JavaScript Logging

This document details logging in JavaScript. For c++ based logging, please see the Gecko Logging document.

Notes About Logging

Logging Ethos

The general ethos is to keep the browser console “clean” of logging output by default, except in situations where there are errors or potentially warnings.

There is also a test which will fail if there are any unexpected logs on startup.

For situations where it is useful to have debug logging available, these are generally covered by off-by-default logging which can be enabled by a preference.

Also be aware that console logging can have a performance impact even when the developer tools are not displayed.

Obsolete Utilities

In the tree, there are two modules that should be considered obsolete: Log.sys.mjs and Console.sys.mjs. Existing uses should be transitioned to use console.createInstance as part of the one logger effort.

The console object is available in all areas of the Firefox code base and has better integration into the developer tools than the existing modules.

Logging using the Console Web API

The Console Web API is available throughout the Firefox code base. It is the best tool to use, as it integrates directly with the developer tools, which provide detailed logging facilities.

By default logs will be output to the browser console, according to the processes and filters selected within the console itself.

Logs may also be output to stdout via the preference. By default, this is set to true for non-official builds, and false for official builds, meaning most of the time, developers do not need to change it.


In some cases, it is useful to attribute logging to a particular module or have it controlled by a preference. In this case, console.createInstance may be used to create a specific logging instance.

For example:

const lazy = {};

ChromeUtils.defineLazyGetter(lazy, "logConsole", () => {
  return console.createInstance({
    maxLogLevelPref: "",
    prefix: "Downloads",

The preference might have a typical default value of Error. The available levels are listed in Console.webidl.

This creates a lazily initialized console instance that can be used as follows:

// Logs by default.
lazy.logConsole.error("Something bad happened");

// Doesn't log unless the preference was changed prior to logging.
lazy.logConsole.debug("foo", 123)

Note: Workers are not able to access preferences, and therefore must use maxLogLevel rather than maxLogLevelPref.

Other Options to console.createInstance

console.createInstance may be passed other options. See ConsoleInstanceOptions in the Web IDL for more details

The most useful of these is probably maxLogLevel which allows manually setting the log level, which may be useful for transitioning to console.createInstance from other logging systems where preferences already exist.