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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) <img width="647" src="https://user-images.githubusercontent.com/71256/29091486-fa38524c-7c37-11e7-895f-e7ec8e1039b6.png"> A tiny JavaScript debugging utility modelled after Node.js core's debugging technique. Works in Node.js and web browsers. ## 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_](./examples/node/app.js): ```js var debug = require('debug')('http') , http = require('http') , name = 'My App'; // fake app debug('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 kind require('./worker'); ``` Example [_worker.js_](./examples/node/worker.js): ```js var a = require('debug')('worker:a') , b = require('debug')('worker:b'); function work() { a('doing lots of uninteresting work'); setTimeout(work, Math.random() * 1000); } work(); function workb() { b('doing some work'); setTimeout(workb, Math.random() * 2000); } workb(); ``` The `DEBUG` environment variable is then used to enable these based on space or comma-delimited names. Here are some examples: <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"> <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"> <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"> #### 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. ## Namespace Colors 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 instance a debug line belongs to. #### Node.js In Node.js, colors are enabled when stderr is a TTY. You also _should_ install the [`supports-color`](https://npmjs.org/supports-color) module alongside debug, otherwise debug will only use a small handful of basic colors. <img width="521" src="https://user-images.githubusercontent.com/71256/29092181-47f6a9e6-7c3a-11e7-9a14-1928d8a711cd.png"> #### Web Browser 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). <img width="524" src="https://user-images.githubusercontent.com/71256/29092033-b65f9f2e-7c39-11e7-8e32-f6f0d8e865c1.png"> ## 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. <img width="647" src="https://user-images.githubusercontent.com/71256/29091486-fa38524c-7c37-11e7-895f-e7ec8e1039b6.png"> When stdout is not a TTY, `Date#toISOString()` is used, making it more useful for logging the debug information as shown below: <img width="647" src="https://user-images.githubusercontent.com/71256/29091956-6bd78372-7c39-11e7-8c55-c948396d6edd.png"> ## 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". 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. ## 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/disables specific debugging namespaces. | | `DEBUG_HIDE_DATE` | Hide date from debug output (non-TTY). | | `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 sh