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js-Logger Build Status npm version npm dependencies

Lightweight, unobtrusive, configurable JavaScript logger.

logger.js will make you rich, famous and want for almost nothing - oh and it's a flexible abstraction over using console.log as well.


js-Logger has zero dependencies and comes with AMD and CommonJS module boilerplate. If the last sentence meant nothing to you then just lob the following into your page:

<script src=""></script>

Have a look at babel-plugin-js-logger, in case your project utilizes Babel, and you want to use js-Logger throughout your entire project efficiently.


Nothing beats the sheer ecstasy of logging! js-Logger does its best to not be awkward and get in the way. If you're the sort of person who just wants to get down and dirty then all you need is one line of code:

// Log messages will be written to the window's console. 

Now, when you want to emit a red-hot log message, just drop one of the following (the syntax is identical to the console object)

Logger.debug("I'm a debug message!");"OMG! Check this window out!", window);
Logger.warn("Purple Alert! Purple Alert!");
Logger.error("HOLY SHI... no carrier.");

Log messages can get a bit annoying; you don't need to tell me, it's all cool. If things are getting too noisy for your liking then it's time you read up on the Logger.setLevel method:

// Only log WARN and ERROR messages. 
Logger.debug("Donut machine is out of pink ones");  // Not a peep. 
Logger.warn("Asteroid detected!");  // Logs "Asteroid detected!", best do something about that! 
// Ah, you know what, I'm sick of all these messages. 
// But I want to see them again later 
var oldLevel = Logger.getLevel();
Logger.error("Hull breach on decks 5 through to 41!");  // ... 
// Actually, maybe those logs were quite useful... 

Log Handler Functions

All log messages are routed through a handler function which redirects filtered messages somewhere. You can configure the handler function via Logger.setHandler noting that the supplied function expects two arguments; the first being the log messages to output and the latter being a context object which can be inspected by the log handler.

Logger.setHandler(function (messages, context) {
    // Send messages to a custom logging endpoint for analysis. 
    // TODO: Add some security? (nah, you worry too much! :P)'/logs', { message: messages[0], level: context.level });

Default Log Handler Function

js-Logger provides a default handler function which writes to your browser's console object using the appropriate logging functions based on the message's log level (ie: will result in a call to The default handler automatically shims for sub-optiomal environments right down to IE7's complete lack of console object (it only appears when you open the DevTools - seriosuly, this is one of the anti-user troll things I've seen!)

Use Logger.createDefaultHandler() to return a new log handler function which can then be supplied to Logger.setHandler().

You can customise the formatting of each log message by supplying a formatter function to createDefaultHandler:

    formatter: function(messages, context) {
        // prefix each log message with a timestamp. 
        messages.unshift(new Date().toUTCString())

You can use functional composition to extend the default handler with your own custom handler logic:

var consoleHandler = Logger.createDefaultHandler();
var myHandler = function (messages, context) {'/logs', { message: messages[0], level: context.level });
Logger.setHandler(function (messages, context) {
    consoleHandler(messages, context);
    myHandler(messages, context);


Logger.useDefaults() is a convenience function which allows you to configure both the default logLevel and handler in one go:

    defaultLevel: Logger.WARN,
    formatter: function (messages, context) {
        messages.unshift(new Date().toUTCString())

Named Loggers

Okay, let's get serious, logging is not for kids, it's for adults with serious software to write and mission critical log messages to trawl through. To help you in your goal, js-Logger provides 'named' loggers which can be configured individual with their own contexts.

// Retrieve a named logger and store it for use. 
var myLogger = Logger.get('ModuleA');"FizzWozz starting up");
// This logger instance can be configured independent of all others (including the global one). 
// As it's the same instance being returned each time, you don't have to store a reference: 
Logger.get('ModuleA').warn('FizzWozz combombulated!');

Note that Logger.setLevel() will also change the current log filter level for all named logger instances; so typically you would configure your logger levels like so:

// Create a couple of named loggers (typically in their own module) 
var loggerA = Logger.get('LoggerA');
var loggerB = Logger.get('LoggerB');
// Configure log levels. 
Logger.setLevel(Logger.WARN);  // Global logging level. 
Logger.get('LoggerB').setLevel(Logger.DEBUG);  // Enable debug logging for LoggerB 


Sometimes its good to know what's taking so damn long; you can use Logger.time() and Logger.timeEnd() to keep tabs on things, the default log handler implementation delegates to the equivalent console methods if they exist, or write to console.log if they don't.

// Start timing something 
Logger.time('self destruct sequence');
// ... Some time passes ... 
// Stop timing something. 
Logger.timeEnd('self destruct sequence'); // logs: 'self destruct sequence: 1022ms'. 

Note that time and timeEnd methods are also provided to named Logger instances.