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A tiny JavaScript debugging utility modelled after Node.js core's debuggingtechnique. Works in Node.js and web browsers.

$ npm install debug

debug exposes a function; simply pass this function the name of your module, and it will return a decorated version ofconsole.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.

Exampleapp.js:

vardebug=require('debug')('http'),http=require('http'),name='My App';// fake appdebug('booting %o',name);http.createServer(function(req,res){debug(req.method+' '+req.url);res.end('hello\n');}).listen(3000,function(){debug('listening');});// fake worker of some kindrequire('./worker');

Exampleworker.js:

vara=require('debug')('worker:a'),b=require('debug')('worker:b');functionwork(){a('doing lots of uninteresting work');setTimeout(work,Math.random()*1000);}work();functionworkb(){b('doing some work');setTimeout(workb,Math.random()*2000);}workb();

TheDEBUG environment variable is then used to enable these based on space orcomma-delimited names.

Here are some examples:

On Windows the environment variable is set using theset command.

setDEBUG=*,-not_this

Example:

setDEBUG=*& node app.js

PowerShell uses different syntax to set environment variables.

$env:DEBUG ="*,-not_this"

Example:

$env:DEBUG='app';node app.js

Then, run the program to be debugged as usual.

npm script example:

"windowsDebug":"@powershell -Command $env:DEBUG='*';node app.js",

Every debug instance has a color generated for it based on its namespace name.This helps when visually parsing the debug output to identify which debug instancea debug line belongs to.

In Node.js, colors are enabled when stderr is a TTY. You alsoshould installthesupports-color module alongside debug,otherwise debug will only use a small handful of basic colors.

Colors are also enabled on "Web Inspectors" that understand the%c formattingoption. These are WebKit web inspectors, Firefox (since version31)and the Firebug plugin for Firefox (any version).

When actively developing an application it can be useful to see when the time spent between onedebug() call and the next. Suppose for example you invokedebug() before requesting a resource, and after as well, the "+NNNms" will show you how much time was spent between calls.

When stdout is not a TTY,Date#toISOString() is used, making it more useful for logging the debug information as shown below:

If you're using this in one or more of your libraries, youshould use the name of your library so that developers may toggle debugging as desired without guessing names. If you have more than one debuggers youshould 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.

The* character may be used as a wildcard. Suppose for example your library hasdebuggers named "connect:bodyParser", "connect:compress", "connect:session",instead of listing all three withDEBUG=connect:bodyParser,connect:compress,connect:session, you may simply doDEBUG=connect:*, or to run everything using this module simply useDEBUG=*.

You can also exclude specific debuggers by prefixing them with a "-" character.For example,DEBUG=*,-connect:* would include all debuggers except thosestarting with "connect:".

When running through Node.js, you can set a few environment variables that willchange the behavior of the debug logging:

Note: The environment variables beginning withDEBUG_ end up beingconverted into an Options object that gets used with%o/%O formatters.See the Node.js documentation forutil.inspect()for the complete list.

Debug usesprintf-style formatting.Below are the officially supported formatters:

You can add custom formatters by extending thedebug.formatters object.For example, if you wanted to add support for rendering a Buffer as hex with%h, you could do something like:

constcreateDebug=require('debug')createDebug.formatters.h=(v)=>{returnv.toString('hex')}// …elsewhereconstdebug=createDebug('foo')debug('this is hex: %h',newBuffer('hello world'))//   foo this is hex: 68656c6c6f20776f726c6421 +0ms

You can build a browser-ready script usingbrowserify,or just use thebrowserify-as-a-servicebuild,if you don't want to build it yourself.

Debug's enable state is currently persisted bylocalStorage.Consider the situation shown below where you haveworker:a andworker:b,and wish to debug both. You can enable this usinglocalStorage.debug:

localStorage.debug='worker:*'

And then refresh the page.

a=debug('worker:a');b=debug('worker:b');setInterval(function(){a('doing some work');},1000);setInterval(function(){b('doing some work');},1200);

In Chromium-based web browsers (e.g. Brave, Chrome, and Electron), the JavaScript console will—by default—only show messages logged bydebug if the "Verbose" log level isenabled.

By defaultdebug will log to stderr, however this can be configured per-namespace by overriding thelog method:

Examplestdout.js:

vardebug=require('debug');varerror=debug('app:error');// by default stderr is usederror('goes to stderr!');varlog=debug('app:log');// set this namespace to log via console.loglog.log=console.log.bind(console);// don't forget to bind to console!log('goes to stdout');error('still goes to stderr!');// set all output to go via console.info// overrides all per-namespace log settingsdebug.log=console.info.bind(console);error('now goes to stdout via console.info');log('still goes to stdout, but via console.info now');

You can simply extend debugger

constlog=require('debug')('auth');//creates new debug instance with extended namespaceconstlogSign=log.extend('sign');constlogLogin=log.extend('login');log('hello');// auth hellologSign('hello');//auth:sign hellologLogin('hello');//auth:login hello

You can also enable debug dynamically by calling theenable() method :

letdebug=require('debug');console.log(1,debug.enabled('test'));debug.enable('test');console.log(2,debug.enabled('test'));debug.disable();console.log(3,debug.enabled('test'));

print :

1 false2 true3 false

Usage :
enable(namespaces)
namespaces can include modes separated by a colon and wildcards.

Note that callingenable() completely overrides previously set DEBUG variable :

$ DEBUG=foo node -e 'var dbg = require("debug"); dbg.enable("bar"); console.log(dbg.enabled("foo"))'=> false

disable()

Will disable all namespaces. The functions returns the namespaces currentlyenabled (and skipped). This can be useful if you want to disable debuggingtemporarily without knowing what was enabled to begin with.

For example:

letdebug=require('debug');debug.enable('foo:*,-foo:bar');letnamespaces=debug.disable();debug.enable(namespaces);

Note: There is no guarantee that the string will be identical to the initialenable string, but semantically they will be identical.

After you've created a debug instance, you can determine whether or not it isenabled by checking theenabled property:

constdebug=require('debug')('http');if(debug.enabled){// do stuff...}

You can also manually toggle this property to force the debug instance to beenabled or disabled.

Due to the waydebug detects if the output is a TTY or not, colors are not shown in child processes whenstderr is piped. A solution is to pass theDEBUG_COLORS=1 environment variable to the child process.
For example:

worker=fork(WORKER_WRAP_PATH,[workerPath],{stdio:[/* stdin: */0,/* stdout: */'pipe',/* stderr: */'pipe','ipc',],env:Object.assign({},process.env,{DEBUG_COLORS:1// without this settings, colors won't be shown}),});worker.stderr.pipe(process.stderr,{end:false});

• TJ Holowaychuk
• Nathan Rajlich
• Andrew Rhyne
• Josh Junon

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(The MIT License)

Copyright (c) 2014-2017 TJ Holowaychuk <tj@vision-media.ca>Copyright (c) 2018-2021 Josh Junon

Permission is hereby granted, free of charge, to any person obtaininga copy of this software and associated documentation files (the'Software'), to deal in the Software without restriction, includingwithout limitation the rights to use, copy, modify, merge, publish,distribute, sublicense, and/or sell copies of the Software, and topermit persons to whom the Software is furnished to do so, subject tothe following conditions:

The above copyright notice and this permission notice shall beincluded in all copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED 'AS IS', WITHOUT WARRANTY OF ANY KIND,EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OFMERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANYCLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT,TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THESOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.