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';
debug('booting %s', name);
http.createServer(function(req, res){
debug(req.method + ' ' + req.url);
res.end('hello\n');
}).listen(3000, function(){
debug('listening');
});
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:
![debug http and worker](http://f.cl.ly/items/18471z1H402O24072r1J/Screenshot.png)
![debug worker](http://f.cl.ly/items/1X413v1a3M0d3C2c1E0i/Screenshot.png)
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.
![](http://f.cl.ly/items/2i3h1d3t121M2Z1A3Q0N/Screenshot.png)
When stdout is not a TTY, Date#toUTCString()
is used, making it more useful for logging the debug information as shown below:
![](http://f.cl.ly/items/112H3i0e0o0P0a2Q2r11/Screenshot.png)
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:
![](https://cloud.githubusercontent.com/assets/71256/3139768/b98c5fd8-e8ef-11e3-862a-f7253b6f47c6.png)
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');
error('goes to stderr!');
var log = debug('app:log');
log.log = console.log.bind(console);
log('goes to stdout');
error('still goes to stderr!');
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 <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.