1# debug
2[![Build Status](https://travis-ci.org/visionmedia/debug.svg?branch=master)](https://travis-ci.org/visionmedia/debug)  [![Coverage Status](https://coveralls.io/repos/github/visionmedia/debug/badge.svg?branch=master)](https://coveralls.io/github/visionmedia/debug?branch=master)  [![Slack](https://visionmedia-community-slackin.now.sh/badge.svg)](https://visionmedia-community-slackin.now.sh/) [![OpenCollective](https://opencollective.com/debug/backers/badge.svg)](#backers)
3[![OpenCollective](https://opencollective.com/debug/sponsors/badge.svg)](#sponsors)
4
5
6
7A tiny node.js debugging utility modelled after node core's debugging technique.
8
9**Discussion around the V3 API is under way [here](https://github.com/visionmedia/debug/issues/370)**
10
11## Installation
12
13```bash
14$ npm install debug
15```
16
17## Usage
18
19`debug` exposes a function; simply pass this function the name of your module, and it will return a decorated version of `console.error` for you to pass debug statements to. This will allow you to toggle the debug output for different parts of your module as well as the module as a whole.
20
21Example _app.js_:
22
23```js
24var debug = require('debug')('http')
25  , http = require('http')
26  , name = 'My App';
27
28// fake app
29
30debug('booting %s', name);
31
32http.createServer(function(req, res){
33  debug(req.method + ' ' + req.url);
34  res.end('hello\n');
35}).listen(3000, function(){
36  debug('listening');
37});
38
39// fake worker of some kind
40
41require('./worker');
42```
43
44Example _worker.js_:
45
46```js
47var debug = require('debug')('worker');
48
49setInterval(function(){
50  debug('doing some work');
51}, 1000);
52```
53
54 The __DEBUG__ environment variable is then used to enable these based on space or comma-delimited names. Here are some examples:
55
56  ![debug http and worker](http://f.cl.ly/items/18471z1H402O24072r1J/Screenshot.png)
57
58  ![debug worker](http://f.cl.ly/items/1X413v1a3M0d3C2c1E0i/Screenshot.png)
59
60#### Windows note
61
62 On Windows the environment variable is set using the `set` command.
63
64 ```cmd
65 set DEBUG=*,-not_this
66 ```
67
68 Note that PowerShell uses different syntax to set environment variables.
69
70 ```cmd
71 $env:DEBUG = "*,-not_this"
72  ```
73
74Then, run the program to be debugged as usual.
75
76## Millisecond diff
77
78  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.
79
80  ![](http://f.cl.ly/items/2i3h1d3t121M2Z1A3Q0N/Screenshot.png)
81
82  When stdout is not a TTY, `Date#toUTCString()` is used, making it more useful for logging the debug information as shown below:
83
84  ![](http://f.cl.ly/items/112H3i0e0o0P0a2Q2r11/Screenshot.png)
85
86## Conventions
87
88  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".
89
90## Wildcards
91
92  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=*`.
93
94  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:".
95
96## Environment Variables
97
98  When running through Node.js, you can set a few environment variables that will
99  change the behavior of the debug logging:
100
101| Name      | Purpose                                         |
102|-----------|-------------------------------------------------|
103| `DEBUG`   | Enables/disables specific debugging namespaces. |
104| `DEBUG_COLORS`| Whether or not to use colors in the debug output. |
105| `DEBUG_DEPTH` | Object inspection depth. |
106| `DEBUG_SHOW_HIDDEN` | Shows hidden properties on inspected objects. |
107
108
109  __Note:__ The environment variables beginning with `DEBUG_` end up being
110  converted into an Options object that gets used with `%o`/`%O` formatters.
111  See the Node.js documentation for
112  [`util.inspect()`](https://nodejs.org/api/util.html#util_util_inspect_object_options)
113  for the complete list.
114
115## Formatters
116
117
118  Debug uses [printf-style](https://wikipedia.org/wiki/Printf_format_string) formatting. Below are the officially supported formatters:
119
120| Formatter | Representation |
121|-----------|----------------|
122| `%O`      | Pretty-print an Object on multiple lines. |
123| `%o`      | Pretty-print an Object all on a single line. |
124| `%s`      | String. |
125| `%d`      | Number (both integer and float). |
126| `%j`      | JSON. Replaced with the string '[Circular]' if the argument contains circular references. |
127| `%%`      | Single percent sign ('%'). This does not consume an argument. |
128
129### Custom formatters
130
131  You can add custom formatters by extending the `debug.formatters` object. For example, if you wanted to add support for rendering a Buffer as hex with `%h`, you could do something like:
132
133```js
134const createDebug = require('debug')
135createDebug.formatters.h = (v) => {
136  return v.toString('hex')
137}
138
139// …elsewhere
140const debug = createDebug('foo')
141debug('this is hex: %h', new Buffer('hello world'))
142//   foo this is hex: 68656c6c6f20776f726c6421 +0ms
143```
144
145## Browser support
146  You can build a browser-ready script using [browserify](https://github.com/substack/node-browserify),
147  or just use the [browserify-as-a-service](https://wzrd.in/) [build](https://wzrd.in/standalone/debug@latest),
148  if you don't want to build it yourself.
149
150  Debug's enable state is currently persisted by `localStorage`.
151  Consider the situation shown below where you have `worker:a` and `worker:b`,
152  and wish to debug both. You can enable this using `localStorage.debug`:
153
154```js
155localStorage.debug = 'worker:*'
156```
157
158And then refresh the page.
159
160```js
161a = debug('worker:a');
162b = debug('worker:b');
163
164setInterval(function(){
165  a('doing some work');
166}, 1000);
167
168setInterval(function(){
169  b('doing some work');
170}, 1200);
171```
172
173#### Web Inspector Colors
174
175  Colors are also enabled on "Web Inspectors" that understand the `%c` formatting
176  option. These are WebKit web inspectors, Firefox ([since version
177  31](https://hacks.mozilla.org/2014/05/editable-box-model-multiple-selection-sublime-text-keys-much-more-firefox-developer-tools-episode-31/))
178  and the Firebug plugin for Firefox (any version).
179
180  Colored output looks something like:
181
182  ![](https://cloud.githubusercontent.com/assets/71256/3139768/b98c5fd8-e8ef-11e3-862a-f7253b6f47c6.png)
183
184
185## Output streams
186
187  By default `debug` will log to stderr, however this can be configured per-namespace by overriding the `log` method:
188
189Example _stdout.js_:
190
191```js
192var debug = require('debug');
193var error = debug('app:error');
194
195// by default stderr is used
196error('goes to stderr!');
197
198var log = debug('app:log');
199// set this namespace to log via console.log
200log.log = console.log.bind(console); // don't forget to bind to console!
201log('goes to stdout');
202error('still goes to stderr!');
203
204// set all output to go via console.info
205// overrides all per-namespace log settings
206debug.log = console.info.bind(console);
207error('now goes to stdout via console.info');
208log('still goes to stdout, but via console.info now');
209```
210
211
212## Authors
213
214 - TJ Holowaychuk
215 - Nathan Rajlich
216 - Andrew Rhyne
217
218## Backers
219
220Support us with a monthly donation and help us continue our activities. [[Become a backer](https://opencollective.com/debug#backer)]
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252
253
254## Sponsors
255
256Become a sponsor and get your logo on our README on Github with a link to your site. [[Become a sponsor](https://opencollective.com/debug#sponsor)]
257
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288
289## License
290
291(The MIT License)
292
293Copyright (c) 2014-2016 TJ Holowaychuk &lt;tj@vision-media.ca&gt;
294
295Permission is hereby granted, free of charge, to any person obtaining
296a copy of this software and associated documentation files (the
297'Software'), to deal in the Software without restriction, including
298without limitation the rights to use, copy, modify, merge, publish,
299distribute, sublicense, and/or sell copies of the Software, and to
300permit persons to whom the Software is furnished to do so, subject to
301the following conditions:
302
303The above copyright notice and this permission notice shall be
304included in all copies or substantial portions of the Software.
305
306THE SOFTWARE IS PROVIDED 'AS IS', WITHOUT WARRANTY OF ANY KIND,
307EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
308MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.
309IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY
310CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT,
311TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE
312SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
313