.
I debug step by step through the jsnlog.js and I can see at the following place
var xhr = new XMLHttpRequest();'POST', ajaxUrl);
xhr.setRequestHeader('Content-Type', 'application/json');
xhr.setRequestHeader('JSNLog-RequestId', JL.requestId);
that the variable json has the right value.
Json = "{\"r\":\"\",\"lg\":[{\"l\":5000,\"m\":\"Exception - \\\"a\\\" ist undefiniert, url: http://localhost:58941/WebForm1.aspx, line number: 36\",\"n\":\"\",\"t\":1424335371528}]}"
But in my log table the record is not appended.
Do you have a tip to fix this problem?
QuestionCan System.Diagnostics be used for logging on the server side? Pin
klachko28-Sep-14 20:54
Memberklachko28-Sep-14 20:54 
AnswerRe: Can System.Diagnostics be used for logging on the server side? Pin
Matt Perdeck29-Sep-14 1:11
MemberMatt Perdeck29-Sep-14 1:11 
QuestionHow can i view log message on Server. Pin
Member 1107703711-Sep-14 22:02
MemberMember 1107703711-Sep-14 22:02 
AnswerRe: How can i view log message on Server. Pin
Matt Perdeck12-Sep-14 16:13
MemberMatt Perdeck12-Sep-14 16:13 

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