README.md (19827B)
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 <img width="647" src="https://user-images.githubusercontent.com/71256/29091486-fa38524c-7c37-11e7-895f-e7ec8e1039b6.png"> 6 7 A tiny JavaScript debugging utility modelled after Node.js core's debugging 8 technique. Works in Node.js and web browsers. 9 10 ## Installation 11 12 ```bash 13 $ npm install debug 14 ``` 15 16 ## Usage 17 18 `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. 19 20 Example [_app.js_](./examples/node/app.js): 21 22 ```js 23 var debug = require('debug')('http') 24 , http = require('http') 25 , name = 'My App'; 26 27 // fake app 28 29 debug('booting %o', name); 30 31 http.createServer(function(req, res){ 32 debug(req.method + ' ' + req.url); 33 res.end('hello\n'); 34 }).listen(3000, function(){ 35 debug('listening'); 36 }); 37 38 // fake worker of some kind 39 40 require('./worker'); 41 ``` 42 43 Example [_worker.js_](./examples/node/worker.js): 44 45 ```js 46 var a = require('debug')('worker:a') 47 , b = require('debug')('worker:b'); 48 49 function work() { 50 a('doing lots of uninteresting work'); 51 setTimeout(work, Math.random() * 1000); 52 } 53 54 work(); 55 56 function workb() { 57 b('doing some work'); 58 setTimeout(workb, Math.random() * 2000); 59 } 60 61 workb(); 62 ``` 63 64 The `DEBUG` environment variable is then used to enable these based on space or 65 comma-delimited names. 66 67 Here are some examples: 68 69 <img width="647" alt="screen shot 2017-08-08 at 12 53 04 pm" src="https://user-images.githubusercontent.com/71256/29091703-a6302cdc-7c38-11e7-8304-7c0b3bc600cd.png"> 70 <img width="647" alt="screen shot 2017-08-08 at 12 53 38 pm" src="https://user-images.githubusercontent.com/71256/29091700-a62a6888-7c38-11e7-800b-db911291ca2b.png"> 71 <img width="647" alt="screen shot 2017-08-08 at 12 53 25 pm" src="https://user-images.githubusercontent.com/71256/29091701-a62ea114-7c38-11e7-826a-2692bedca740.png"> 72 73 #### Windows note 74 75 On Windows the environment variable is set using the `set` command. 76 77 ```cmd 78 set DEBUG=*,-not_this 79 ``` 80 81 Note that PowerShell uses different syntax to set environment variables. 82 83 ```cmd 84 $env:DEBUG = "*,-not_this" 85 ``` 86 87 Then, run the program to be debugged as usual. 88 89 90 ## Namespace Colors 91 92 Every debug instance has a color generated for it based on its namespace name. 93 This helps when visually parsing the debug output to identify which debug instance 94 a debug line belongs to. 95 96 #### Node.js 97 98 In Node.js, colors are enabled when stderr is a TTY. You also _should_ install 99 the [`supports-color`](https://npmjs.org/supports-color) module alongside debug, 100 otherwise debug will only use a small handful of basic colors. 101 102 <img width="521" src="https://user-images.githubusercontent.com/71256/29092181-47f6a9e6-7c3a-11e7-9a14-1928d8a711cd.png"> 103 104 #### Web Browser 105 106 Colors are also enabled on "Web Inspectors" that understand the `%c` formatting 107 option. These are WebKit web inspectors, Firefox ([since version 108 31](https://hacks.mozilla.org/2014/05/editable-box-model-multiple-selection-sublime-text-keys-much-more-firefox-developer-tools-episode-31/)) 109 and the Firebug plugin for Firefox (any version). 110 111 <img width="524" src="https://user-images.githubusercontent.com/71256/29092033-b65f9f2e-7c39-11e7-8e32-f6f0d8e865c1.png"> 112 113 114 ## Millisecond diff 115 116 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. 117 118 <img width="647" src="https://user-images.githubusercontent.com/71256/29091486-fa38524c-7c37-11e7-895f-e7ec8e1039b6.png"> 119 120 When stdout is not a TTY, `Date#toISOString()` is used, making it more useful for logging the debug information as shown below: 121 122 <img width="647" src="https://user-images.githubusercontent.com/71256/29091956-6bd78372-7c39-11e7-8c55-c948396d6edd.png"> 123 124 125 ## Conventions 126 127 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". If you append a "*" to the end of your name, it will always be enabled regardless of the setting of the DEBUG environment variable. You can then use it for normal output as well as debug output. 128 129 ## Wildcards 130 131 The `*` character may be used as a wildcard. Suppose for example your library has 132 debuggers named "connect:bodyParser", "connect:compress", "connect:session", 133 instead of listing all three with 134 `DEBUG=connect:bodyParser,connect:compress,connect:session`, you may simply do 135 `DEBUG=connect:*`, or to run everything using this module simply use `DEBUG=*`. 136 137 You can also exclude specific debuggers by prefixing them with a "-" character. 138 For example, `DEBUG=*,-connect:*` would include all debuggers except those 139 starting with "connect:". 140 141 ## Environment Variables 142 143 When running through Node.js, you can set a few environment variables that will 144 change the behavior of the debug logging: 145 146 | Name | Purpose | 147 |-----------|-------------------------------------------------| 148 | `DEBUG` | Enables/disables specific debugging namespaces. | 149 | `DEBUG_HIDE_DATE` | Hide date from debug output (non-TTY). | 150 | `DEBUG_COLORS`| Whether or not to use colors in the debug output. | 151 | `DEBUG_DEPTH` | Object inspection depth. | 152 | `DEBUG_SHOW_HIDDEN` | Shows hidden properties on inspected objects. | 153 154 155 __Note:__ The environment variables beginning with `DEBUG_` end up being 156 converted into an Options object that gets used with `%o`/`%O` formatters. 157 See the Node.js documentation for 158 [`util.inspect()`](https://nodejs.org/api/util.html#util_util_inspect_object_options) 159 for the complete list. 160 161 ## Formatters 162 163 Debug uses [printf-style](https://wikipedia.org/wiki/Printf_format_string) formatting. 164 Below are the officially supported formatters: 165 166 | Formatter | Representation | 167 |-----------|----------------| 168 | `%O` | Pretty-print an Object on multiple lines. | 169 | `%o` | Pretty-print an Object all on a single line. | 170 | `%s` | String. | 171 | `%d` | Number (both integer and float). | 172 | `%j` | JSON. Replaced with the string '[Circular]' if the argument contains circular references. | 173 | `%%` | Single percent sign ('%'). This does not consume an argument. | 174 175 176 ### Custom formatters 177 178 You can add custom formatters by extending the `debug.formatters` object. 179 For example, if you wanted to add support for rendering a Buffer as hex with 180 `%h`, you could do something like: 181 182 ```js 183 const createDebug = require('debug') 184 createDebug.formatters.h = (v) => { 185 return v.toString('hex') 186 } 187 188 // …elsewhere 189 const debug = createDebug('foo') 190 debug('this is hex: %h', new Buffer('hello world')) 191 // foo this is hex: 68656c6c6f20776f726c6421 +0ms 192 ``` 193 194 195 ## Browser Support 196 197 You can build a browser-ready script using [browserify](https://github.com/substack/node-browserify), 198 or just use the [browserify-as-a-service](https://wzrd.in/) [build](https://wzrd.in/standalone/debug@latest), 199 if you don't want to build it yourself. 200 201 Debug's enable state is currently persisted by `localStorage`. 202 Consider the situation shown below where you have `worker:a` and `worker:b`, 203 and wish to debug both. You can enable this using `localStorage.debug`: 204 205 ```js 206 localStorage.debug = 'worker:*' 207 ``` 208 209 And then refresh the page. 210 211 ```js 212 a = debug('worker:a'); 213 b = debug('worker:b'); 214 215 setInterval(function(){ 216 a('doing some work'); 217 }, 1000); 218 219 setInterval(function(){ 220 b('doing some work'); 221 }, 1200); 222 ``` 223 224 225 ## Output streams 226 227 By default `debug` will log to stderr, however this can be configured per-namespace by overriding the `log` method: 228 229 Example [_stdout.js_](./examples/node/stdout.js): 230 231 ```js 232 var debug = require('debug'); 233 var error = debug('app:error'); 234 235 // by default stderr is used 236 error('goes to stderr!'); 237 238 var log = debug('app:log'); 239 // set this namespace to log via console.log 240 log.log = console.log.bind(console); // don't forget to bind to console! 241 log('goes to stdout'); 242 error('still goes to stderr!'); 243 244 // set all output to go via console.info 245 // overrides all per-namespace log settings 246 debug.log = console.info.bind(console); 247 error('now goes to stdout via console.info'); 248 log('still goes to stdout, but via console.info now'); 249 ``` 250 251 ## Checking whether a debug target is enabled 252 253 After you've created a debug instance, you can determine whether or not it is 254 enabled by checking the `enabled` property: 255 256 ```javascript 257 const debug = require('debug')('http'); 258 259 if (debug.enabled) { 260 // do stuff... 261 } 262 ``` 263 264 You can also manually toggle this property to force the debug instance to be 265 enabled or disabled. 266 267 268 ## Authors 269 270 - TJ Holowaychuk 271 - Nathan Rajlich 272 - Andrew Rhyne 273 274 ## Backers 275 276 Support us with a monthly donation and help us continue our activities. 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License 346 347 (The MIT License) 348 349 Copyright (c) 2014-2017 TJ Holowaychuk <tj@vision-media.ca> 350 351 Permission is hereby granted, free of charge, to any person obtaining 352 a copy of this software and associated documentation files (the 353 'Software'), to deal in the Software without restriction, including 354 without limitation the rights to use, copy, modify, merge, publish, 355 distribute, sublicense, and/or sell copies of the Software, and to 356 permit persons to whom the Software is furnished to do so, subject to 357 the following conditions: 358 359 The above copyright notice and this permission notice shall be 360 included in all copies or substantial portions of the Software. 361 362 THE SOFTWARE IS PROVIDED 'AS IS', WITHOUT WARRANTY OF ANY KIND, 363 EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF 364 MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. 365 IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY 366 CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, 367 TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE 368 SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.