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# debug [](https://travis-ci.org/visionmedia/debug) [](https://coveralls.io/github/visionmedia/debug?branch=master) [](https://visionmedia-community-slackin.now.sh/) [](#backers) [](#sponsors) A tiny node.js debugging utility modelled after node core's debugging technique. **Discussion around the V3 API is under way [here](https://github.com/visionmedia/debug/issues/370)** ## Installation ```bash $ npm install debug ``` ## Usage `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. Example _app.js_: ```js var debug = require('debug')('http') , http = require('http') , name = 'My App'; // fake app debug('booting %s', name); http.createServer(function(req, res){ debug(req.method + ' ' + req.url); res.end('hello\n'); }).listen(3000, function(){ debug('listening'); }); // fake worker of some kind require('./worker'); ``` Example _worker.js_: ```js var debug = require('debug')('worker'); setInterval(function(){ debug('doing some work'); }, 1000); ``` The __DEBUG__ environment variable is then used to enable these based on space or comma-delimited names. Here are some examples:   #### Windows note On Windows the environment variable is set using the `set` command. ```cmd set DEBUG=*,-not_this ``` Note that PowerShell uses different syntax to set environment variables. ```cmd $env:DEBUG = "*,-not_this" ``` Then, run the program to be debugged as usual. ## Millisecond diff 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.  When stdout is not a TTY, `Date#toUTCString()` is used, making it more useful for logging the debug information as shown below:  ## Conventions 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". ## Wildcards The `*` character may be used as a wildcard. Suppose for example your library has debuggers named "connect:bodyParser", "connect:compress", "connect:session", instead of listing all three with `DEBUG=connect:bodyParser,connect:compress,connect:session`, you may simply do `DEBUG=connect:*`, or to run everything using this module simply use `DEBUG=*`. You can also exclude specific debuggers by prefixing them with a "-" character. For example, `DEBUG=*,-connect:*` would include all debuggers except those starting with "connect:". ## Environment Variables When running through Node.js, you can set a few environment variables that will change the behavior of the debug logging: | Name | Purpose | |-----------|-------------------------------------------------| | `DEBUG` | Enables/disabled specific debugging namespaces. | | `DEBUG_COLORS`| Whether or not to use colors in the debug output. | | `DEBUG_DEPTH` | Object inspection depth. | | `DEBUG_SHOW_HIDDEN` | Shows hidden properties on inspected objects. | __Note:__ The environment variables beginning with `DEBUG_` end up being converted into an Options object that gets used with `%o`/`%O` formatters. See the Node.js documentation for [`util.inspect()`](https://nodejs.org/api/util.html#util_util_inspect_object_options) for the complete list. ## Formatters Debug uses [printf-style](https://wikipedia.org/wiki/Printf_format_string) formatting. Below are the officially supported formatters: | Formatter | Representation | |-----------|----------------| | `%O` | Pretty-print an Object on multiple lines. | | `%o` | Pretty-print an Object all on a single line. | | `%s` | String. | | `%d` | Number (both integer and float). | | `%j` | JSON. Replaced with the string '[Circular]' if the argument contains circular references. | | `%%` | Single percent sign ('%'). This does not consume an argument. | ### Custom formatters You can add custom formatters by extending the `debug.formatters` object. For example, if you wanted to add support for rendering a Buffer as hex with `%h`, you could do something like: ```js const createDebug = require('debug') createDebug.formatters.h = (v) => { return v.toString('hex') } // …elsewhere const debug = createDebug('foo') debug('this is hex: %h', new Buffer('hello world')) // foo this is hex: 68656c6c6f20776f726c6421 +0ms ``` ## Browser support You can build a browser-ready script using [browserify](https://github.com/substack/node-browserify), or just use the [browserify-as-a-service](https://wzrd.in/) [build](https://wzrd.in/standalone/debug@latest), if you don't want to build it yourself. Debug's enable state is currently persisted by `localStorage`. Consider the situation shown below where you have `worker:a` and `worker:b`, and wish to debug both. You can enable this using `localStorage.debug`: ```js localStorage.debug = 'worker:*' ``` And then refresh the page. ```js a = debug('worker:a'); b = debug('worker:b'); setInterval(function(){ a('doing some work'); }, 1000); setInterval(function(){ b('doing some work'); }, 1200); ``` #### Web Inspector Colors Colors are also enabled on "Web Inspectors" that understand the `%c` formatting option. These are WebKit web inspectors, Firefox ([since version 31](https://hacks.mozilla.org/2014/05/editable-box-model-multiple-selection-sublime-text-keys-much-more-firefox-developer-tools-episode-31/)) and the Firebug plugin for Firefox (any version). Colored output looks something like:  ## Output streams By default `debug` will log to stderr, however this can be configured per-namespace by overriding the `log` method: Example _stdout.js_: ```js var debug = require('debug'); var error = debug('app:error'); // by default stderr is used error('goes to stderr!'); var log = debug('app:log'); // set this namespace to log via console.log log.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 settings debug.log = console.info.bind(console); error('now goes to stdout via console.info'); log('still goes to stdout, but via console.info now'); ``` ## Authors - TJ Holowaychuk - Nathan Rajlich - Andrew Rhyne ## Backers Support us with a monthly donation and help us continue our activities. [[Become a backer](https://opencollective.com/debug#backer)] <a href="https://opencollective.com/debug/backer/0/website" target="_blank"><img src="https://opencollective.com/debug/backer/0/avatar.svg"></a> <a href="https://opencollective.com/debug/backer/1/website" targ