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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
5<img width="647" src="https://user-images.githubusercontent.com/71256/29091486-fa38524c-7c37-11e7-895f-e7ec8e1039b6.png">
6
7A tiny JavaScript debugging utility modelled after Node.js core's debugging
8technique. 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
20Example [_app.js_](./examples/node/app.js):
21
22```js
23var debug = require('debug')('http')
24 , http = require('http')
25 , name = 'My App';
26
27// fake app
28
29debug('booting %o', name);
30
31http.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
40require('./worker');
41```
42
43Example [_worker.js_](./examples/node/worker.js):
44
45```js
46var a = require('debug')('worker:a')
47 , b = require('debug')('worker:b');
48
49function work() {
50 a('doing lots of uninteresting work');
51 setTimeout(work, Math.random() * 1000);
52}
53
54work();
55
56function workb() {
57 b('doing some work');
58 setTimeout(workb, Math.random() * 2000);
59}
60
61workb();
62```
63
64The `DEBUG` environment variable is then used to enable these based on space or
65comma-delimited names.
66
67Here 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
77On Windows the environment variable is set using the `set` command.
78
79```cmd
80set DEBUG=*,-not_this
81```
82
83Example:
84
85```cmd
86set DEBUG=* & node app.js
87```
88
89##### PowerShell (VS Code default)
90
91PowerShell uses different syntax to set environment variables.
92
93```cmd
94$env:DEBUG = "*,-not_this"
95```
96
97Example:
98
99```cmd
100$env:DEBUG='app';node app.js
101```
102
103Then, run the program to be debugged as usual.
104
105npm script example:
106```js
107 "windowsDebug": "@powershell -Command $env:DEBUG='*';node app.js",
108```
109
110## Namespace Colors
111
112Every debug instance has a color generated for it based on its namespace name.
113This helps when visually parsing the debug output to identify which debug instance
114a debug line belongs to.
115
116#### Node.js
117
118In Node.js, colors are enabled when stderr is a TTY. You also _should_ install
119the [`supports-color`](https://npmjs.org/supports-color) module alongside debug,
120otherwise 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
126Colors are also enabled on "Web Inspectors" that understand the `%c` formatting
127option. These are WebKit web inspectors, Firefox ([since version
12831](https://hacks.mozilla.org/2014/05/editable-box-model-multiple-selection-sublime-text-keys-much-more-firefox-developer-tools-episode-31/))
129and 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
136When 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
140When 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
147If 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
151The `*` character may be used as a wildcard. Suppose for example your library has
152debuggers named "connect:bodyParser", "connect:compress", "connect:session",
153instead 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
157You can also exclude specific debuggers by prefixing them with a "-" character.
158For example, `DEBUG=*,-connect:*` would include all debuggers except those
159starting with "connect:".
160
161## Environment Variables
162
163When running through Node.js, you can set a few environment variables that will
164change 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
176converted into an Options object that gets used with `%o`/`%O` formatters.
177See the Node.js documentation for
178[`util.inspect()`](https://nodejs.org/api/util.html#util_util_inspect_object_options)
179for the complete list.
180
181## Formatters
182
183Debug uses [printf-style](https://wikipedia.org/wiki/Printf_format_string) formatting.
184Below 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
198You can add custom formatters by extending the `debug.formatters` object.
199For example, if you wanted to add support for rendering a Buffer as hex with
200`%h`, you could do something like:
201
202```js
203const createDebug = require('debug')
204createDebug.formatters.h = (v) => {
205 return v.toString('hex')
206}
207
208// …elsewhere
209const debug = createDebug('foo')
210debug('this is hex: %h', new Buffer('hello world'))
211// foo this is hex: 68656c6c6f20776f726c6421 +0ms
212```
213
214
215## Browser Support
216
217You can build a browser-ready script using [browserify](https://github.com/substack/node-browserify),
218or just use the [browserify-as-a-service](https://wzrd.in/) [build](https://wzrd.in/standalone/debug@latest),
219if you don't want to build it yourself.
220
221Debug's enable state is currently persisted by `localStorage`.
222Consider the situation shown below where you have `worker:a` and `worker:b`,
223and wish to debug both. You can enable this using `localStorage.debug`:
224
225```js
226localStorage.debug = 'worker:*'
227```
228
229And then refresh the page.
230
231```js
232a = debug('worker:a');
233b = debug('worker:b');
234
235setInterval(function(){
236 a('doing some work');
237}, 1000);
238
239setInterval(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
249Example [_stdout.js_](./examples/node/stdout.js):
250
251```js
252var debug = require('debug');
253var error = debug('app:error');
254
255// by default stderr is used
256error('goes to stderr!');
257
258var log = debug('app:log');
259// set this namespace to log via console.log
260log.log = console.log.bind(console); // don't forget to bind to console!
261log('goes to stdout');
262error('still goes to stderr!');
263
264// set all output to go via console.info
265// overrides all per-namespace log settings
266debug.log = console.info.bind(console);
267error('now goes to stdout via console.info');
268log('still goes to stdout, but via console.info now');
269```
270
271## Extend
272You can simply extend debugger
273```js
274const log = require('debug')('auth');
275
276//creates new debug instance with extended namespace
277const logSign = log.extend('sign');
278const logLogin = log.extend('login');
279
280log('hello'); // auth hello
281logSign('hello'); //auth:sign hello
282logLogin('hello'); //auth:login hello
283```
284
285## Set dynamically
286
287You can also enable debug dynamically by calling the `enable()` method :
288
289```js
290let debug = require('debug');
291
292console.log(1, debug.enabled('test'));
293
294debug.enable('test');
295console.log(2, debug.enabled('test'));
296
297debug.disable();
298console.log(3, debug.enabled('test'));
299
300```
301
302print :
303```
3041 false
3052 true
3063 false
307```
308
309Usage :
310`enable(namespaces)`
311`namespaces` can include modes separated by a colon and wildcards.
312
313Note 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
322Will disable all namespaces. The functions returns the namespaces currently
323enabled (and skipped). This can be useful if you want to disable debugging
324temporarily without knowing what was enabled to begin with.
325
326For example:
327
328```js
329let debug = require('debug');
330debug.enable('foo:*,-foo:bar');
331let namespaces = debug.disable();
332debug.enable(namespaces);
333```
334
335Note: There is no guarantee that the string will be identical to the initial
336enable string, but semantically they will be identical.
337
338## Checking whether a debug target is enabled
339
340After you've created a debug instance, you can determine whether or not it is
341enabled by checking the `enabled` property:
342
343```javascript
344const debug = require('debug')('http');
345
346if (debug.enabled) {
347 // do stuff...
348}
349```
350
351You can also manually toggle this property to force the debug instance to be
352enabled or disabled.
353
354
355## Authors
356
357 - TJ Holowaychuk
358 - Nathan Rajlich
359 - Andrew Rhyne
360
361## Backers
362
363Support us with a monthly donation and help us continue our activities. [[Become a backer](https://opencollective.com/debug#backer)]
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395
396
397## Sponsors
398
399Become 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)]
400
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431
432## License
433
434(The MIT License)
435
436Copyright (c) 2014-2017 TJ Holowaychuk &lt;tj@vision-media.ca&gt;
437
438Permission is hereby granted, free of charge, to any person obtaining
439a copy of this software and associated documentation files (the
440'Software'), to deal in the Software without restriction, including
441without limitation the rights to use, copy, modify, merge, publish,
442distribute, sublicense, and/or sell copies of the Software, and to
443permit persons to whom the Software is furnished to do so, subject to
444the following conditions:
445
446The above copyright notice and this permission notice shall be
447included in all copies or substantial portions of the Software.
448
449THE SOFTWARE IS PROVIDED 'AS IS', WITHOUT WARRANTY OF ANY KIND,
450EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
451MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.
452IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY
453CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT,
454TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE
455SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
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