debug

What is debug?

The debug npm package is a flexible debugging utility that allows developers to log messages in a structured and conditional way. It is designed to be simple to use and to support modular applications by allowing developers to enable or disable logging on a per-module or per-scope basis.

What are debug's main functionalities?

Conditional Logging

This feature allows developers to create conditional logging statements that can be enabled or disabled based on the namespace. The '%o' formatter can be used to pretty-print objects.

const debug = require('debug')('http');
debug('booting %o', 'My App');
if (error) debug('Error: %o', error);

Namespacing Logs

Namespacing allows developers to categorize logs into different sections, which can be individually toggled on or off. This is useful for separating logs from different parts of an application.

const debug = require('debug');
const log = debug('myapp:log');
const error = debug('myapp:error');
log('This will be logged under the myapp:log namespace');
error('This will be logged under the myapp:error namespace');

Environment Variable Control

The debug package can be controlled using the DEBUG environment variable, which allows developers to specify which namespaces should be logged. This can be set before running the application.

DEBUG=myapp:* node app.js

Browser Support

The debug package also works in the browser. Namespaces can be enabled by setting the debug key in localStorage to the desired namespace.

localStorage.debug = 'myapp:*';

Other packages similar to debug

winston

Winston is a multi-transport async logging library for Node.js. Unlike debug, which is primarily for enabling/disabling logging, winston provides more features like multiple storage options for logs, custom log levels, and log formatting.

pino

Pino is a very low overhead Node.js logger, which focuses on performance. It provides similar functionality to debug but is optimized for speed and includes features like child loggers and custom serializers.

log4js

Log4js is a logging framework for Node.js, which is inspired by the Java library log4j. It supports multiple appenders, log categories, and layouts. It is more complex and configurable compared to debug.

README

debug

tiny node.js debugging utility modelled after node core's debugging technique.

Installation

$ npm install debug

Usage

With debug you simply invoke the exported function to generate your debug function, passing it a name which will determine if a noop function is returned, or a decorated console.error, so all of the console format string goodies you're used to work fine. A unique color is selected per-function for visibility.

Example app.js:

var debug = require('debug')('http')
  , http = require('http')
  , name = 'My App';

// fake app

debug('booting %s', name);

http.createServer(function(req, res){
  debug(req.method + ' ' + req.url);
  res.end('hello\n');
}).listen(3000, function(){
  debug('listening');
});

// fake worker of some kind

require('./worker');

Example worker.js:

var debug = require('debug')('worker');

setInterval(function(){
  debug('doing some work');
}, 1000);

The DEBUG environment variable is then used to enable these based on space or comma-delimited names. Here are some examples:

Windows note

On Windows the environment variable is set using the set command.

set DEBUG=*,-not_this

Then, run the program to be debugged as usual.

Millisecond diff

When actively developing an application it can be useful to see when the time spent between one debug() call and the next. Suppose for example you invoke debug() before requesting a resource, and after as well, the "+NNNms" will show you how much time was spent between calls.

When stdout is not a TTY, Date#toUTCString() is used, making it more useful for logging the debug information as shown below:

Conventions

If you're using this in one or more of your libraries, you should use the name of your library so that developers may toggle debugging as desired without guessing names. If you have more than one debuggers you should prefix them with your library name and use ":" to separate features. For example "bodyParser" from Connect would then be "connect:bodyParser".

Wildcards

The * character may be used as a wildcard. Suppose for example your library has debuggers named "connect:bodyParser", "connect:compress", "connect:session", instead of listing all three with DEBUG=connect:bodyParser,connect.compress,connect:session, you may simply do DEBUG=connect:*, or to run everything using this module simply use DEBUG=*.

You can also exclude specific debuggers by prefixing them with a "-" character. For example, DEBUG=*,-connect:* would include all debuggers except those starting with "connect:".

Browser support

Debug works in the browser as well, currently persisted by localStorage. Consider the situation shown below where you have worker:a and worker:b, and wish to debug both. Somewhere in the code on your page, include:

window.myDebug = require("debug");

("debug" is a global object in the browser so we give this object a different name.) When your page is open in the browser, type the following in the console:

myDebug.enable("worker:*")

Refresh the page. Debug output will continue to be sent to the console until it is disabled by typing myDebug.disable() in the console.

a = debug('worker:a');
b = debug('worker:b');

setInterval(function(){
  a('doing some work');
}, 1000);

setInterval(function(){
  b('doing some work');
}, 1200);

Web Inspector Colors

Colors are also enabled on "Web Inspectors" that understand the %c formatting option. These are WebKit web inspectors, Firefox (since version 31) and the Firebug plugin for Firefox (any version).

Colored output looks something like:

stderr vs stdout

You can set an alternative logging method per-namespace by overriding the log method on a per-namespace or globally:

Example stdout.js:

var debug = require('debug');
var error = debug('app:error');

// by default stderr is used
error('goes to stderr!');

var log = debug('app:log');
// set this namespace to log via console.log
log.log = console.log.bind(console); // don't forget to bind to console!
log('goes to stdout');
error('still goes to stderr!');

// set all output to go via console.info
// overrides all per-namespace log settings
debug.log = console.info.bind(console);
error('now goes to stdout via console.info');
log('still goes to stdout, but via console.info now');

Authors

• TJ Holowaychuk
• Nathan Rajlich

License

(The MIT License)

Copyright (c) 2014 TJ Holowaychuk <[e-mail]>

Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the 'Software'), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED 'AS IS', WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.