README.md
1 # debug 2 [](https://travis-ci.org/debug-js/debug) [](https://coveralls.io/github/debug-js/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 command prompt notes 74 75 ##### CMD 76 77 On Windows the environment variable is set using the `set` command. 78 79 ```cmd 80 set DEBUG=*,-not_this 81 ``` 82 83 Example: 84 85 ```cmd 86 set DEBUG=* & node app.js 87 ``` 88 89 ##### PowerShell (VS Code default) 90 91 PowerShell uses different syntax to set environment variables. 92 93 ```cmd 94 $env:DEBUG = "*,-not_this" 95 ``` 96 97 Example: 98 99 ```cmd 100 $env:DEBUG='app';node app.js 101 ``` 102 103 Then, run the program to be debugged as usual. 104 105 npm script example: 106 ```js 107 "windowsDebug": "@powershell -Command $env:DEBUG='*';node app.js", 108 ``` 109 110 ## Namespace Colors 111 112 Every debug instance has a color generated for it based on its namespace name. 113 This helps when visually parsing the debug output to identify which debug instance 114 a debug line belongs to. 115 116 #### Node.js 117 118 In Node.js, colors are enabled when stderr is a TTY. You also _should_ install 119 the [`supports-color`](https://npmjs.org/supports-color) module alongside debug, 120 otherwise debug will only use a small handful of basic colors. 121 122 <img width="521" src="https://user-images.githubusercontent.com/71256/29092181-47f6a9e6-7c3a-11e7-9a14-1928d8a711cd.png"> 123 124 #### Web Browser 125 126 Colors are also enabled on "Web Inspectors" that understand the `%c` formatting 127 option. These are WebKit web inspectors, Firefox ([since version 128 31](https://hacks.mozilla.org/2014/05/editable-box-model-multiple-selection-sublime-text-keys-much-more-firefox-developer-tools-episode-31/)) 129 and the Firebug plugin for Firefox (any version). 130 131 <img width="524" src="https://user-images.githubusercontent.com/71256/29092033-b65f9f2e-7c39-11e7-8e32-f6f0d8e865c1.png"> 132 133 134 ## Millisecond diff 135 136 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. 137 138 <img width="647" src="https://user-images.githubusercontent.com/71256/29091486-fa38524c-7c37-11e7-895f-e7ec8e1039b6.png"> 139 140 When stdout is not a TTY, `Date#toISOString()` is used, making it more useful for logging the debug information as shown below: 141 142 <img width="647" src="https://user-images.githubusercontent.com/71256/29091956-6bd78372-7c39-11e7-8c55-c948396d6edd.png"> 143 144 145 ## Conventions 146 147 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. 148 149 ## Wildcards 150 151 The `*` character may be used as a wildcard. Suppose for example your library has 152 debuggers named "connect:bodyParser", "connect:compress", "connect:session", 153 instead of listing all three with 154 `DEBUG=connect:bodyParser,connect:compress,connect:session`, you may simply do 155 `DEBUG=connect:*`, or to run everything using this module simply use `DEBUG=*`. 156 157 You can also exclude specific debuggers by prefixing them with a "-" character. 158 For example, `DEBUG=*,-connect:*` would include all debuggers except those 159 starting with "connect:". 160 161 ## Environment Variables 162 163 When running through Node.js, you can set a few environment variables that will 164 change the behavior of the debug logging: 165 166 | Name | Purpose | 167 |-----------|-------------------------------------------------| 168 | `DEBUG` | Enables/disables specific debugging namespaces. | 169 | `DEBUG_HIDE_DATE` | Hide date from debug output (non-TTY). | 170 | `DEBUG_COLORS`| Whether or not to use colors in the debug output. | 171 | `DEBUG_DEPTH` | Object inspection depth. | 172 | `DEBUG_SHOW_HIDDEN` | Shows hidden properties on inspected objects. | 173 174 175 __Note:__ The environment variables beginning with `DEBUG_` end up being 176 converted into an Options object that gets used with `%o`/`%O` formatters. 177 See the Node.js documentation for 178 [`util.inspect()`](https://nodejs.org/api/util.html#util_util_inspect_object_options) 179 for the complete list. 180 181 ## Formatters 182 183 Debug uses [printf-style](https://wikipedia.org/wiki/Printf_format_string) formatting. 184 Below are the officially supported formatters: 185 186 | Formatter | Representation | 187 |-----------|----------------| 188 | `%O` | Pretty-print an Object on multiple lines. | 189 | `%o` | Pretty-print an Object all on a single line. | 190 | `%s` | String. | 191 | `%d` | Number (both integer and float). | 192 | `%j` | JSON. Replaced with the string '[Circular]' if the argument contains circular references. | 193 | `%%` | Single percent sign ('%'). This does not consume an argument. | 194 195 196 ### Custom formatters 197 198 You can add custom formatters by extending the `debug.formatters` object. 199 For example, if you wanted to add support for rendering a Buffer as hex with 200 `%h`, you could do something like: 201 202 ```js 203 const createDebug = require('debug') 204 createDebug.formatters.h = (v) => { 205 return v.toString('hex') 206 } 207 208 // …elsewhere 209 const debug = createDebug('foo') 210 debug('this is hex: %h', new Buffer('hello world')) 211 // foo this is hex: 68656c6c6f20776f726c6421 +0ms 212 ``` 213 214 215 ## Browser Support 216 217 You can build a browser-ready script using [browserify](https://github.com/substack/node-browserify), 218 or just use the [browserify-as-a-service](https://wzrd.in/) [build](https://wzrd.in/standalone/debug@latest), 219 if you don't want to build it yourself. 220 221 Debug's enable state is currently persisted by `localStorage`. 222 Consider the situation shown below where you have `worker:a` and `worker:b`, 223 and wish to debug both. You can enable this using `localStorage.debug`: 224 225 ```js 226 localStorage.debug = 'worker:*' 227 ``` 228 229 And then refresh the page. 230 231 ```js 232 a = debug('worker:a'); 233 b = debug('worker:b'); 234 235 setInterval(function(){ 236 a('doing some work'); 237 }, 1000); 238 239 setInterval(function(){ 240 b('doing some work'); 241 }, 1200); 242 ``` 243 244 245 ## Output streams 246 247 By default `debug` will log to stderr, however this can be configured per-namespace by overriding the `log` method: 248 249 Example [_stdout.js_](./examples/node/stdout.js): 250 251 ```js 252 var debug = require('debug'); 253 var error = debug('app:error'); 254 255 // by default stderr is used 256 error('goes to stderr!'); 257 258 var log = debug('app:log'); 259 // set this namespace to log via console.log 260 log.log = console.log.bind(console); // don't forget to bind to console! 261 log('goes to stdout'); 262 error('still goes to stderr!'); 263 264 // set all output to go via console.info 265 // overrides all per-namespace log settings 266 debug.log = console.info.bind(console); 267 error('now goes to stdout via console.info'); 268 log('still goes to stdout, but via console.info now'); 269 ``` 270 271 ## Extend 272 You can simply extend debugger 273 ```js 274 const log = require('debug')('auth'); 275 276 //creates new debug instance with extended namespace 277 const logSign = log.extend('sign'); 278 const logLogin = log.extend('login'); 279 280 log('hello'); // auth hello 281 logSign('hello'); //auth:sign hello 282 logLogin('hello'); //auth:login hello 283 ``` 284 285 ## Set dynamically 286 287 You can also enable debug dynamically by calling the `enable()` method : 288 289 ```js 290 let debug = require('debug'); 291 292 console.log(1, debug.enabled('test')); 293 294 debug.enable('test'); 295 console.log(2, debug.enabled('test')); 296 297 debug.disable(); 298 console.log(3, debug.enabled('test')); 299 300 ``` 301 302 print : 303 ``` 304 1 false 305 2 true 306 3 false 307 ``` 308 309 Usage : 310 `enable(namespaces)` 311 `namespaces` can include modes separated by a colon and wildcards. 312 313 Note that calling `enable()` completely overrides previously set DEBUG variable : 314 315 ``` 316 $ DEBUG=foo node -e 'var dbg = require("debug"); dbg.enable("bar"); console.log(dbg.enabled("foo"))' 317 => false 318 ``` 319 320 `disable()` 321 322 Will disable all namespaces. The functions returns the namespaces currently 323 enabled (and skipped). This can be useful if you want to disable debugging 324 temporarily without knowing what was enabled to begin with. 325 326 For example: 327 328 ```js 329 let debug = require('debug'); 330 debug.enable('foo:*,-foo:bar'); 331 let namespaces = debug.disable(); 332 debug.enable(namespaces); 333 ``` 334 335 Note: There is no guarantee that the string will be identical to the initial 336 enable string, but semantically they will be identical. 337 338 ## Checking whether a debug target is enabled 339 340 After you've created a debug instance, you can determine whether or not it is 341 enabled by checking the `enabled` property: 342 343 ```javascript 344 const debug = require('debug')('http'); 345 346 if (debug.enabled) { 347 // do stuff... 348 } 349 ``` 350 351 You can also manually toggle this property to force the debug instance to be 352 enabled or disabled. 353 354 ## Usage in child processes 355 356 Due to the way `debug` detects if the output is a TTY or not, colors are not shown in child processes when `stderr` is piped. A solution is to pass the `DEBUG_COLORS=1` environment variable to the child process. 357 For example: 358 359 ```javascript 360 worker = fork(WORKER_WRAP_PATH, [workerPath], { 361 stdio: [ 362 /* stdin: */ 0, 363 /* stdout: */ 'pipe', 364 /* stderr: */ 'pipe', 365 'ipc', 366 ], 367 env: Object.assign({}, process.env, { 368 DEBUG_COLORS: 1 // without this settings, colors won't be shown 369 }), 370 }); 371 372 worker.stderr.pipe(process.stderr, { end: false }); 373 ``` 374 375 376 ## Authors 377 378 - TJ Holowaychuk 379 - Nathan Rajlich 380 - Andrew Rhyne 381 - Josh Junon 382 383 ## Backers 384 385 Support us with a monthly donation and help us continue our activities. 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License 455 456 (The MIT License) 457 458 Copyright (c) 2014-2017 TJ Holowaychuk <tj@vision-media.ca> 459 Copyright (c) 2018-2021 Josh Junon 460 461 Permission is hereby granted, free of charge, to any person obtaining 462 a copy of this software and associated documentation files (the 463 'Software'), to deal in the Software without restriction, including 464 without limitation the rights to use, copy, modify, merge, publish, 465 distribute, sublicense, and/or sell copies of the Software, and to 466 permit persons to whom the Software is furnished to do so, subject to 467 the following conditions: 468 469 The above copyright notice and this permission notice shall be 470 included in all copies or substantial portions of the Software. 471 472 THE SOFTWARE IS PROVIDED 'AS IS', WITHOUT WARRANTY OF ANY KIND, 473 EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF 474 MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. 475 IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY 476 CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, 477 TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE 478 SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.