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