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:

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. For example if you have worker:a and worker:b as shown below, and wish to debug both type debug.enable('worker:*') in the console and refresh the page, this will remain until you disable with debug.disable().

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 stderr.js:

var debug = require('../');
var log = debug('app:log');

// by default console.log is used
log('goes to stdout!');

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

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

Authors

• TJ Holowaychuk
• Nathan Rajlich

License

(The MIT License)

Copyright (c) 2014 TJ Holowaychuk <[email protected]>

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.