README.md
1 # debug 2 [](https://travis-ci.org/visionmedia/debug) [](https://coveralls.io/github/visionmedia/debug?branch=master) [](https://visionmedia-community-slackin.now.sh/) [](#backers) 3 [](#sponsors) 4 5 6 7 A 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 21 Example _app.js_: 22 23 ```js 24 var debug = require('debug')('http') 25 , http = require('http') 26 , name = 'My App'; 27 28 // fake app 29 30 debug('booting %s', name); 31 32 http.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 41 require('./worker'); 42 ``` 43 44 Example _worker.js_: 45 46 ```js 47 var debug = require('debug')('worker'); 48 49 setInterval(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  57 58  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 74 Then, 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  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  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 134 const createDebug = require('debug') 135 createDebug.formatters.h = (v) => { 136 return v.toString('hex') 137 } 138 139 // …elsewhere 140 const debug = createDebug('foo') 141 debug('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 155 localStorage.debug = 'worker:*' 156 ``` 157 158 And then refresh the page. 159 160 ```js 161 a = debug('worker:a'); 162 b = debug('worker:b'); 163 164 setInterval(function(){ 165 a('doing some work'); 166 }, 1000); 167 168 setInterval(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  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: 188 189 Example _stdout.js_: 190 191 ```js 192 var debug = require('debug'); 193 var error = debug('app:error'); 194 195 // by default stderr is used 196 error('goes to stderr!'); 197 198 var log = debug('app:log'); 199 // set this namespace to log via console.log 200 log.log = console.log.bind(console); // don't forget to bind to console! 201 log('goes to stdout'); 202 error('still goes to stderr!'); 203 204 // set all output to go via console.info 205 // overrides all per-namespace log settings 206 debug.log = console.info.bind(console); 207 error('now goes to stdout via console.info'); 208 log('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 219 220 Support us with a monthly donation and help us continue our activities. 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License 290 291 (The MIT License) 292 293 Copyright (c) 2014-2016 TJ Holowaychuk <tj@vision-media.ca> 294 295 Permission is hereby granted, free of charge, to any person obtaining 296 a copy of this software and associated documentation files (the 297 'Software'), to deal in the Software without restriction, including 298 without limitation the rights to use, copy, modify, merge, publish, 299 distribute, sublicense, and/or sell copies of the Software, and to 300 permit persons to whom the Software is furnished to do so, subject to 301 the following conditions: 302 303 The above copyright notice and this permission notice shall be 304 included in all copies or substantial portions of the Software. 305 306 THE SOFTWARE IS PROVIDED 'AS IS', WITHOUT WARRANTY OF ANY KIND, 307 EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF 308 MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. 309 IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY 310 CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, 311 TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE 312 SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.