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Logger & events


Requires @compas/stdlib to be installed


Although the stdlib package exports more than just a logger, this document will focus on the logger.


Provided by @compas/stdlib


This function constructs a new logger. It accepts an optional context object that is included in each log line for that logger. A log formatter is automatically configured based on the environment that your application is running in. If NODE_ENV is set to development a pretty printer will be used else it defaults to newline delimited json. Which boils down to a single stringified json object per log call.


  • options:
    • ctx (object): Any context to add to log lines. This value is copied immediately on logger creation, so changes made via a reference, will not be reflected.


Make sure to have a .env file with NODE_ENV=development in it for local development so log lines are readable.

A logger is a plain JavaScript object with 2 functions:

info and error:

The info and error function accept a single argument that is logged. This happens in a single write call when pretty printing or not.


An application is able to gradually add more information to the shared context used by loggers. All properties added via this function will end up in each log line like the ctx option of newLogger. You can still overwrite values via the ctx option of newLogger. The added properties will only show up in loggers created after calling this function.


The logger uses Pino under the hood, and thus is able to use its destination and transport system. This affects all loggers created via newLogger after setting a new destination.

On log levels and processing

Log levels are used to get some ranking based on importance in your log lines. Compas only knows 2 log levels: info and error. A log line is either important enough to warn you, or it is not. By simplifying this decision, you can spend a bit more time deciding if you really need to log that information or not.

There is much more information in logs than just the contrasting error- and info-'level', however this always requires further processing. Compas prefers that this happens outside the current process. That is why by default we write everything to stdout. To accommodate processing, every log line is a valid JSON string, also called newline delimited JSON or NDJSON for short. The log line is structured as follows:

// Created with `{ hello: "world" });`
  // The log level, either 'info' or 'error'
  level: "info",
  // Milliseconds since the unix epoch
  time: 1634375371114,
  // The `ctx` that is passed in `newLogger`
  // In this case the default by `mainFn` from `@compas/stdlib`
  context: {
    // File name without extension
    type: "api",
    // Value of process.env.APP_NAME, see environment variable docs
    application: "compas",
  // The value passed in to the log function
  // So can be an object, array, string, boolean, ...
  message: {
    hello: "world",

The log-processor can take these lines and do whatever is needed to check on 'indirect' errors like a spike of unauthorized requests.


Events are used for manually tracing call stacks. This way you can check how long your async functions took to run or analyze how often some route is called.

Events are created with newEvent(logger), most of the time you are not constructing that event or logger your self, but use some of the provided events:

  • newTestEvent(t) in tests
  • ctx.event in server middleware
  • event as the first argument passed in to queue functions

Let's check an example of a function that expects an event:

 * Add a & b, but before returning wait for a random amount of time
 * @param {InsightEvent} event
 * @param {number} a
 * @param {number} b
 * @returns {number}
async function addWithWait(event, a, b) {
  eventStart(event, "add");

  const result = a + b;
  await asyncSleep(Math.random() * 100); // Wait between 0 & 100 milliseconds

  // Always the last statement before the return statement

  return result;

As you can see we can use eventStart and eventStop to wrap our function body. We can also create 'sub-events' to follow the nested callstack. It looks something like:

function fooAdd(event, a, b) {
  eventStart(event, "foo.add");

  const result = fooAddAll(newEventFromEvent(event), a, b);


  return result;

function fooAddAll(event, ...args) {
  eventStart(event, "foo.addAll");

  const result = args.reduce((sum, next) => sum + next, 0);


  return result;

When eventStop is called on the 'root-event' i.e the event created by newEvent, it will automatically log the callstack via its logger. That will look something like:

11:11:13.390 info[script_name] {
  type: 'event_callstack',
  aborted: false,
  callStack: [
      type: 'start',
      name: 'foo.add',
      time: 1630401073390,
      duration: 0
        type: 'start',
        name: 'foo.addAll',
        time: 1630401073390,
        duration: 0
      { type: 'stop', name: 'foo.addAll', time: 1630401073390 }
    { type: 'stop', name: 'foo.add', time: 1630401073390 }

As you can see, both foo.add and foo.addAll events took 0 milliseconds. This is why we recommend only using events with async functions, as these most likely contain some network / database calls.