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User projections you create in JavaScript have a bonus that debugging is easier via any browser that ships with debugging capabilities. The following screenshots show the use of Chrome, but we have tested debugging with all major browsers which includes Firefox, Microsoft Edge and Safari.

Logging from within a Projection

For debugging purposes, projections includes a log method which will, when called, send messages to the configured Event Store logger (the default is NLog and to a file as well as stdout).

You might find printing out the structure of the event body for inspection useful.

For example:

    $any: function(s,e){

Starting and Configuring Event Store for Projections

The following configuration starts Event Store with all the projection modes enabled (user and system defined) and uses an in memory database which is suitable for development purposes.

EventStore.ClusterNode.exe --run-projections=all --mem-db

Creating a sample projection for debugging purposes

Filename: stats-counter.json


    $init: function(){
        return {
            count: 0
    $any: function(s,e){
        s.count += 1;

You create the projection by making a call to the API and providing it with the definition of the projection.

curl -i -d@stats-counter.json http://localhost:2113/projections/continuous?name=stats-counter%26type=js%26enabled=true%26emit=true%26trackemittedstreams=true -u admin:changeit

Debugging your first projection

Once the projection is running, open your browser and enable the developer tools. Once you have the developer tools open, visit your projection URL and you should see a button labelled Debug.

Projections Debugging Part 1

After clicking the projection “Debug” button, you will see the debugging interface with the definition of the projection and information about the events the projection is processing on the right hand side.

At the top there are couple of buttons to take note of, specifically the Run Step and Update buttons. You use Run Step to step through the event waiting in the queue, placing you in projection debugging mode. The Update button provides you with a way to update the projection definition without having to go back to the projection itself and leave the context of the debugger.

Projections Debugging Part 2

If the Run Step button is not greyed out and you click it, you will see that you browser has hit a breakpoint.

Projections Debugging Part 3

You are now able to step through the projection, the important method to step into so that you can debug your projection is to step into the handler(state, eventEnvelope) method.