README.md
1 # debug 2 [](#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 In Chromium-based web browsers (e.g. Brave, Chrome, and Electron), the JavaScript console will—by default—only show messages logged by `debug` if the "Verbose" log level is _enabled_. 245 246 <img width="647" src="https://user-images.githubusercontent.com/7143133/152083257-29034707-c42c-4959-8add-3cee850e6fcf.png"> 247 248 ## Output streams 249 250 By default `debug` will log to stderr, however this can be configured per-namespace by overriding the `log` method: 251 252 Example [_stdout.js_](./examples/node/stdout.js): 253 254 ```js 255 var debug = require('debug'); 256 var error = debug('app:error'); 257 258 // by default stderr is used 259 error('goes to stderr!'); 260 261 var log = debug('app:log'); 262 // set this namespace to log via console.log 263 log.log = console.log.bind(console); // don't forget to bind to console! 264 log('goes to stdout'); 265 error('still goes to stderr!'); 266 267 // set all output to go via console.info 268 // overrides all per-namespace log settings 269 debug.log = console.info.bind(console); 270 error('now goes to stdout via console.info'); 271 log('still goes to stdout, but via console.info now'); 272 ``` 273 274 ## Extend 275 You can simply extend debugger 276 ```js 277 const log = require('debug')('auth'); 278 279 //creates new debug instance with extended namespace 280 const logSign = log.extend('sign'); 281 const logLogin = log.extend('login'); 282 283 log('hello'); // auth hello 284 logSign('hello'); //auth:sign hello 285 logLogin('hello'); //auth:login hello 286 ``` 287 288 ## Set dynamically 289 290 You can also enable debug dynamically by calling the `enable()` method : 291 292 ```js 293 let debug = require('debug'); 294 295 console.log(1, debug.enabled('test')); 296 297 debug.enable('test'); 298 console.log(2, debug.enabled('test')); 299 300 debug.disable(); 301 console.log(3, debug.enabled('test')); 302 303 ``` 304 305 print : 306 ``` 307 1 false 308 2 true 309 3 false 310 ``` 311 312 Usage : 313 `enable(namespaces)` 314 `namespaces` can include modes separated by a colon and wildcards. 315 316 Note that calling `enable()` completely overrides previously set DEBUG variable : 317 318 ``` 319 $ DEBUG=foo node -e 'var dbg = require("debug"); dbg.enable("bar"); console.log(dbg.enabled("foo"))' 320 => false 321 ``` 322 323 `disable()` 324 325 Will disable all namespaces. The functions returns the namespaces currently 326 enabled (and skipped). This can be useful if you want to disable debugging 327 temporarily without knowing what was enabled to begin with. 328 329 For example: 330 331 ```js 332 let debug = require('debug'); 333 debug.enable('foo:*,-foo:bar'); 334 let namespaces = debug.disable(); 335 debug.enable(namespaces); 336 ``` 337 338 Note: There is no guarantee that the string will be identical to the initial 339 enable string, but semantically they will be identical. 340 341 ## Checking whether a debug target is enabled 342 343 After you've created a debug instance, you can determine whether or not it is 344 enabled by checking the `enabled` property: 345 346 ```javascript 347 const debug = require('debug')('http'); 348 349 if (debug.enabled) { 350 // do stuff... 351 } 352 ``` 353 354 You can also manually toggle this property to force the debug instance to be 355 enabled or disabled. 356 357 ## Usage in child processes 358 359 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. 360 For example: 361 362 ```javascript 363 worker = fork(WORKER_WRAP_PATH, [workerPath], { 364 stdio: [ 365 /* stdin: */ 0, 366 /* stdout: */ 'pipe', 367 /* stderr: */ 'pipe', 368 'ipc', 369 ], 370 env: Object.assign({}, process.env, { 371 DEBUG_COLORS: 1 // without this settings, colors won't be shown 372 }), 373 }); 374 375 worker.stderr.pipe(process.stderr, { end: false }); 376 ``` 377 378 379 ## Authors 380 381 - TJ Holowaychuk 382 - Nathan Rajlich 383 - Andrew Rhyne 384 - Josh Junon 385 386 ## Backers 387 388 Support us with a monthly donation and help us continue our activities. 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License 458 459 (The MIT License) 460 461 Copyright (c) 2014-2017 TJ Holowaychuk <tj@vision-media.ca> 462 Copyright (c) 2018-2021 Josh Junon 463 464 Permission is hereby granted, free of charge, to any person obtaining 465 a copy of this software and associated documentation files (the 466 'Software'), to deal in the Software without restriction, including 467 without limitation the rights to use, copy, modify, merge, publish, 468 distribute, sublicense, and/or sell copies of the Software, and to 469 permit persons to whom the Software is furnished to do so, subject to 470 the following conditions: 471 472 The above copyright notice and this permission notice shall be 473 included in all copies or substantial portions of the Software. 474 475 THE SOFTWARE IS PROVIDED 'AS IS', WITHOUT WARRANTY OF ANY KIND, 476 EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF 477 MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. 478 IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY 479 CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, 480 TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE 481 SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.