debug

tiny node.js debugging utility.

Installation

$ npm install debug

Example

This module is modelled after node core's debugging technique, allowing you to enable one or more topic-specific debugging functions, for example core does the following within many modules:

var debug;
if (process.env.NODE_DEBUG && /cluster/.test(process.env.NODE_DEBUG)) {
  debug = function(x) {
    var prefix = process.pid + ',' +
        (process.env.NODE_WORKER_ID ? 'Worker' : 'Master');
    console.error(prefix, x);
  };
} else {
  debug = function() { };
}

This concept is extremely simple but it works well. 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 useDEBUG=*`.

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(){
  a('doing some work');
}, 1200);

License

(The MIT License)

Copyright (c) 2011 TJ Holowaychuk <tj@vision-media.ca>

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.